Chemistry · Secondary school · Matter and Mixtures

Matter and Mixtures

The kitchen and the water treatment works are our laboratories, the first with the saucepan, the sieve and the salt, the second with a whole city separating mud and microorganisms from the water it is going to drink. We will ask how we recognise a substance, how we know whether something is a mixture and how we separate what has been mixed. On every bench a magnifier shows the particles behind what we see with the naked eye.

1States of matter
2Melting and boiling points
3Density
4Substances and mixtures
5Phases
6Heterogeneous mixtures
7Homogeneous mixtures
8Water treatment
✓Challenges

STEP 1

What changes when matter changes state?

We call matter anything that has mass and takes up space, from the water in a glass to the air that fills a balloon. At secondary school we deal with three states of matter. A solid has its own shape and volume, a liquid has its own volume and takes the shape of its container, and a gas fills the whole container and can be compressed.

The difference lies in the particles. In a solid they are held in place and vibrate around a fixed position, in a liquid they stay together and slide over one another, and in a gas they are far apart and fly about until they hit a neighbour or the wall. The agitation grows with temperature, and we can think of a change of state as the moment when this agitation overcomes the attraction that held the particles in their previous arrangement.

Each change has a name. From solid to liquid we have melting, and the way back is freezing, or solidification. From liquid to gas we have vaporisation, which is undone by condensation. When a solid turns into a gas without passing through the liquid, like mothballs in the wardrobe, we speak of sublimation, and the reverse is deposition, also called desublimation.

In all of these changes the substance remains itself. The water in ice, in the glass and in steam has the same molecules, closer together or further apart, and so a change of state is a physical change, which alters the arrangement of the particles and leaves the substance as it was.

solid ⇄ liquidmelting → · ← freezingliquid ⇄ gasvaporisation → · ← condensationsolid ⇄ gassublimation → · ← depositionThe changes to the right take in energy, and the changes to the left give energy back to the surroundings. Deposition is also called desublimation.

Let's discuss

With the water at 25 °C, click "Push the piston". Why does it stop when it touches the liquid, and what does the magnifier show of the particles just below the surface?
Raise the temperature slowly until the water boils and follow the magnifier through the change. What seems to happen to the distance between the particles, and to how fast they move?
Take the magnifier to the space between the surface and the piston. It shows "nothing" because we leave the vapour for step 2, but what reasons would we have to suspect that there are a few water molecules there?
Choose iron and look for the temperature at which it melts. What relation does this temperature seem to have with the fact that water is liquid in the kitchen and iron is not?

Temperature

State

Own shape?

Own volume?

Temperature

Exercises for step 1 10 questions · 4 basic · 4 intermediate · 2 challenge

basic
What state is each of these in at room temperature: an ice cube, the juice in a glass, a coin, the air inside a football and the honey in a jar?

Show solution
The ice cube and the coin have their own shape and volume, and are solids.
The juice and the honey have their own volume and take the shape of the container, and are liquids, although honey flows much more slowly.
The air fills the whole ball and can be compressed when we pump it up, and is a gas.
Solids, the ice and the coin. Liquids, the juice and the honey. Gas, the air in the ball.
basic
Name the change of state in each case: dry ice that shrinks without leaving a puddle, clothes drying on the line, dew appearing on the grass in the early morning and a mothball that vanishes in the wardrobe.

Show solution
Dry ice is solid carbon dioxide and turns straight into gas, with no puddle, which is sublimation. The white "smoke" around it is water vapour from the air that has condensed into tiny droplets.
The water in the clothes turns into vapour without boiling, and this is vaporisation, of the kind we call evaporation.
Water vapour in the air cools on the grass and turns into liquid, which is condensation.
The mothball goes from solid to gas, another sublimation.
Sublimation, vaporisation, condensation and sublimation.
basic
At 1 atm, water melts at 0 °C and boils at 100 °C. What state is it in at −10 °C, at 50 °C and at 120 °C?

Show solution
Below the melting point water is solid, and −10 °C < 0 °C.
Between the melting point and the boiling point it is liquid, and 0 °C < 50 °C < 100 °C.
Above the boiling point it is a gas, and 120 °C > 100 °C.
Solid at −10 °C, liquid at 50 °C and gas at 120 °C.
basic
Why can we compress the air inside a sealed syringe, and hardly compress water at all in the same syringe?

Show solution
In a gas the particles are far apart, with plenty of empty space between them, and the piston merely brings them closer.
In a liquid the particles are already practically touching one another, and there is no room left to bring them closer.
A gas has empty space between its particles and a liquid has almost none, and so only the gas can be compressed to any appreciable extent.
intermediate
At 1 atm, ethanol melts at −114.1 °C and boils at 78.3 °C, and iron melts at 1538 °C and boils at 2861 °C. What state is ethanol in at −120 °C, 25 °C and 90 °C? And iron at 25 °C, 1600 °C and 3000 °C?

Show solution
We compare each temperature with the MP and the BP of the substance.
For ethanol, −120 °C is below −114.1 °C (solid), 25 °C lies between the two points (liquid) and 90 °C is above 78.3 °C (gas).
For iron, 25 °C is below 1538 °C (solid), 1600 °C lies between 1538 °C and 2861 °C (liquid) and 3000 °C is above 2861 °C (gas).
Ethanol is solid, liquid and gas, and so is iron, in that order.
intermediate
Why do we say that melting an ice cube is a physical change, and burning a sheet of paper is a chemical change?

Show solution
When ice melts, the water molecules stay the same, they only leave the fixed arrangement of the solid and start to slide over one another. If we cool the water again, the ice comes back.
When paper burns, the cellulose reacts with oxygen in the air and forms new substances, such as carbon dioxide and water vapour, and the ashes do not turn back into paper.
In a physical change the substance stays the same, and in a chemical change new substances appear.
intermediate
A puddle of water dries on the ground on a 30 °C day, and the water in a saucepan boils on the hob at 100 °C. Both are vaporisations. What distinguishes evaporation from boiling?

Show solution
In evaporation only the most agitated particles at the surface escape into the air. It happens at any temperature below the boiling point and tends to be slow.
In boiling the liquid turns into vapour in its interior as well, forming bubbles, at a temperature fixed for each pressure, 100 °C for water at 1 atm.
Evaporation happens at the surface, below the BP and slowly, and boiling happens throughout the liquid, with bubbles, at the BP.
intermediate
On the bench, with the water above 100 °C, we push the piston and the volume of the gas falls to a third. What happens to the number of particles in the cylinder, to the distance between them and to the density of the gas?

Show solution
The cylinder is closed and no particle gets in or out, so the number of particles and the mass stay the same.
The same particles now take up a third of the volume, and so they are closer to one another.
Density is mass divided by volume, and with the mass fixed and the volume divided by three it becomes three times greater.
The number of particles does not change, the distance between them shrinks and the density triples.
challenge
On Mars the pressure at the surface is less than 1% of the pressure at the surface of the Earth, and water ice exposed there tends to turn straight into vapour, without melting. What does this suggest about how the melting and boiling points depend on pressure?

Show solution
The melting and boiling points depend on pressure, and the boiling point falls considerably when the pressure drops. That is why water boils below 100 °C at the top of a mountain.
With very low pressure, the boiling point comes down close to the melting point, and the range of temperatures in which water stays liquid shrinks until it almost disappears.
It seems plausible, then, that Martian ice skips the liquid and sublimes, since it finds no temperature range in which it could stay liquid.
Sublimation on Mars suggests that, with low enough pressure, the BP approaches the MP and the liquid state no longer appears.
challenge
Use the agitation of the particles to explain why a gas fills any container it is put in, however large, while a liquid stays at the bottom.

Show solution
In a gas the particles move very fast and are so far apart that the attraction between them hardly counts.
Each particle goes in a straight line until it hits a neighbour or the wall, and nothing pulls it back towards the others.
Over time the particles spread through all the available space, and the gas comes to fill the whole container.
In a liquid the attraction between neighbouring particles still holds the whole together, and gravity keeps it at the bottom.
In a gas the agitation overcomes the attraction and the particles spread through the whole volume, and in a liquid the attraction keeps them together.

STEP 2

How do we recognise a substance with a thermometer?

Mass and volume tell us how much material we have, and so we call them general properties. A glass and a swimming pool full of water have very different masses and the same water inside. To find out which material it is, we look at its specific properties, such as the melting point (MP), the boiling point (BP) and the density, which work like a fingerprint of the substance. At 1 atm pure water melts at 0 °C and boils at 100 °C, in the glass or in the pool.

When we heat a pure substance over a steady flame, the temperature rises, stops during melting, rises again and stops once more during boiling. We call these flat stretches of the curve plateaus. While there is solid melting or liquid boiling, the heat that arrives goes into pulling the particles apart and does not increase their agitation, and the thermometer stands still. The physics of this heat is in the lesson on Heat and Temperature.

In a mixture the curve changes shape. Salt water starts to boil above 100 °C and the temperature keeps rising while it boils, because the vapour that leaves is only water and the salt stays behind. The solution becomes more and more concentrated, and its boiling point rises with it. The plateau turns into a ramp, perhaps the simplest sign that we have a mixture on our hands.

The MPs and BPs in the tables hold at 1 atm, and pressure matters. In very high cities, such as La Paz, in Bolivia, the air presses less on the saucepan and water boils well below 100 °C, which tends to make cooking slower.

\(Q = m\,c\,\Delta T\)\(Q = m\,L\)On the plateaus the temperature does not change, and all the heat goes into \(Q = m\,L\), with \(L\) the latent heat of fusion or of vaporisation. Away from them, \(Q = m\,c\,\Delta T\) holds.

