Chapter 3 of 8

All 8 chapters
  1. 1Elements and compounds, readunderstood
  2. 2Pure substances and mixtures, readunderstood
  3. 3Separating mixtures, readunderstood
  4. 4Physical and chemical changes, readunderstood
  5. 5Rusting and other reactions, readunderstood
  6. 6Burning and fire safety, readunderstood
  7. 7Acids and alkalis, readunderstood
  8. 8Neutralisation, readunderstood

S3.6.3 Chapter 3

Separating mixtures

How can you get salt back out of sea water, or take the mud out of muddy water?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

  • Choose sieving, filtering or evaporating to separate a mixture
  • Get salt back from salty water by leaving it in the sun
  • Separate the colours in ink using paper and water

The parts of a mixture are not joined. That is the good news. It means you can separate them again, without any chemistry. You choose a method that suits the mixture.

Each method uses a difference between the parts. They may be different in size. One may dissolve and one may not. Or the liquid may turn to a gas while the other part stays.

Draw it first

Draw salt coming back from salty water

  1. A shallow dish in the sun, drawn three times, one picture for each step. In the first picture, the dish is full of salty water, with the Sun above it. In the second picture, the water in the dish is lower, and arrows rise from it, labelled water vapour. In the third picture, the water has gone and white lumps are left in the dish, labelled salt. In each picture the labels are Sun, dish, and either salty water or salt. In the second picture there is also water vapour.Sundishsalty water

    1 Draw a shallow dish of salty water in the sun.

  2. A shallow dish in the sun, drawn three times, one picture for each step. In the first picture, the dish is full of salty water, with the Sun above it. In the second picture, the water in the dish is lower, and arrows rise from it, labelled water vapour. In the third picture, the water has gone and white lumps are left in the dish, labelled salt. In each picture the labels are Sun, dish, and either salty water or salt. In the second picture there is also water vapour.Sundishsalty waterwater vapour

    2 Draw it later. The water is lower. Draw arrows rising from it. The water is leaving as water vapour.

  3. A shallow dish in the sun, drawn three times, one picture for each step. In the first picture, the dish is full of salty water, with the Sun above it. In the second picture, the water in the dish is lower, and arrows rise from it, labelled water vapour. In the third picture, the water has gone and white lumps are left in the dish, labelled salt. In each picture the labels are Sun, dish, and either salty water or salt. In the second picture there is also water vapour.Sundishsalt

    3 Draw it later still. The water has all gone. The salt is left in the dish.

To evaporate is to turn from a liquid into a gas. The water in the dish evaporates, and its vapour goes into the air. Salt does not evaporate in the sun, so it stays in the dish. This is how salt is made from sea water in hot places.

Now a different mixture: small stones and sand. They do not dissolve, and they are different in size. A sieve is a bowl with a mesh of holes. It lets small pieces fall through and holds big pieces back.

A sieve with three stones resting on its wires, and a bowl below it. Sand grains are falling through the holes and a few lie in the bowl. Labels: stones, sieve, sand.stonessievesand
The holes are wider than a grain of sand and narrower than a stone. The stones stay on top.

Next, muddy water. Mud does not dissolve, but it is too small for a sieve. Use a filter. Filter paper has holes so small that mud cannot get through. The mud stays on the paper.

A funnel holding a folded cone of filter paper, with muddy water in it. Small grains of mud rest on the paper near the point. Under the funnel, a beaker holds clear water, with a few drops falling into it. Labels: filter paper, funnel, residue, filtrate.filter paperfunnelresiduefiltrate
The solid left on the paper is the residue. The clear liquid that passes through is the filtrate.

Filtering takes out a solid that does not dissolve. It cannot take out dissolved salt. Salt particles are so small that they pass through the paper with the water. The filtrate of salty water is still salty.

See it three ways

With things

Think of what each method uses. A sieve uses size. A filter uses the fact that a solid does not dissolve. Evaporating uses the fact that water becomes a gas and salt does not.

