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Primary Science · Year 5Chapter 6 · Solutions and separating mixtures
Year 5 · Chapter 6

Solutions and separating mixtures

By the end of this chapter you will be able to

  • Say which solids dissolve in water to form a solution, and which do not
  • Name the solute, the solvent and the solution in a salt water mixture
  • Describe how to recover a dissolved substance from a solution, by evaporating the water
  • Choose a method, filtering, sieving or evaporating, to separate a mixture, and give a reason
  • Use a fair test to find out which solid dissolves faster or more fully

You should already know

  • Materials and their properties (Year 5 Science, Chapter 5)
  • Solids, liquids and gases, and the water cycle (Year 4 Science)

Where this leads

  • Reversible and irreversible changes, including dissolving and changes of state (Year 5 Science, Chapter 7)
  • Working scientifically, planning and reporting (Year 5 Science, Chapter 1)

National Curriculum for England, Key Stage 2 science, Year 5. Content: M3 know that some materials will dissolve in liquid to form a solution, and describe how to recover a substance from a solution, M4 use knowledge of solids, liquids and gases to decide how mixtures might be separated, including through filtering, sieving and evaporating.

Exercise P  Do you remember?

1
FWhich state of matter has a fixed shape: solid, liquid or gas? Write the word.
2
FWhich state of matter takes the shape of its container, and flows: solid, liquid or gas? Write the word.
3
FWhat is the name for the process in which water turns into a gas, so that it rises into the air? Write the word.
4
FWhich kind of material is sand: a solid, a liquid or a gas? Write the word.
5
FWrite the word for a mixture that can be separated by a sieve, such as sand and pebbles.
6.1

Dissolving solids in water

Year 5
M3: know that some materials will dissolve in liquid to form a solution
Video lesson: Dissolving solids in waterComing soon
Story. Aisha stirs a spoon of salt into a cup of water. After a minute, the salt has gone, and the water looks clear. “Where has the salt gone?” she asks. Her teacher says: “It has dissolved, so it is still there, but too small to see.”

Some solids dissolve in water. When a solid dissolves, its particles spread out evenly through the liquid, and you can no longer see them. Salt and sugar dissolve in water. Sand and pebbles do not dissolve.

A fair test for dissolving uses the same amount of each solid, the same amount of water, and the same stirring for the same time.

Solid stirred into waterDoes it dissolve?
saltyes
sugaryes
sandno
pebblesno

Key ideas

  • Some solids dissolve in water, and others do not.
  • Salt and sugar dissolve in water.
  • Sand and pebbles do not dissolve in water.
  • A fair test keeps the solid, the water and the stirring the same.
Worked example 6.1.1
Look at the table in section 6.1. Which solid dissolves in water: sand, or sugar? Write the word.

Method. Read the second column.

  1. Read the rows. The sugar row says yes, and the sand row says no.
    yes and no
  2. Choose the solid that dissolves. So the solid that dissolves is sugar.
    sugar
  3. Write the answer.
    sugar

Check. Sugar dissolves in water. ✓

Worked example 6.1.2
How many of the four solids in the table dissolve in water? Write the number.

Method. Count the yes rows.

  1. Read the column. The yes rows are salt and sugar.
    two yes rows
  2. Count them. So two solids dissolve.
    2
  3. Write the answer.
    2

Check. The other two, sand and pebbles, do not dissolve. ✓

Worked example 6.1.3
Does sand dissolve in water? Write yes or no.

Method. Read the sand row.

  1. Read the row. The sand row says no.
    no
  2. Decide. So sand does not dissolve.
    no
  3. Write the answer.
    no

Check. Sand stays as grains in the water. ✓

Watch out

Watch out: when a solid dissolves, it does not disappear. Its particles are still in the water, just too small to see.

Try it (with your teacher). With your teacher, stir one teaspoon of salt into a cup of room-temperature water for one minute. Repeat with one teaspoon of sand. Look at each cup. Which solid dissolved? Write yes or no for each. Do not taste anything.

Exam tip

Grown-up tip. Ask: “Where has the salt gone? Can you still see it?” Let your child stir for the same time each time.