Let's discuss

With pure water, let the heater run to the end. How many plateaus appear on the graph, and at what temperatures are they?
Switch to salt water and follow the zoom near boiling. At what temperature does it start to boil, and what does the thermometer do after that?
Restart pure water with 50 W and then with 500 W. What changes in the length of each plateau, and what seems not to change?
Heat sample X, read the two plateaus and look for the row of the table that matches them before you make your guess. During boiling, take the magnifier to the liquid and to the space above it.

Heater power

Temperature

Phase

Melting plateau

Boiling plateau

Time (min:s)

MP and BP at 1 atm
Substance	MP (°C)	BP (°C)

Exercises for step 2 10 questions · 4 basic · 4 intermediate · 2 challenge

basic
Sort these into general and specific properties: mass, boiling point, volume, density and melting point.

Show solution
A general property depends on the amount of material and does not tell us which material it is. Mass and volume grow with the sample and apply to any material.
A specific property has its own value for each substance, under the same conditions, whether the sample is large or small.
General, mass and volume. Specific, the boiling point, the density and the melting point.
basic
A pure substance, solid at the start, was heated over a steady flame at 1 atm, and the temperature was written down every 2 minutes: 0, 10, 17, 17, 17, 50, 85, 118, 118, 118 (in °C). What are the MP and the BP of this substance?

Show solution
The plateaus appear where the temperature repeats while the heating goes on.
The first plateau, at 17 °C, is melting, because the substance started as a solid. The second, at 118 °C, is boiling.
MP = 17 °C and BP = 118 °C.
basic
At 1 atm, isopropanol melts at −87.9 °C and boils at 82.3 °C. What state is it in at −100 °C, at 25 °C and at 90 °C?

Show solution
Below the MP the substance is solid, and −100 °C < −87.9 °C.
Between the MP and the BP it is liquid, and −87.9 °C < 25 °C < 82.3 °C.
Above the BP it is a gas, and 90 °C > 82.3 °C.
Solid at −100 °C, liquid at 25 °C and gas at 90 °C.
basic
While ice is melting the flame stays lit and the temperature stays at 0 °C. Where does the heat the water receives in this interval go?

Show solution
Temperature measures the average agitation of the particles, and it does not change on the plateau.
The heat goes into freeing the molecules from the fixed arrangement of the ice, overcoming the attraction between them, and it is the latent heat of fusion.
The heat pulls the particles of the solid apart instead of agitating them more, and so the temperature stands still until the last piece of ice has melted.
intermediate
A colourless liquid melts at −88 °C and boils at 82 °C, at 1 atm. Using the table on the bench (water, ethanol, acetone and isopropanol), say which substance it seems to be, and why looking only at the BP could mislead us.

Show solution
In the table, water has an MP of 0 °C and a BP of 100 °C, ethanol −114.1 °C and 78.3 °C, acetone −94.7 °C and 56.1 °C and isopropanol −87.9 °C and 82.3 °C.
Both measured values lie within half a degree of isopropanol.
The BP of ethanol, 78.3 °C, is 4 °C from the measured value, and a careless measurement could confuse the two. The MP, which differs by more than 25 °C, settles the doubt.
The liquid seems to be isopropanol, and checking both points is safer than checking just one.
intermediate
Two liquid samples were heated at 1 atm. During boiling, sample A stayed at 100.0 °C from start to finish, and sample B started to boil at 101.8 °C and reached 103.5 °C. Which of them is pure water, and what might the other be?

Show solution
A pure substance boils at constant temperature, with a plateau, and that is the case of A, at the BP of water at 1 atm.
In B the temperature rises while the liquid boils, a ramp. This happens in a mixture in which the vapour carries away only one of the components and the other becomes concentrated in what is left, as in salt water.
A is pure water, and B is a mixture, probably water with something dissolved in it that does not evaporate along with it.
intermediate
In a pressure cooker the sealed lid keeps a pressure inside greater than the one outside. What does this do to the boiling point of water, and why do beans tend to cook faster?

Show solution
The boiling point rises when the pressure on the liquid increases, and the water in the cooker boils above 100 °C.
While it boils, water stays at its BP, and in an ordinary saucepan at sea level this ceiling is 100 °C. In the pressure cooker the ceiling is higher.
With the food immersed in hotter water, it is plausible that the changes that cook it go faster.
The higher pressure raises the BP, the water gets hotter than 100 °C and the beans tend to cook in less time.
intermediate
How much heat is needed to melt 50 g of ice that is already at 0 °C? Use \(L_f = 334\ \text{J/g}\).

Show solution
The ice is already at its MP, and all the heat goes into melting, without changing the temperature.
\(Q = m\,L_f = 50 \cdot 334 = 16\,700\ \text{J}\).
\(Q = 16\,700\ \text{J} \approx 16.7\ \text{kJ}\)
challenge
Work out the energy to take 100 g of ice from −20 °C to steam at 100 °C, stage by stage. Use \(c_{\text{ice}} = 2.09\), \(c_{\text{water}} = 4.18\) (in J/(g·°C)), \(L_f = 334\ \text{J/g}\) and \(L_v = 2257\ \text{J/g}\). Which stage costs the most?

Show solution
Warming the ice from −20 °C to 0 °C: \(Q_1 = 100 \cdot 2.09 \cdot 20\) \(= 4180\ \text{J}\).
Melting the ice at 0 °C: \(Q_2 = 100 \cdot 334 = 33\,400\ \text{J}\).
Warming the water from 0 °C to 100 °C: \(Q_3 = 100 \cdot 4.18 \cdot 100\) \(= 41\,800\ \text{J}\).
Boiling all the water at 100 °C: \(Q_4 = 100 \cdot 2257\) \(= 225\,700\ \text{J}\).
Adding up, \(Q = 4180 + 33\,400\) \({}+ 41\,800 + 225\,700\) \(= 305\,080\ \text{J}\).
Boiling alone takes \(225\,700 / 305\,080 \approx 74\%\) of the total, and it is the longest plateau on the bench.
\(Q \approx 305\ \text{kJ}\), and boiling the water costs about three quarters of this energy.
challenge
We dissolve 10 g of NaCl (58.44 g/mol) in 100 g of water. Estimate the BP of the solution at 1 atm with \(\Delta T = i\,K_b\,b\), where \(K_b = 0.512\ \text{°C·kg/mol}\), \(i = 2\) and \(b\) is the molality. What assumptions does the model carry?

Show solution
The amount of salt is \(n = 10 / 58.44 \approx 0.171\ \text{mol}\), in \(0.100\ \text{kg}\) of water, and the molality is \(b \approx 1.71\ \text{mol/kg}\).
Each NaCl splits into two ions, Na⁺ and Cl⁻, and so \(i = 2\).
\(\Delta T = 2 \cdot 0.512 \cdot 1.71 \approx 1.75\ \text{°C}\), and the BP comes out close to 101.8 °C.
The model assumes ideal ions, which spread out without attracting one another. In such a concentrated solution they attract each other a little, and the effective \(i\) probably falls below 2, so the measured value tends to be a little lower.
The calculation also holds only for the start of boiling, because the water that evaporates concentrates the solution and the BP keeps rising.
\(\text{BP} \approx 101.8\ \text{°C}\) at the start, in the ideal-ion model, which may overestimate the rise.

STEP 3

Does density identify a material?

Density is mass divided by volume, \(d = m/V\), and we usually write it in g/cm³, which gives the same number as g/mL. It is a specific property. A piece of aluminium with twice the mass also takes up twice the volume, and the ratio between the two is still 2.70 g/cm³ at 20 °C.

The mass comes from the balance. The volume of an irregular piece is hard to measure with a ruler, and so we drop it into a measuring cylinder with water and read how far the level has risen, since the piece pushes aside a volume of water equal to its own.

With the density measured, we look it up in a table. A piece of brass, which is an alloy of copper and zinc, gives a value that falls between those in the table and matches none of them, and that is information too, because it suggests that the piece is not made of any of the pure metals on the list.

Density at 20 °C
Metal	g/cm³

Liquids that do not mix, or that mix very slowly, stack up by density, with the densest at the bottom. Honey dissolves in water, but so slowly, because it is very viscous, that when poured carefully it forms a layer that lasts a good while, whereas alcohol would mix with water almost at once. An object dropped into such a column sinks as long as it meets liquid less dense than itself and stops on the first denser layer. The reason why, with buoyancy, is in the lesson on Hydrostatics.

\(d = \dfrac{m}{V}\)\(V_{\text{piece}} = V_{\text{final}} - V_{\text{initial}}\)Since \(1\ \text{mL} = 1\ \text{cm}^3\), the density in g/mL and in g/cm³ is the same number.

Let's discuss

Drag piece A from the balance into the measuring cylinder and read how far the level has risen. What density comes out of it, and which metal in the table does it match?
Change the size of piece A while it is inside the cylinder. What happens to the mass, to the volume and to the density?
Measure piece E. Why does the bench answer "none in the table", and what clue does that give us about the material?
In Column mode, try to predict where the grape will stop before you drop it into the jar, and then drop the other objects and check each prediction.

Size (mass)

Mass

Displaced volume

Density

Likely material

Exercises for step 3 10 questions · 4 basic · 4 intermediate · 2 challenge

basic
A piece has a mass of 270 g and a volume of 100 cm³. What is its density?

Show solution
\(d = \dfrac{m}{V} = \dfrac{270}{100} = 2.70\ \text{g/cm}^3\).
\(d = 2.70\ \text{g/cm}^3\), the value for aluminium in the table.
basic
What is the mass of a 10 cm³ piece of copper, if the density of copper is \(8.96\ \text{g/cm}^3\)?