As a picture

A table with the columns mixture and method. Stones and sand: sieve. Muddy water: filter. Salty water: evaporate.mixturemethodstones and sandsievemuddy waterfiltersalty waterevaporate

In words

Match the method to the mixture. Sieve when the pieces are different sizes. Filter when a solid is in a liquid. Evaporate when a solid is dissolved.

One more mixture is a drop of ink. The black ink in a felt-tip pen is often a mixture of several coloured substances. Chromatography separates them. A strip of paper and a little water are all you need.

A paper strip stands in a glass with a little water. A ruler runs up the side in cm, from 0 at the start line to 7. A dark ink spot sits on the start line, above the water level. Labels: ink spot, start line, water level, cm.start linewater levelink spot01234567cm
The ink spot starts above the water. If the spot were under the water, the ink would wash into the glass.

The water soaks up the paper and carries the ink with it. Each colour is carried a different distance. Some colours cling to the paper and travel only a little. Others dissolve well in water and travel far.

A paper strip stands in a glass with a little water. A ruler runs up the side in cm, from 0 at the start line to 7. Three small bands sit above the start line, with a dashed line near the top where the water reached. The band labelled yellow is at 4 cm, the band labelled red is at 2.5 cm and the band labelled blue is at 1 cm. Other labels: start line, water level, cm.yellowredbluestart linewater level01234567cm
The colours have separated. Yellow travelled 4 cm, red 2.5 cm and blue 1 cm from the start line.

Try it

Get salt back from salty water

You need: a dark plate or a flat dish, a cup of water, a spoon of salt, a sunny, safe place.

Be safe: Do not taste the salt. Keep the plate away from small children and pets.

  1. Stir a spoon of salt into half a cup of water until it has all dissolved.
  2. Pour a thin layer of the salty water onto the plate. Put the plate in a sunny place where nobody will knock it.
  3. Wait until the water has all gone. This may take a day or two. Look at the plate.

What you will see: The water disappears into the air. A white crust of salt is left on the plate. The salt was in the water all the time.

If you cannot try it, this is what happens.
A shallow dish in the sun with the water gone. White lumps are left in the dish, labelled salt. Labels: Sun, dish, salt.Sundishsalt

Try it

Split an ink into colours

You need: a strip of coffee filter paper or kitchen paper, about 3 cm wide, a pencil, a felt-tip pen with washable ink, a tall glass, water.

Be safe: Use only washable pens. Do not drink the water.

  1. Draw a pencil line across the strip, 2 cm from one end. Use a pencil, not a pen. Pencil does not dissolve in water, so the line stays put. Put a dot of ink on the line.
  2. Pour water into the glass, about 1 cm deep. Hang the strip so its end dips in the water. The ink spot must stay above the water.
  3. Wait about 10 minutes. Take the strip out and lay it flat. Look at the colours, and measure how far each one went from the line.

What you will see: The water climbs the paper. A black or brown ink often splits into several colours. Each colour stops at its own height. Your colours will differ from the picture.

If you cannot try it, this is what happens.
A paper strip stands in a glass with a little water. A ruler runs up the side in cm, from 0 at the start line to 7. Three small bands sit above the start line, with a dashed line near the top where the water reached. The band labelled yellow is at 4 cm, the band labelled red is at 2.5 cm and the band labelled blue is at 1 cm. Other labels: start line, water level, cm.yellowredbluestart linewater level01234567cm

A wrong idea many people have

Filtering salty water takes the salt out.

Why it feels right: Filter paper takes the mud out of muddy water. So it seems it should take salt out of salty water too.

The test that breaks it: Pour salty water through filter paper. Taste is not a safe test, so leave the filtrate in a dish in the sun. A white crust of salt is left. The salt went through the paper.