Show solution
Solving \(d = m/V\) for the mass, we get \(m = d \cdot V\).
\(m = 8.96 \cdot 10 = 89.6\ \text{g}\).
\(m = 89.6\ \text{g}\)
basic
A measuring cylinder holds 60 mL of water. After we drop a stone into it, the level rises to 75 mL. What is the volume of the stone?

Show solution
The stone pushes aside a volume of water equal to its own.
\(V_{\text{stone}} = V_{\text{final}} - V_{\text{initial}}\) \(= 75 - 60 = 15\ \text{mL}\).
\(V = 15\ \text{mL} = 15\ \text{cm}^3\)
basic
A label says that the density of a liquid is \(1.00\ \text{g/mL}\), and a book gives the same density in g/cm³. What is it in that unit, and what is the mass of 1 L of this liquid?

Show solution
A millilitre is the volume of a cube with 1 cm edges, and so \(1\ \text{mL} = 1\ \text{cm}^3\) and \(1.00\ \text{g/mL} = 1.00\ \text{g/cm}^3\).
A litre has 1000 mL, and \(m = d \cdot V\) \(= 1.00 \cdot 1000 = 1000\ \text{g}\).
\(1.00\ \text{g/cm}^3\), and 1 L has a mass of 1000 g, or 1 kg.
intermediate
A 393.5 g metal piece makes the level in a measuring cylinder rise from 100 mL to 150 mL. Using the table in this step, what metal does it seem to be made of?

Show solution
The volume is \(V = 150 - 100 = 50\ \text{cm}^3\).
The density is \(d = \dfrac{393.5}{50} = 7.87\ \text{g/cm}^3\).
In the table, 7.87 g/cm³ is the value for iron, and no other metal on the list comes close.
The piece seems to be made of iron.
intermediate
Two pieces of the same material have 54 g and 20 cm³, the smaller, and 135 g and 50 cm³, the larger. Work out the density of each and comment on the result.

Show solution
Smaller: \(d = \dfrac{54}{20} = 2.70\ \text{g/cm}^3\).
Larger: \(d = \dfrac{135}{50} = 2.70\ \text{g/cm}^3\).
The larger piece has 2.5 times the mass and 2.5 times the volume of the smaller one, and the ratio between the two stays the same.
Both have \(2.70\ \text{g/cm}^3\), because density is a property of the material and does not depend on the size of the piece.
intermediate
In the column on the bench the layers are, from bottom to top, honey (1.42 g/cm³), water (1.00), oil (0.92) and alcohol (0.79). Where does a bead of density 0.85 g/cm³ dropped at the top come to rest?

Show solution
The bead is denser than alcohol, 0.85 > 0.79, and sinks in it.
It is less dense than oil, 0.85 < 0.92, and cannot enter that layer.
The bead rests on the oil, at the bottom of the alcohol layer.
intermediate
Why does oil sit on top of water in a glass, and why, in the column on the bench, does the oil have to be between the water and the alcohol?

Show solution
Oil and water do not mix, and oil is less dense, 0.92 against 1.00 g/cm³. The less dense liquid stays on top.
Alcohol, on the other hand, mixes with water almost at once. If the two touched, they would form a single layer.
Honey also dissolves in water, but it is so viscous that, poured carefully, it takes a good while to mix, and so the honey layer can sit against the water.
Oil, which mixes with neither of them, stays in the middle and keeps the alcohol away from the water.
Oil floats on water because it is less dense and immiscible, and in the column it separates the water from the alcohol, which would otherwise mix.
challenge
A 1000 g crown displaces 60 mL of water. Gold has \(19.3\ \text{g/cm}^3\) and silver, \(10.5\ \text{g/cm}^3\). Assuming that the crown is an alloy of the two and that the volumes add up, what fraction of the mass is gold?

Show solution
The density of the crown is \(d = \dfrac{1000}{60} \approx 16.7\ \text{g/cm}^3\), below that of gold, and so it is not pure gold.
Calling the mass of gold \(x\), the mass of silver is \(1000 - x\), and the volumes add up to 60, \(\dfrac{x}{19.3} + \dfrac{1000 - x}{10.5} = 60\).
This gives \(95.24 - x\left(\dfrac{1}{10.5} - \dfrac{1}{19.3}\right) = 60\), or \(0.04343\,x = 35.24\).
Hence \(x \approx 811\ \text{g}\).
About 81% of the mass is gold, in the model in which the volumes add up.
challenge
Piece E on the bench is brass, with \(8.5\ \text{g/cm}^3\). Show that this value lies between that of zinc (\(7.1\ \text{g/cm}^3\)) and that of copper (\(8.96\ \text{g/cm}^3\)) and estimate the fraction of the volume that is copper, assuming that the volumes add up.

Show solution
We have \(7.1 < 8.5 < 8.96\), and the alloy lies between the two metals, closer to copper.
Let \(f\) be the fraction of the volume taken up by copper. With the volumes added, the density of the alloy is the average weighted by the volumes, \(8.96\,f + 7.1\,(1 - f) = 8.5\).
Hence \(1.86\,f = 1.4\) and \(f \approx 0.75\).
By mass, copper would be \(\dfrac{8.96 \cdot 0.75}{8.5} \approx 0.79\) of the total.
The model probably overstates the copper. Ordinary brass has about 65% copper by mass, and the difference comes from the fact that, in the alloy, the volumes of copper and zinc do not add up exactly.
About three quarters of the volume, or close to 80% of the mass, would be copper in the model in which the volumes add up, above the roughly 65% of ordinary brass.

STEP 4

Element, compound or mixture?

All matter is made of atoms, and each kind of atom is a chemical element, such as oxygen (O), hydrogen (H) and carbon (C). A pure substance has a single kind of particle, and so it has fixed properties, such as the MP, the BP and the density we measured in steps 2 and 3.

When each particle has atoms of one element only, the substance is an element, such as oxygen gas (O₂), ozone (O₃) and nitrogen gas (N₂). When the particle joins atoms of more than one element, the substance is a compound, such as water (H₂O) and carbon dioxide (CO₂). O₂ and O₃ are made of the same element and are nonetheless different substances, with molecules of two and of three atoms and properties of their own, a case of what we call allotropy.

A mixture has more than one kind of particle, each coming from a substance. Its properties vary with the proportion between the parts, and that is why the salt water in step 2 boils along a ramp instead of a plateau, since the proportion changes as the water evaporates. Air, with nitrogen, oxygen, argon and small amounts of other gases, and seawater, with salt and other substances dissolved in it, are mixtures.

Salt dissolved in water appears in the magnifier as ions, Na⁺ and Cl⁻, separated from one another. They are two kinds of particle coming from a single substance, sodium chloride (NaCl), and salt water has two substances, water and salt.

\(\text{O}_2,\ \text{O}_3,\ \text{N}_2\) elements\(\text{H}_2\text{O},\ \text{CO}_2\) compoundsThe formula tells us which elements make up the particle and how many atoms of each it has. More than one substance in the same flask means a mixture.

Let's discuss

With the magnifier over flask 1, count the atoms in each particle and how many different elements appear in it. In which box would you put this flask?
By eye, the gas flasks look the same. Pass the magnifier over each of them and say which have a single kind of particle.
Compare in the magnifier the two flasks that contain only oxygen. Why do they go into the same box, and why do they nonetheless hold different substances?
In the flask with dissolved salt the magnifier shows three kinds of particle. Why do we count two substances there, and what does the bench answer if you put it among the compounds?

Correct

Attempts

Exercises for step 4 10 questions · 4 basic · 4 intermediate · 2 challenge

basic
Classify each material as an element, a compound or a mixture: oxygen gas (O₂), distilled water (H₂O), air, 24-carat gold and mineral water.

Show solution
Oxygen gas has a single kind of molecule, with two atoms of the same element, and is an element.
Distilled water has only H₂O molecules, which join hydrogen and oxygen, and is a compound.
Air brings together nitrogen, oxygen, argon and other gases, each with its own particle, and is a mixture.
24-carat gold is practically all gold, a single element, and so counts as an element.
Mineral water carries dissolved salts, listed on the label, and is a mixture.
Elements, O₂ and 24-carat gold. Compound, distilled water. Mixtures, air and mineral water.
basic
What is the difference between a chemical element and a substance? Use oxygen as an example.

Show solution
An element is a kind of atom, and oxygen, with symbol O, is one of them.
A substance is a material with a single kind of particle and fixed properties. On its own, the element oxygen exists in two forms, oxygen gas (O₂) and ozone (O₃), which are its allotropes, and it appears alongside other elements in compounds, such as water (H₂O) and carbon dioxide (CO₂).
The element is the kind of atom, and the substance is the material these atoms form, on their own or combined with atoms of other elements.
basic
How many chemical elements make up carbon dioxide (CO₂), sulfuric acid (H₂SO₄) and sodium chloride (NaCl)? Which are they?

Show solution
Each different symbol in the formula is an element, and the subscript only tells us how many of its atoms go in.
CO₂ has carbon and oxygen, two elements.
H₂SO₄ has hydrogen, sulfur and oxygen, three elements.
NaCl has sodium and chlorine, two elements.
Two, three and two elements, and all three are compounds.
basic
What does the formula H₂O tell us about water?

Show solution
The formula tells us which elements make up the particle and how many atoms of each it has.
Each water molecule has two hydrogen atoms, from the subscript 2, and one oxygen atom, since the subscript 1 is not written.
There are two elements in the same particle, and so water is a compound.
Each water molecule has 2 atoms of H and 1 of O, and water is a compound.
intermediate
Oxygen gas (O₂) and ozone (O₃) are made only of oxygen atoms. Why do we say they are two different substances?

Show solution
We recognise a substance by its particle and by its properties. The O₂ molecule has two atoms and the O₃ molecule has three, and this difference is enough to change the properties.
O₂ is the gas we breathe, odourless, and O₃ has a strong smell and forms the ozone layer, which filters out part of the Sun's ultraviolet radiation.
When the same element exists in different forms, we speak of allotropy, and O₂ and O₃ are allotropes of oxygen.
The molecules are different and so are the properties, and so O₂ and O₃ are allotropes, two different forms of the same element.
intermediate
A pure substance melts at a fixed temperature. A white solid, heated slowly at 1 atm, starts to melt at 40 °C and only finishes at 46 °C. What does this suggest about the sample?

Show solution
When a pure substance melts, the temperature stands still on the plateau until the last piece has melted, as we saw in step 2.
Here the temperature rose 6 °C during melting, along a ramp, and this indicates that the proportion between the parts changed while the solid melted.
The sample is probably a mixture, or a substance with impurities, and hardly a pure substance.
intermediate
An advert calls mineral water "pure water". From the point of view of chemistry, is the phrase correct?

Show solution
For chemistry, pure water is water that has only H₂O molecules, like distilled water.
The label on mineral water lists the salts dissolved in it, with the amount of each, and so it has more than one substance and is a mixture.
In the advert, "pure" seems to mean clean and fit to drink, which is something else.
For chemistry mineral water is a mixture, and the "pure" in the advert speaks of cleanliness.
intermediate
In a particle drawing, each ball is an atom, and balls that touch form a molecule. Flask A has only pairs of white balls. Flask B has only trios, each with a black ball between two white ones. Flask C has pairs of white balls and loose black balls. Classify the contents of each flask.

Show solution
In A all the particles are the same, and each has two atoms of the same element, which gives an element.
In B the particles are also all the same, except that each joins atoms of two elements, the white and the black, and we have a compound.
In C there are two kinds of particle, the white molecules and the loose black atoms, each kind from a different element, and the flask holds a mixture.
A is an element, B is a compound and C is a mixture of two elements.
challenge
Glucose has the formula C₆H₁₂O₆ and sucrose, table sugar, C₁₂H₂₂O₁₁. How many atoms and how many elements are there in each molecule?

Show solution
We add up the subscripts to count the atoms, and count the different symbols to count the elements.
Glucose: \(6 + 12 + 6 = 24\) atoms, of three elements, carbon, hydrogen and oxygen.
Sucrose: \(12 + 22 + 11 = 45\) atoms, of the same three elements.
The two molecules have the same elements in different amounts, and they are different substances, both compounds.
Glucose, 24 atoms and 3 elements. Sucrose, 45 atoms and 3 elements.
challenge
The carat of gold says how many parts of gold there are in 24 parts of the mass of the piece. Why is 18-carat gold a mixture, while 24-carat gold is practically a single substance?

Show solution
18-carat gold has 18 parts of gold in 24, or \(18/24 = 75\%\) of the mass. The rest is other metals, usually copper and silver, and the piece is an alloy, a mixture of metals.
24-carat gold has 24 parts in 24, almost pure gold, with only traces of other elements.
We say "practically" because no real sample is 100% pure. What we call a pure substance, in practice, is a material whose impurities are too small to change the properties we measure.
18-carat gold is an alloy with 75% gold, a mixture, and 24-carat gold is gold with almost nothing mixed in, practically an element.

STEP 5

How many phases does this system have?

We call a system the portion of matter we choose to study, such as the contents of a beaker. Each substance present in it is a component, and each region with the same properties at every point is a phase. Water with oil has two components and two phases, and the boundary between them can be seen with the naked eye. Cooking oil is itself a mixture of several substances, and here we count it as a single component.

A system with a single phase is homogeneous, and one with more than one is heterogeneous. Mixtures are divided in the same way, into homogeneous and heterogeneous. Salt dissolves in water and vanishes from sight, and salt water is homogeneous up to a limit. At 20 °C, 100 g of water dissolve about 36 g of salt, and whatever goes beyond that stays at the bottom as a new phase, a solid one.

Components and phases are different counts. Ice and liquid water are the same substance in two states, and a glass of water with ice has one component and two phases. Textbooks also agree to leave out the air above the liquid and the container itself, and that is the convention the bench follows when it counts.

\(\text{homogeneous} \Leftrightarrow 1\ \text{phase}\)\(\text{solubility of NaCl (20 °C)}\) \(\approx 36\ \text{g}/100\ \text{g of water}\)Solubility is the maximum amount that dissolves. It changes with temperature, and this value holds at 20 °C.

Let's discuss

Put in water and add salt 10 g at a time. On which click does the second phase appear, and what does that have to do with the limit of 36 g per 100 g of water?
Make water with ice. How many components and how many phases does the bench count, and why do the two numbers not coincide?
Add oil and then ice. Where does the ice end up, and what does that suggest about its density compared with that of water and that of oil?
Take the magnifier to the salt water, which by eye is a single phase. Milk and blood also look homogeneous by eye, and what would you expect to see in them with a microscope?

Components

Phases

System

Phases present

Exercises for step 5 10 questions · 4 basic · 4 intermediate · 2 challenge

basic
How many components and how many phases are there in each system: water with oil, and water with a little salt dissolved in it?

Show solution
In water with oil there are two substances, which do not mix and show a boundary, and we count two components and two phases.
In water with a little dissolved salt there are also two substances, water and sodium chloride, except that the salt vanishes into the water and the liquid is the same at every point, and we count two components and one phase.
Water with oil, 2 components and 2 phases. Water with dissolved salt, 2 components and 1 phase.
basic
A glass has liquid water with three ice cubes. How many components and how many phases does this system have?

Show solution
Ice and liquid water are the same substance, H₂O, and count as one component.
The three cubes have the same properties and together form a single phase, a solid one. The liquid water is the other phase.
1 component and 2 phases, and the number of cubes does not change the count.
basic
Granite is a rock with grains of different colours, and textbooks usually describe it as quartz, feldspar and mica. How many phases does it have, and is it homogeneous or heterogeneous?

Show solution
Each kind of grain has the same properties at all its points and differs from the others, and so it counts as a phase. Grains of the same mineral scattered through the rock form a single phase.
With three minerals, textbooks count three phases.
3 phases, and granite is heterogeneous, which can already be seen with the naked eye.
basic
Say whether each system is homogeneous or heterogeneous: water with sand, water with a little sugar dissolved in it, water with oil and alcohol with water.

Show solution
Sand does not dissolve and stays at the bottom, and water with sand has two phases.
Sugar vanishes into the water and the liquid is the same at every point, a single phase.
Water with oil forms two layers.
Alcohol and water mix in any proportion, in a single phase.
Heterogeneous, water with sand and water with oil. Homogeneous, water with sugar and alcohol with water.
intermediate
At 20 °C, 100 g of water dissolve about 36 g of NaCl. How much salt dissolves in 250 g of water, and how much is left at the bottom if we add 100 g of salt?

Show solution
The amount that dissolves grows in proportion to the water, \(250 \cdot \dfrac{36}{100} = 90\ \text{g}\).
Of the 100 g added, \(100 - 90 = 10\ \text{g}\) stay undissolved, at the bottom.
The system now has two phases, the saturated solution and the solid salt.
90 g dissolve, and 10 g are left at the bottom, at 20 °C.
intermediate
A beaker contains water, ice, oil and sand. How many components and how many phases does the system have?

Show solution
The components are water, which counts only once for the liquid and the ice, oil and sand, three in all.
The phases are liquid water, ice, oil and sand, four in all.
3 components and 4 phases.
intermediate
A freshly opened bottle of sparkling water gives off bubbles. How many phases are there in the liquid while the bubbles rise, and how many were there with the bottle closed, with no bubbles at all?

Show solution
With the bottle closed, the carbon dioxide is dissolved in the water and the liquid is the same at every point, a single phase.
Once it is opened, part of the gas leaves the solution and forms bubbles, which are a gas phase in the middle of the liquid.
The components are still two in both cases, water and carbon dioxide, if we leave aside the salts the water may contain.
Closed, 1 phase. With bubbles, 2 phases, the solution and the gas in the bubbles.
intermediate
Why does the air above the water in a glass not enter the count of phases, and what would change if it did?

Show solution
It is a textbook convention. The system is the material we are studying, the contents of the glass, and the air around it and the glass itself are left out.
Gases always mix with one another, and the whole of the air would count as a single gas phase, with several components, such as nitrogen, oxygen, argon and water vapour.
Almost every open system would gain the same extra phase, and the count would say little about what is in the glass.
By convention, the air and the container are left out of the system. If they came in, every open system would gain a gas phase and a few extra components.
challenge
A beaker has water with salt dissolved up to the limit, plus undissolved salt at the bottom, ice cubes, a layer of oil and iron filings. How many components and how many phases does this system have?

Show solution
The components are water, including the ice, sodium chloride, dissolved and at the bottom, oil and iron, four in all.
The phases are the salt-water solution, the ice, the undissolved salt, the oil and the filings, five in all.
The salt appears in two phases and counts as a single component, like the water, which appears in the solution and in the ice.
4 components and 5 phases.
challenge
Milk and blood look homogeneous to the naked eye. What changes when we look at them under a microscope, and how does that affect the classification?