What is true: A filter holds back only solids that are too big to pass through. Dissolved salt is far smaller, so it passes through. To get salt back, evaporate the water.

Key words

evaporate
turn from a liquid into a gas.
sieve
a mesh of holes that lets small pieces fall through.
filter
paper with tiny holes that holds back solid pieces.
residue
the solid left on the filter paper.
filtrate
the liquid that passes through the filter.
chromatography
a way to separate coloured substances using paper and water.

Back to your guess

The answer is A: Leave the water in the sun. The water goes into the air and the salt stays behind.

The water evaporates into the air as vapour. The salt does not evaporate, so it is left behind. Dissolved salt is still there, and you can get it back.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. You have small stones mixed with sand. Which method separates them?

    Tap your answer.

    1. Not this one. Neither sand nor stones turn to gas in the sun. Evaporating separates a dissolved solid from its liquid. Go to S3.6.3. Try another.

    2. Not this one. Sand grains are too big to pass through filter paper, so it blocks. A sieve is the right tool for solids of different size. Go to S2.4.7. Try another.

    3. Yes, that is right. The sand grains are smaller than the holes, and the stones are bigger. Size is the difference a sieve uses.

    See the answer

    Sieve them. The sand falls through and the stones stay.

    The sand grains are smaller than the holes, and the stones are bigger. Size is the difference a sieve uses.

    • If you chose A: Neither sand nor stones turn to gas in the sun. Evaporating separates a dissolved solid from its liquid. Go to S3.6.3.
    • If you chose B: Sand grains are too big to pass through filter paper, so it blocks. A sieve is the right tool for solids of different size. Go to S2.4.7.

  2. Muddy water is poured through filter paper. What happens?

    Tap your answer.

    1. Not this one. The holes in filter paper are too small for mud to pass through. It stays on the paper. Go to S2.4.7. Try another.

    2. Not this one. Water is made of tiny particles and passes through the holes easily. The mud is too big. Go to S3.6.3. Try another.

    3. Yes, that is right. The mud does not dissolve, and its pieces are bigger than the holes in the paper. The water is not held back.

    See the answer

    The mud stays on the paper as the residue, and clear water passes through as the filtrate.

    The mud does not dissolve, and its pieces are bigger than the holes in the paper. The water is not held back.

    • If you chose A: The holes in filter paper are too small for mud to pass through. It stays on the paper. Go to S2.4.7.
    • If you chose B: Water is made of tiny particles and passes through the holes easily. The mud is too big. Go to S3.6.3.

  3. How can you get the salt back from a cup of salty water?

    Tap your answer.

    1. Yes, that is right. The water evaporates into the air and the salt does not. So the salt is left in the dish.

    2. Not this one. Dissolved salt is far too small to be held back. It passes through the paper with the water. Go to S3.5.7. Try another.

    3. Not this one. The salt has dissolved, so its particles are spread through the water. They pass through the holes of a sieve with ease. Go to S3.5.7. Try another.

    See the answer

    Leave it in a dish in the sun until the water has gone. The salt is left behind.

    The water evaporates into the air and the salt does not. So the salt is left in the dish.

    • If you chose B: Dissolved salt is far too small to be held back. It passes through the paper with the water. Go to S3.5.7.
    • If you chose C: The salt has dissolved, so its particles are spread through the water. They pass through the holes of a sieve with ease. Go to S3.5.7.

  4. A dish of salty water stands in the sun until it is dry. What is in the dish at the end?

    Tap your answer.

    1. Not this one. Water evaporates in the sun. Salt does not. It stays in the dish. Go to S2.4.4. Try another.

    2. Yes, that is right. Water particles leave the dish as vapour. The salt does not evaporate, so it stays.

    3. Not this one. The question says the dish is dry. The water has all gone into the air as vapour. Go to S2.4.4. Try another.

    See the answer

    White salt.

    Water particles leave the dish as vapour. The salt does not evaporate, so it stays.