Show solution
Under the microscope milk shows droplets of fat scattered through a watery liquid, and blood shows cells, such as red blood cells, suspended in the plasma.
With magnification, regions with different properties appear, and so both have more than one phase.
The classification depends, then, on the scale at which we look. Textbooks usually treat milk and blood as heterogeneous mixtures, because the separation shows up under the microscope, even though by eye they look like a single thing.
They are homogeneous by eye and heterogeneous under the microscope, and textbooks tend to classify them as heterogeneous.

STEP 6

How do we separate a heterogeneous mixture?

In a heterogeneous mixture the parts can be told apart, by eye or under the microscope, and differ in some property. To separate them we choose the property in which they differ and a piece of equipment that takes advantage of it. The bench explores three, particle size, density and magnetism, and no method works for every mixture.

Filtration separates a solid that has not dissolved from the liquid it is in, like sand in water. Filter paper is a mesh of fibres with very small pores, which holds back whatever is bigger than the pores and lets the liquid through to the flask below. The coffee filter does the same job in the kitchen.

When the parts have different densities, gravity does the separating for us, and we call this decanting. Sand goes to the bottom of a glass of water in a few seconds, and water and oil, which do not mix, form two layers, with the less dense oil on top. A separating funnel is a vessel with a tap at the bottom, through which we let the lower layer out, and we close the tap when the boundary reaches it.

Centrifugation speeds up decanting. The tube spins very fast, and the parts of the mixture behave as if gravity were many times stronger. This is how, in minutes, we separate the cream from milk and the cells from blood plasma, mixtures that, left standing, would take hours to separate, or would never separate fully.

size → filtrationdensity → decanting, centrifugemagnetism → magnetEach method takes advantage of a property in which the parts differ. If they do not differ in that property, the method separates nothing.

Magnetic separation uses a magnet to pull out the part it attracts, like iron filings mixed with sand. There are also methods that need no equipment. In hand-picking we choose the parts by hand, like someone taking the little stones out of the beans, in sieving the smaller grains pass through the mesh and the larger ones stay behind, and in winnowing a current of air carries away the lighter part, like the husk that comes loose from grains of rice.

Let's discuss

Choose sand + water and try out the four tools. Which gives the highest purity, and why does the magnet separate nothing?
Pass water + oil through the filter paper and read the bench's explanation. In what property do the two parts differ, and which tool takes advantage of that difference?
With the milk, compare the separating funnel with the centrifuge. What does the centrifuge do in minutes that milk standing still does slowly, or hardly does at all?
After filtering the sand, take the magnifier to the water flask. What particles appear, and what do they suggest about a purity of 99 %?

Does the method work?

Purity

Exercises for step 6 10 questions · 4 basic · 4 intermediate · 2 challenge

basic
What method separates sand from water, and what is held back by the filter paper?

Show solution
Sand does not dissolve in water, and the grains are much bigger than the pores of the paper.
In filtration the paper holds back the grains, and the water passes through the pores and is collected in the flask below.
We can let the sand settle first and carefully pour off most of the water, which is decanting, and filter only the rest, which makes the filtration quicker.
Filtration. The paper holds back the sand, the solid with particles larger than the pores, and lets the water through.
basic
What equipment separates water from oil, and which of the two comes out first?

Show solution
Water and oil do not mix and form two layers, with the less dense oil on top.
In the separating funnel we open the tap at the bottom, and the water comes out first. We close the tap when the boundary reaches it, and the oil stays in the funnel.
The separating funnel, and the water comes out first, because it is the lower layer.
basic
How do we separate iron filings mixed with powdered sulfur?

Show solution
Both parts are powdered solids, with grains of similar sizes, and neither the sieve nor the filter paper can tell them apart.
Iron is attracted by a magnet and sulfur is not.
We pass a magnet over the mixture, and it pulls out the filings and leaves the sulfur.
Magnetic separation, with a magnet.
basic
Before cooking beans, many people spread the beans out on the table and take out the little stones and the spoilt beans by hand. What method is this?

Show solution
The parts can be told apart by eye, by colour and shape, and are big enough to pick up with the fingers.
Separating by choosing the parts by hand is hand-picking.
Hand-picking.
intermediate
A glass contains sand, iron filings and water. In what order would you use the methods on the bench to get the three parts separated?

Show solution
In filtration the paper holds back the sand and the filings together, which are the two solids, and the water passes into the flask below.
Once dry, the solid left on the paper is a mixture of sand and filings, and the magnet pulls out the filings and leaves the sand.
We could also pass the magnet over first, through the wet mixture, and filter afterwards, but the filings would come out wet and with grains of sand stuck to them, which seems less clean.
Filter, dry the solid and separate the filings with the magnet.
intermediate
Why does the centrifuge separate in minutes what ordinary decanting would take hours to separate?

Show solution
In ordinary decanting the denser part sinks because of gravity, and very small particles sink very slowly.
In the centrifuge the tube spins fast, and the effect on the mixture is that of a gravity many times stronger, which pushes the denser part to the bottom of the tube.
The separation is still by density, only faster.
The rotation plays the part of a much stronger gravity, and decanting that would take hours happens in minutes.
intermediate
A mixture of water and petrol was put into a separating funnel. Petrol is less dense than water and does not mix with it. Which liquid comes out of the tap first, and how do we know when to close it?

Show solution
The less dense petrol forms the upper layer, and the water stays below, next to the tap.
When we open the tap, the water comes out first. The boundary between the layers moves down, and we close the tap when it reaches the outlet.
In practice, a little of the liquid near the boundary is sacrificed so as not to mix the two parts again.
The water comes out first, and we close the tap when the boundary with the petrol reaches it.
intermediate
Laboratories use "vacuum filtration", with a porcelain funnel fitted into a flask connected to a pump. What does the pump seem to do, and in what situation is this set-up worth it?

Show solution
The pump removes part of the air from the flask below, and the pressure there becomes lower than that of the air above the paper.
The outside air then pushes the liquid through the pores, and the filtration becomes faster than one that depends only on the weight of the liquid.
The set-up is probably worth it when ordinary filtration would be very slow, with a lot of solid or with fine particles that clog the pores.
The pump lowers the pressure below the paper, and the pressure difference pushes the liquid through faster, which helps with slow filtrations.
challenge
Plan the separation of a mixture of sand, salt, iron filings and oil, recovering all four parts.

Show solution
The magnet pulls out the iron filings, which are the only part it attracts.
We add water to what is left. The salt dissolves, the sand goes to the bottom and the oil forms a layer on top.
In filtration the paper holds back the sand, and the oil and the salt water, which are liquids, pass through.
In the separating funnel the denser salt water comes out through the tap, and the oil stays in the funnel.
Dissolved salt passes through any filter paper and forms no layer, and to recover it we need to evaporate the water, the subject of step 7.
Magnet, water, filtration, separating funnel and evaporation, in that order.
challenge
A student described the separation of sand, salt and water like this. "I filter the mixture and the sand stays on the paper. I pass the liquid through a second, finer filter paper to hold back the salt. I dry the second paper and collect the salt." Where is the mistake, and how do we correct it?

Show solution
The first filtration is right, because the sand does not dissolve and the grains stay on the paper.
The mistake is in the second step. The dissolved salt is spread through the water as ions, particles much smaller than the pores of any filter paper, and salt water is a homogeneous mixture, which filtration does not separate.
To recover the salt, we evaporate the water from the filtrate, and the salt stays at the bottom of the container.
The second paper lets the salt through with the water. The correction is to evaporate the water from the filtrate.

STEP 7

How do we separate a homogeneous mixture?

In a homogeneous mixture the parts cannot be told apart, not even under the microscope, and the methods of step 6 are of no use. Salt dissolved in water passes through filter paper, forms no layer in the funnel and does not sink in the centrifuge. To separate it we take advantage of another property, the temperature at which each part turns into vapour.

In evaporation we let the liquid go and keep the solid. This is what salt pans do, holding seawater in shallow tanks and letting the sun and the wind evaporate the water until the salt appears at the bottom. The water is lost to the air, and the method only works when what we want is the solid.

To recover the water as well we use simple distillation. The mixture boils in a flask, the vapour rises and passes through a condenser, a tube cooled from the outside by running water, and turns back into liquid, which drips into the receiving flask. The salt does not turn into vapour and stays in the flask. A thermometer at the top, level with the outlet to the condenser, reads the temperature of the vapour, and with water and salt it stays at 100 °C at 1 atm, because the vapour is pure water, while the flask, saltier and saltier, boils above that.

\(y_{\text{ethanol}} > x_{\text{ethanol}}\)\(\text{limit} \approx 96\ \%\ \text{by volume}\)\(x\) is the fraction of ethanol in the boiling liquid and \(y\), the fraction in the vapour that leaves it. The vapour comes out richer in ethanol than the liquid, up to the limit of about 96 % by volume.

Two liquids that mix, like water and ethanol, call for fractional distillation. The vapour that leaves a boiling mixture is richer in the more volatile liquid, the one with the lower boiling point, and the fractionating column repeats the boiling several times. The vapour condenses on the packing of the column, boils again a little higher up the column, where it is already cooler, and becomes richer in ethanol at each stage, until it reaches the top at about 78 °C.

There is a limit. A mixture of water and ethanol does not get beyond about 96 % ethanol by volume through distillation, because at that proportion the vapour comes out with the same composition as the liquid (we call the mixture at that proportion an azeotrope), and that is the 96 % alcohol sold in chemists' shops. An oil refinery carries out fractional distillation on a large scale, with columns tens of metres tall that separate crude oil into fractions such as bottled gas (LPG), petrol, kerosene and diesel, each drawn off at a different height of the column.

Let's discuss

Distil the salt water and compare the temperature at the top with that of the flask. Why does the top stay at 100 °C while the flask gets hotter, and what appears at the bottom at the end?
With water + ethanol, compare simple with fractional distillation. At what temperature does the top start in each, and how much ethanol does the first vapour contain?
In the fractional one, take the magnifier to different heights of the column. How does the proportion of ethanol in the vapour change, from bottom to top?
Let the fractional distillation run to the end and look at the graph. What happens to the temperature at the top when the ethanol in the flask runs out, and why would no column, however tall, get beyond about 96 %v?

Heating

Top (thermometer)

Ethanol in the vapour

Ethanol in the receiver

Ethanol in the flask

Fraction collected

Exercises for step 7 10 questions · 4 basic · 4 intermediate · 2 challenge

basic
What method separates water from the salt dissolved in it, recovering the water?

Show solution
Salt water is homogeneous, and the dissolved salt passes through filter paper.
When the mixture boils, only the water turns into vapour. The vapour passes through the condenser, turns back into liquid and is collected in the receiving flask, and the salt stays in the flask.
Simple distillation.
basic
If we only want the salt from seawater and do not mind losing the water, what method is enough?

Show solution
We let the water evaporate, and the salt, which does not turn into vapour, stays at the bottom of the container.
This is what salt pans do, with the heat of the sun and the wind.
Evaporation.
basic
What is the job of the condenser in a still?

Show solution
The condenser is a tube surrounded by a jacket through which cold water runs.
The hot vapour coming from the flask loses heat to the cold walls of the tube and returns to the liquid state, which runs down into the receiving flask.
The condenser cools the vapour and turns it back into liquid, which is condensation.
basic
The thermometer in a still sits at the top, level with the outlet to the condenser, and not immersed in the liquid in the flask. What does it measure?

Show solution
The vapour on its way to the condenser passes at the level of the outlet, and the bulb of the thermometer is bathed in it.
The thermometer measures the temperature of this vapour, which tells us what is distilling. With water and salt it reads 100 °C, at 1 atm, because the vapour is pure water.
The temperature of the vapour leaving for the condenser.
intermediate
Why does fractional distillation separate water and ethanol better than simple distillation?

Show solution
In simple distillation the vapour is collected after a single boiling, and it comes out richer in ethanol than the liquid in the flask, but much less rich than after several boilings. As the flask is depleted, the distillate also gets weaker.
In fractional distillation the vapour condenses and boils again several times as it rises through the column, and becomes richer in ethanol at each stage.
On the bench, starting from the same mixture, the first vapour of the simple distillation has about 72 %v ethanol, and that of the fractional one, about 93 %v.
The column repeats the boiling several times, and the vapour at the top comes out much richer in ethanol.
intermediate
A mixture of two liquids that mix, with BPs of 56 °C and 100 °C at 1 atm, goes through fractional distillation. Which comes out first, and at what temperature should the thermometer at the top stay meanwhile?

Show solution
The more volatile liquid comes out first, the one with a BP of 56 °C, which could be acetone.
With an efficient column, the vapour at the top is almost entirely this liquid, and the thermometer stays close to 56 °C.
When it runs out in the flask, the temperature at the top rises to close to 100 °C, and the other liquid starts to distil.
The one with a BP of 56 °C comes out first, with the top close to 56 °C, and then the top jumps to close to 100 °C.
intermediate
Why do salt pans use wide, shallow tanks, and why are they usually found in regions with strong sun, wind and little rain, like part of the coast of Rio Grande do Norte, in Brazil?

Show solution
Water evaporates from its surface, and a wide, shallow tank exposes a lot of surface for a small volume of water.
The sun supplies the energy for evaporation, and the wind carries away the humid air above the water, which tends to speed up evaporation.
Rain would put water back into the tanks and undo the work.
Plenty of surface, plenty of sun, wind and little rain make the water evaporate quickly, and the salt stays at the bottom.
intermediate
In the distillation of a mixture, in a column good enough for each liquid to come out almost pure, the temperature at the top stayed at 56 °C while the first 40 % of the volume of distillate was being collected, rose quickly and stayed at 100 °C until the end. Here we count the distillate by volume, and not in moles as the bench does. What does the curve tell us about the mixture?

Show solution
Each plateau indicates that, in that stretch, an almost pure liquid with that boiling point is distilling.
The first plateau suggests a liquid with a BP of 56 °C, perhaps acetone, and the second, water, at 1 atm.
The quick rise between the plateaus marks the moment when the first liquid ran out in the flask.
The mixture seems to have two components, about 40 % by volume of a liquid with a BP of 56 °C and the rest water.
challenge
Why can we not obtain 100 % ethanol by distilling a mixture of water and ethanol, however tall the column?

Show solution
The column works because the vapour comes out richer in ethanol than the liquid. The more ethanol the mixture has, the smaller this advantage.
Near 96 % by volume it disappears, and the vapour comes out with the same composition as the liquid. The mixture at that proportion is the azeotrope, and boiling it again changes nothing.
To go further, industry needs another resource, such as substances that hold on to water, drying agents, or other separation tricks.
At about 96 %v the vapour has the same composition as the liquid, and distillation stops enriching the mixture.
challenge
We distil 1 L of a mixture of water and ethanol with 26 % ethanol by volume, and collect 200 mL of distillate with 70 % ethanol by volume. Assuming that the volumes add up, which is not exactly true for water and ethanol, how much ethanol is left in the flask, and at what percentage by volume?

Show solution
At the start there were \(0.26 \cdot 1000 = 260\ \text{mL}\) of ethanol.
The distillate took \(0.70 \cdot 200 = 140\ \text{mL}\).
Left in the flask were \(260 - 140 = 120\ \text{mL}\) of ethanol, in \(1000 - 200 = 800\ \text{mL}\) of mixture.
The percentage is \(\dfrac{120}{800} = 0.15\), that is, 15 %v.
120 mL of ethanol were left in the flask, at 15 % by volume, assuming that the volumes add up.

STEP 8

From the reservoir to the tap

The water that comes out of the tap came from a river or a reservoir, with branches, mud and microorganisms, and passed first through a water treatment works. Almost everything that happens there we have already seen in steps 6 and 7, in tanks the size of swimming pools. We can think of the works as a queue of separation methods, each removing from the water what the previous one let through, and only two stages, coagulation and chlorination, are chemical.

The raw water first goes through screens, which catch branches, leaves and rubbish, a form of sieving on a large scale. The fine mud gets through the screens. Clay particles are so small that they would stay suspended for a long time, perhaps days, and neither decanting nor a sand filter could deal with them alone. In coagulation the works adds aluminium sulfate to the water, with rapid stirring, and a chemical reaction makes the fine particles start to stick to one another. In flocculation slow stirring gathers these particles into larger and larger flocs, without breaking them up.

The flocs are denser than water and sink in a calm tank, which is decanting, leaving at the bottom a sludge of mud with trapped microorganisms. The water at the top moves on to filtration, through layers of charcoal, sand and gravel, which catch the small flocs that did not sink. Portaria GM/MS 888/2021, Brazil's drinking-water regulation from the Ministry of Health, requires filtered water to have a turbidity of at most 0.5 uT, the unit of turbidity that the bench writes as NTU, in 95 % of each month's samples.

physical screening · flocculation · decanting · filtrationchemical coagulation · chlorinationThe physical stages separate parts of the mixture by size or by density. The chemical ones add to the water substances that react, aluminium sulfate in coagulation and chlorine in disinfection.

None of these stages guarantees that the microorganisms left over are dead. In disinfection the works adds chlorine, which kills bacteria and other microorganisms, and leaves a little of it dissolved to protect the water on its way through the pipes to people's homes. In fluoridation a fluorine compound goes in, in a small dose, which helps to prevent tooth decay. The water comes out drinkable, safe to drink, which is not the same as pure in the sense of step 4. It is still a mixture, with dissolved mineral salts, the chlorine and the fluoride, and pure water, a single substance, is found almost only in the laboratory.

The numbers on the bench are illustrative, chosen to show what each stage does, and every works measures its own. The magnifier also exaggerates the proportions, because even the muddiest water has far more water molecules than clay particles.

Let's discuss

Go forward stage by stage and follow the turbidity meter. At which stages does it really fall, and why do coagulation and flocculation, with so much work, not change the reading?
Compare in the magnifier the raw-water tank with the flocculation tank. What has happened to the clay particles, and why do large flocs help the next stage?
In decanting the meter still reads a few hundred coliforms per 100 mL, and the magnifier no longer shows any microorganism. Why might a sample as small as the magnifier's catch none?
Take the magnifier to the last tank and look for the chlorine. Why does the works leave a little of it in the water, if the coliforms have already reached zero?

Stage

Type

Turbidity

Coliforms

Exercises for step 8 10 questions · 4 basic · 4 intermediate · 2 challenge

basic
Put the stages of a water treatment works in order: filtration, coagulation, chlorination, screening, decanting and flocculation.

Show solution
Screening comes before everything else, because branches and leaves would clog the following tanks.
Coagulation makes the fine particles stick together, and flocculation gathers these particles into large flocs.
The flocs sink during decanting, and the filter catches the small ones left over.
Chlorine goes in at the end, into water that is already clean, to kill the microorganisms that got through.
Screening, coagulation, flocculation, decanting, filtration and chlorination.
basic
Which stages of the treatment works are chemical changes, and why?