    • If you chose A: Water evaporates in the sun. Salt does not. It stays in the dish. Go to S2.4.4.
    • If you chose C: The question says the dish is dry. The water has all gone into the air as vapour. Go to S2.4.4.

  5. A spot of black ink is put on a strip of paper. After the water has climbed up, the spot has split into yellow, red and blue. What does this show?

    Tap your answer.

    1. Yes, that is right. Each coloured substance in the ink travels a different distance. So they are separated. That shows the ink was a mixture.

    2. Not this one. The water only carried the colours. They were in the ink before the water reached it. Go to S3.6.3. Try another.

    3. Not this one. A pure substance would stay as one spot. Splitting into several colours shows the ink is a mixture. Go to S3.6.2. Try another.

    See the answer

    The black ink is a mixture of different coloured substances.

    Each coloured substance in the ink travels a different distance. So they are separated. That shows the ink was a mixture.

    • If you chose B: The water only carried the colours. They were in the ink before the water reached it. Go to S3.6.3.
    • If you chose C: A pure substance would stay as one spot. Splitting into several colours shows the ink is a mixture. Go to S3.6.2.

  6. How much farther above the start line did the yellow colour travel than the blue colour?

    A paper strip stands in a glass with a little water. A ruler runs up the side in cm, from 0 at the start line to 7. Three small bands sit above the start line, with a dashed line near the top where the water reached. The band labelled yellow is at 4 cm, the band labelled red is at 2.5 cm and the band labelled blue is at 1 cm. Other labels: start line, water level, cm.yellowredbluestart linewater level01234567cm
    Tap your answer.

    1. Yes, that is right. The yellow band is at 4 on the ruler and the blue band is at 1. 4 − 1 = 3.

    2. Not this one. That is how far the yellow went from the start line. The question asks how much farther it went than the blue. Go to S3.6.3. Try another.

    3. Not this one. That is yellow plus blue added together. To compare two distances, take the smaller from the larger. Go to S3.6.3. Try another.

    See the answer

    3 cm

    The yellow band is at 4 on the ruler and the blue band is at 1. 4 − 1 = 3.

    • If you chose B: That is how far the yellow went from the start line. The question asks how much farther it went than the blue. Go to S3.6.3.
    • If you chose C: That is yellow plus blue added together. To compare two distances, take the smaller from the larger. Go to S3.6.3.

  7. From an earlier chapter

    Why is sea water a mixture?

    Tap your answer.

    1. Not this one. The salt is not joined to the water. It is spread through it, and you can get it back by evaporating. Go to S3.6.2. Try another.

    2. Yes, that is right. Water and salt keep their own properties. Because they are not joined, we can separate them by evaporating.

    3. Not this one. Clear does not mean pure. The salt is dissolved, so it cannot be seen. Go to S3.6.2. Try another.

    See the answer

    It has water and dissolved salt that are not joined, and the salt can be got back.

    Water and salt keep their own properties. Because they are not joined, we can separate them by evaporating.

    • If you chose A: The salt is not joined to the water. It is spread through it, and you can get it back by evaporating. Go to S3.6.2.
    • If you chose C: Clear does not mean pure. The salt is dissolved, so it cannot be seen. Go to S3.6.2.

  8. An empty dish has a mass of 40 g. After salty water has dried in it, the dish and salt have a mass of 55 g. What is the mass of the salt?

    See the answer

    15 g

    The salt is the extra mass. 55 − 40 = 15.

Keep it

Copy this into your exercise book.

Sieve to separate by size. Filter to remove a solid. Evaporate to get a dissolved solid back. Chromatography splits an ink into its colours.

Draw: Draw a funnel with paper in it, mud on the paper, and clear water in a beaker below. Label residue and filtrate.

Reading with a child? A short page for the adult: what this chapter teaches and three questions to ask.

No internet tomorrow? Save the whole book as one file (936 KB). It opens on any phone.