Show solution
In coagulation the aluminium sulfate reacts in the water, and it is this reaction that makes the clay particles stick together.
In chlorination the chlorine reacts with the microorganisms and kills them.
Screening, flocculation, decanting and filtration only separate parts of the mixture by size or by density, without forming new substances.
Coagulation and chlorination, because they involve chemical reactions.
basic
What does the filter of a treatment works hold back, and what does it let through?

Show solution
The filter holds back whatever is larger than the spaces between the grains, the small flocs that did not sink during decanting and part of the microorganisms.
It lets through the water and what is dissolved in it, such as the mineral salts, which form a homogeneous mixture with the water.
On the bench, the turbidity falls from 4 to 0.3 NTU in filtration, and a few coliforms still remain.
It holds back suspended particles and lets through the water with what is dissolved in it.
basic
Why does the works add chlorine to the water at the end of the treatment?

Show solution
Decanting and filtration remove most of the microorganisms, but not all, and some cause diseases, such as diarrhoea.
Chlorine kills the ones left over.
Part of the chlorine stays dissolved and keeps acting in the pipes all the way to the tap. Portaria GM/MS 888/2021 requires at least 0.2 mg/L of free residual chlorine throughout the network, or an equivalent dose of another form of chlorine.
To kill the microorganisms that got through the filter and to protect the water on its way to people's homes.
intermediate
Why does the works coagulate and flocculate the water before decanting it? Would it not be enough to leave the raw water standing in a tank?

Show solution
Clay particles are very small, and gravity hardly overcomes them. Left standing, they would stay suspended for a long time, probably days.
Coagulation makes these particles stick together, and flocculation gathers them into large flocs, which sink quickly.
On the bench, the turbidity stays at 55 NTU during coagulation and flocculation, because nothing leaves the water, and only falls, to 4 NTU, during decanting.
Large flocs settle in a short time, and loose fine particles hardly settle at all.
intermediate
Tap water is drinkable. Is it a pure substance?

Show solution
Drinkable means fit to drink, within the limits of Portaria GM/MS 888/2021, without disease-causing microorganisms and with little turbidity.
A pure substance, in the sense of step 4, is a single substance.
Tap water carries dissolved mineral salts, the chlorine that remained and the fluoride from fluoridation, and is a homogeneous mixture.
No. Drinking water is a homogeneous mixture that is safe to drink, and not a pure substance.
intermediate
Many works put a layer of activated charcoal in the filter. What is it for?

Show solution
Activated charcoal is full of tiny pores, and one gram of it has an enormous internal surface.
Many molecules that give water a bad smell and taste get stuck on this surface, which we call adsorption.
As a layer of the filter, charcoal also holds back small flocs, just like sand, and what sets it apart from the other layers is adsorption.
It holds back molecules that give the water smell and taste, trapped on its surface.
intermediate
What builds up at the bottom of the settling tank of a treatment works, and why can this sludge not simply go back into the river?

Show solution
At the bottom the flocs that sank build up, with the clay, the remains of the coagulant and the microorganisms trapped in them.
This sludge concentrates in a small amount of material what was spread through a large volume of water.
Returned to the river untreated, it would probably make the water cloudy and contaminated again for those who live downstream.
The sludge has concentrated clay, coagulant and microorganisms, and needs treatment and proper disposal.
challenge
In a city, the turbidity of the filtered water rises from 0.3 to 0.8 NTU and stays at that value for several days, above the 0.5 the regulation requires in 95 % of the samples, while the decanted water still has 4 NTU. Which stage should the technicians think of first, and why?

Show solution
The water reaches the filter as cloudy as before, with the same 4 NTU, and so coagulation, flocculation and decanting seem to be working.
What changed was the output of the filter. With use, the grains fill up with the flocs they hold back, and a dirty filter tends to let particles through, which calls for a wash.
If the decanted water had also got worse, it would be worth looking first at the dose of aluminium sulfate, which might be forming flocs that are too small.
Filtration, because only the output of the filter got worse, and the filter probably needs washing.
challenge
The distillation of step 7 removes everything dissolved in the water. Why do cities not distil the water they drink? Estimate the energy to distil 1 m³ of water starting from 20 °C, with \(c = 4.18\ \text{J/(g·°C)}\), \(L = 2257\ \text{J/g}\) and \(1\ \text{kWh} = 3.6 \cdot 10^6\ \text{J}\).

Show solution
With the density of water, 1 m³ has \(1000\ \text{kg} = 10^6\ \text{g}\).
To heat it from 20 °C to 100 °C, \(Q_1 = m\,c\,\Delta T = 10^6 \cdot 4.18 \cdot 80\), and \(Q_1 \approx 3.3 \cdot 10^8\ \text{J}\).
To vaporise it, \(Q_2 = m\,L = 10^6 \cdot 2257\), and \(Q_2 \approx 2.3 \cdot 10^9\ \text{J}\).
In total, \(Q \approx 2.6 \cdot 10^9\ \text{J}\), and \(\dfrac{2.6 \cdot 10^9}{3.6 \cdot 10^6} \approx 720\ \text{kWh}\) for each cubic metre.
A city uses thousands, or even millions, of cubic metres a day, and the energy bill would be enormous, while the treatment works runs almost entirely on gravity. Distilled water would also come out without the mineral salts that drinking water may contain.
About \(2.6 \cdot 10^9\ \text{J}\), or 720 kWh per cubic metre, too much energy for the water of a whole city.

WRAP-UP

Challenges

Boiling point and density · Steps 2 and 3

An unlabelled bottle holds a colourless liquid. A 50 mL sample has a mass of 39.5 g, and the liquid boils at 78 °C at 1 atm. At 20 °C, the densities are 1.00 g/mL for water, 0.789 for ethanol, 0.784 for acetone and 0.786 for isopropanol, and the boiling points are in the table of step 2. What is this liquid?

Show solution

The density is \(d = \dfrac{39.5}{50} = 0.79\ \text{g/mL}\), which rules out water, with 1.00 g/mL.
Ethanol, acetone and isopropanol have densities between 0.78 and 0.79 g/mL, and density alone cannot decide between them.
The boiling point decides. Acetone boils at 56.1 °C and isopropanol at 82.3 °C, and only ethanol boils close to 78 °C, at 78.3 °C.
It is ethanol, and we need both properties together to say so.

Planning the separation · Steps 5, 6 and 7

A bucket holds sand, salt, iron filings and water, and all the salt is dissolved. How many phases does the system have? In what order do we separate everything, recovering the water as well?

Show solution

The dissolved salt forms a single phase with the water, and the sand and the filings are two more. That makes 4 components in 3 phases.
A magnet passed through the mixture, or over the solid once it is dry, pulls out the filings and leaves the sand.
Filtration holds back the sand, and the salt water passes through the paper, because the dissolved salt goes through the pores.
Simple distillation separates the water, which turns into vapour and condenses in the receiver, from the salt, which stays in the flask.
Three phases. Magnet, filtration and simple distillation, with distillation at the end, because only it separates what is dissolved.

Distillation curve · Steps 2 and 7

In the fractional distillation of a mixture of two liquids, the thermometer at the top stays at 56 °C for a good while, rises quickly and stays at 82 °C until the end. Which liquids does the mixture seem to contain, going by the table of step 2? What would the curve of a pure liquid show?

Show solution

Each plateau indicates an almost pure liquid distilling. The one at 56 °C matches acetone, with a BP of 56.1 °C, and the one at 82 °C matches isopropanol, with a BP of 82.3 °C, at 1 atm.
The quick rise between the plateaus marks the moment when the acetone ran out in the flask.
A pure liquid boils at a single temperature and would give a single plateau, from start to finish.
Acetone and isopropanol, very probably. A pure liquid would give a single plateau.

Classifying and separating · Steps 4, 5 and 6

A glass holds water, oil and ice cubes. How many components and how many phases does the system have, is it homogeneous or heterogeneous, and how do we take the oil out? As in step 5, we count the oil as a single component.

Show solution

Ice is solid water, the same compound, \(\text{H}_2\text{O}\), and does not count as a new component. There are 2 components, water and oil.
Liquid water, oil and ice can be told apart by eye, and are 3 phases. The system is heterogeneous.
We wait for the ice to melt, two layers are left, and the separating funnel lets out the water at the bottom and keeps the oil.
2 components, 3 phases, heterogeneous, and the oil comes out with the separating funnel once the ice has melted.

The treatment works as a chain of methods · Steps 6 and 8

Match each stage of the treatment works to a method from step 6, where there is one, and say which property it takes advantage of: screening, coagulation, flocculation, decanting, filtration and chlorination.

Show solution

Screening is coarse sieving, and takes advantage of the size of the branches and leaves.
Flocculation prepares for decanting, and decanting takes advantage of the density of the flocs, greater than that of water.
Filtration takes advantage of particle size, like the filter paper of step 6.
Coagulation and chlorination are not separation methods. They are chemical stages, in which aluminium sulfate and chlorine react in the water.
Sieving, decanting and filtration separate by size or by density, and the two chemical stages prepare the separation or disinfect.

Density and separation · Steps 3 and 6

Sawdust has fallen onto a heap of sand. How do we separate the two without a sieve, without a magnet and without a filter, with only a bucket of water?

Show solution

Sand, at about 2.6 g/cm³, is much denser than water and goes to the bottom.
Sawdust from most woods is less dense than water and tends to float, at least until it gets soaked.
We tip the mixture into the water, stir and collect the sawdust from the surface, and the sand stays at the bottom.
By density. The sawdust floats and the sand sinks.

Heat in changes of state · Steps 1 and 2

How much heat melts 200 g of ice at 0 °C, and how much takes this water from 0 °C to 100 °C? Use \(L_f = 334\ \text{J/g}\) and \(c = 4.18\ \text{J/(g·°C)}\).

Show solution

To melt it, \(Q = m\,L_f = 200 \cdot 334\), and \(Q = 66\,800\ \text{J}\), about 67 kJ, with the temperature standing at 0 °C.
To heat it, \(Q = m\,c\,\Delta T = 200 \cdot 4.18 \cdot 100\), and \(Q = 83\,600\ \text{J}\), about 84 kJ.
Melting the ice costs almost as much as heating the water by 100 °C, and that is why the melting plateau in step 2 lasts so long.
About 67 kJ to melt it and 84 kJ to heat it.

ENEM-style questions

The ENEM is Brazil's national secondary-school exam, which most students sit to get into university, and we wrote the five questions below in its format, with an everyday context, a question and five options, each one returning to a different step of the lesson. Further down we list real exam questions on the separation of mixtures, specific properties and water treatment, which we checked against the official booklets and which may well be the best practice once these are done.

Heating curve · Step 2

In a practical class, Lia was given two bottles, one with distilled water and the other with water in which someone had dissolved table salt, but the labels had fallen off. She put 100 g of each sample into two identical beakers, on heaters of the same power, at sea level, and wrote down the temperature every 2 minutes.

Temperature (°C)
Time (min)	Sample 1	Sample 2
0	20	20
2	77	77
4	100	101.9
6	100	102.1
8	100	102.4
10	100	102.8

Which sample is the distilled water, and why?

Sample 1, because its temperature stops rising while it boils, as happens with a pure substance.
Sample 2, because it boils at a higher temperature, a sign that the water is purer.
Both, because they start at 20 °C and reach 77 °C in the same time.
Sample 2, because its temperature keeps rising, a sign that it has not yet started to boil.
Neither, because pure water would boil above 100 °C at sea level.

Show solution

Answer: A.
A pure substance boils at a fixed temperature for each pressure, and the heating curve shows a plateau while it boils. At sea level water boils at 100 °C, and sample 1 stays there from 4 to 10 minutes.
In salt water the boiling point starts above 100 °C and rises as the water evaporates and the solution becomes more concentrated, with no plateau.
Option B swaps cause and effect, since it is the salt that raises the boiling point. C looks only at the stretch in which both are still liquid and warm up equally. D forgets that sample 2 is already boiling, only above 100 °C, and E contradicts the boiling point of water itself.

Density · Step 3
For the science fair, Bia built a column in a measuring cylinder with four liquids that do not mix straight away, poured carefully, from the densest to the least dense. Then she dropped a 3.4 g, 4.0 cm³ piece of rubber into the cylinder.

Density at 20 °C
Liquid d (g/cm³)

Honey 1.42

Water 1.00

Oil 0.92

Alcohol 0.79

Where does the piece of rubber come to rest?
1. At the bottom of the cylinder, below the honey.
2. On the honey, below the water.
3. On the water, below the oil.
4. On the oil, below the alcohol.
5. Floating at the top of the column, on the alcohol.
Show solution

Answer: D.
\(d = \dfrac{m}{V} = \dfrac{3.4}{4.0} = 0.85\ \text{g/cm}^3\)
The piece sinks in the liquids less dense than itself and floats on the denser ones. It is denser than alcohol, at 0.79, and less dense than oil, at 0.92, and so it passes through the alcohol and comes to rest on the oil.
Anyone who divides the volume by the mass gets 1.18 g/cm³ and puts the piece on the honey, in option B. C would hold for a density between 0.92 and 1.00, and A and E for pieces denser than honey or less dense than alcohol.
Separating heterogeneous mixtures · Step 6
At a recycling cooperative, drinks cans arrive crushed and mixed up, some made of steel and others of aluminium, and each metal is sold separately. Steel is an alloy made almost entirely of iron, with a density of about 7.9 g/cm³, and aluminium has a density of 2.7 g/cm³. The cooperative wants to separate tonnes of cans a day on a conveyor belt.

What equipment carries out this separation, and what property does it take advantage of?
1. A sieve, because the steel cans are bigger than the aluminium ones.
2. A water tank, because the aluminium cans, less dense than water, float.
3. An electromagnet above the belt, because steel is attracted by the magnet and aluminium is not.
4. A paper filter, which holds back the aluminium cans and lets the steel ones through.
5. A centrifuge, because steel is denser and separates from the aluminium when spun.
Show solution

Answer: C.
The iron in steel is attracted by the magnet, and aluminium is not, and this difference in magnetism is enough to separate the cans on the belt, like the iron filings from the sand in step 6.
The sieve in A would fail even if the cans had different sizes, because the size of a crushed can depends on how it was made and squashed, not on the metal. B gets the density wrong, because aluminium, at 2.7 g/cm³, is denser than water, and a crushed can sinks. The filter in D separates a solid from a liquid, and the centrifuge in E speeds up the decanting of parts that are in a liquid, which is not the case for whole cans.
Distillation · Step 7
In communities of Brazil's semi-arid north-east that only have brackish water, a solar still can provide drinking water. It is a dark, shallow box with the brackish water at the bottom, covered by a sloping sheet of glass. The sun warms the water, the vapour rises, condenses on the cooler glass, and the drops run down into a gutter that carries them to a container.

What describes the process and the water that reaches the container?
1. It is a filtration, in which the glass holds back the salt and lets the water through.
2. It is a simple distillation, in which only the water turns into vapour, the salt stays in the box and the water in the container comes out with much less salt.
3. It is a decanting, in which the salt sinks to the bottom of the box and the clean water rises up the glass.
4. It is a fractional distillation, which separates two liquids with different boiling points.
5. It is an evaporation, and the water in the container carries the same salt that was in the box.
Show solution

Answer: B.
The dissolved salt does not turn into vapour at the temperatures in the box, and the vapour that rises is water. It condenses on the glass, as in the condenser of step 7, and the water collected comes out almost free of salt, while the salt builds up in the box.
The glass in A is not a filter, because what reaches it is already vapour. The dissolved salt in C does not settle, since it forms a homogeneous mixture with the water. D calls for two liquids, and here there is one liquid with a dissolved solid. E gets the evaporation right, but forgets that the salt stays behind, and evaporation alone, without collecting the vapour, would lose the water to the air.
Water treatment · Step 8
Dona Cida noticed a smell of chlorine in her tap water and phoned the water company, thinking something had gone wrong. The technician who answered explained that Portaria GM/MS 888/2021, Brazil's drinking-water regulation from the Ministry of Health, requires at least 0.2 mg/L of free residual chlorine, or an equivalent dose of another form of chlorine, to be kept throughout the distribution network and at the points of use, and that a faint smell of chlorine is common.

Why does the company leave chlorine in water that has already gone through every stage of treatment?
1. To make the water pure, with no other substance at all.
2. To keep disinfecting the water on its way through the reservoirs and the pipes, where it can be contaminated again.
3. To reduce the turbidity left after filtration.
4. To make the clay particles stick together and settle inside the pipes.
5. To prevent tooth decay in the residents.
Show solution

Answer: B.
The water travels kilometres of pipes and passes through reservoirs before reaching people's homes, and on the way it can pick up microorganisms through a leak or a badly made connection. The chlorine that stayed dissolved keeps acting and kills these microorganisms before the tap.
Option A confuses drinkable with pure, and the chlorine itself is one more substance in the mixture. The turbidity in C is the job of decanting and filtration, and D describes coagulation, which happens at the works. Tooth decay, in E, is a matter for fluoride, through fluoridation.

Density at 20 °C
Liquid	d (g/cm³)
Honey	1.42
Water	1.00
Oil	0.92
Alcohol	0.79

Real ENEM questions on these topics

We checked each question against the official booklet published by INEP (in Portuguese), by year, day, booklet colour and number.

ENEM 2011, day 1, blue booklet, question 52. The riverside communities on the islands around Belém drink river water and suffer from diarrhoea, and the question asks which stage of treatment tackles the microorganisms.
ENEM 2011, day 1, blue booklet, question 54. Standards set a density range for the tin and lead alloys used in solder, and with the compositions of five samples we decide which meet the standard.
ENEM 2013, day 1, blue booklet, question 81. Aluminium sulfate forms particles in water to which the impurities stick, and the question asks for the method that removes these particles.
ENEM 2014, day 1, blue booklet, question 51. Chlorinated water with organic matter can form chloroform, and the question asks which treatment removes these molecules, which leads us to activated charcoal.
ENEM 2022, day 2, blue booklet, question 97. Uranium enrichment separates two compounds that differ only slightly in mass, and the question asks for a procedure that takes advantage of this difference.
ENEM 2022, day 2, blue booklet, question 126. The question lists the stages of a water treatment works, in order, and asks in which of them most of the suspended solids are removed.

ENEM papers and answer keys on the INEP website →

Cheat sheet

Density\(d = m/V\)

Specific properties, at 1 atmMP and BP

Heating curveplateau = pure substance, ramp = mixture

Sensible heat\(Q = m\,c\,\Delta T\)

Latent heat\(Q = m\,L\)

Element, compound, mixtureatoms of a single element · atoms of several elements bonded together · several substances

Components and phasesdifferent substances · parts that can be told apart

Solubility of NaCl, at 20 °C≈ 36 g / 100 g of water

Limit of distilling water + ethanol≈ 96 %v

Mixture → method
Mixture	Method
Solid + liquid, undissolved	filtration
Immiscible liquids	separating funnel
Magnetic solid + another solid	magnet
Solid dissolved in a liquid	evaporation or simple distillation
Miscible liquids	fractional distillation