Exercise 6A  Dissolving solids in water

1
FWhich two solids in the table in section 6.1 dissolve in water? Write the two names.
2
FDoes pebble dissolve in water? Write yes or no.
3
FFill in the gap: when a solid dissolves, its particles spread out ___ through the liquid. Write one word.
4
FTrue or false: a solid that dissolves disappears completely. Write true or false.
5
FHow many of the four solids in the table do not dissolve? Write the number.
6
PSWhich solid in the table is the same as the one used in a salt cellar, and dissolves? Write the word.
7
PSWorking scientifically. Predict, then check. Priya predicts that sugar will dissolve faster than sand. Is she right? Write yes or no.
8
PSLook at the table in section 6.1. Name a solid that does not dissolve in water, other than sand. Write the word.
9
RExplain why a fair test for dissolving keeps the same amount of solid and water. Write one sentence.
10
RExplain why we stir the solid and water in a dissolving test. Write one sentence.
6.2

Solutions: solute, solvent and solution

Year 5
M3: know that some materials will dissolve in liquid to form a solution
Video lesson: Solutions: solute, solvent and solutionComing soon
Story. Mo makes salt water for a science experiment. “Which part is the salt, which part is the water, and what is the mixture called?” he asks. His teacher says: “Learn the three words, and you can explain any solution.”

A solution is a mixture in which a solid has dissolved in a liquid. The solid that dissolves is the solute. The liquid it dissolves into is the solvent.

In salt water, the salt is the solute, the water is the solvent, and the mixture is the solution. Water is the most common solvent, but other liquids can also be solvents.

solid saltthe salt dissolves: the water is now a solutionsolvent: watersolute: salt
A beaker of water with salt that has dissolved. The water is the solvent, the salt is the solute, and the mixture is a solution.

Key ideas

  • A solution is a mixture in which a solid has dissolved in a liquid.
  • The solute is the solid that dissolves.
  • The solvent is the liquid it dissolves into.
  • In salt water, the salt is the solute, the water is the solvent, and the mixture is the solution.
Worked example 6.2.1
In salt water, which substance is the solvent? Write the word.

Method. Find the liquid.

  1. Find the liquid. The liquid that the salt dissolves into is water.
    water
  2. Name the solvent. The liquid it dissolves into is the solvent, so the solvent is water.
    water
  3. Write the answer.
    water

Check. The solvent is always the liquid. ✓

Worked example 6.2.2
In a sugar and water solution, which substance is the solute? Write the word.

Method. Find the solid that dissolves.

  1. Find the solid. The sugar is the solid that dissolves.
    sugar dissolves
  2. Name the solute. The solid that dissolves is the solute, so the solute is sugar.
    sugar
  3. Write the answer.
    sugar

Check. Sugar is the solute. ✓

Worked example 6.2.3
Is the mixture of salt and water a solution, or a mixture of solids? Write the word.

Method. Think about what happens to the salt.

  1. Think about the salt. The salt dissolves in the water, so the mixture is a solution.
    dissolves
  2. Name the mixture. The mixture is a solution.
    solution
  3. Write the answer.
    solution

Check. Salt water is a solution. ✓

Watch out

Watch out: a solution is not a mixture of solids. In a solution the solid has dissolved, so its particles are spread evenly through the liquid.

Try it (with your teacher). Make a cup of sugar water with a teaspoon of sugar and a cup of water. Stir it until the sugar has gone. Point to the solute and the solvent, and say what the mixture is called. Do not taste it, because you must not taste anything in a science lab.

Exam tip

Grown-up tip. Ask: “What is the solid? What is the liquid? What do we call the mixture?” Let your child use the words solute, solvent and solution.

Exercise 6B  Solutions: solute, solvent and solution

1
FWhat is the name for a mixture in which a solid has dissolved in a liquid? Write the word.
2
FWhat is the name for the solid that dissolves in a liquid? Write the word.
3
FWhat is the name for the liquid that a solid dissolves into? Write the word.
4
FIn a salt and water solution, what is the solute? Write the word.
5
FIn a salt and water solution, what is the solvent? Write the word.
6
PSLook at the picture in section 6.2. What does the word solution describe? Write the word.
solid saltthe salt dissolves: the water is now a solutionsolvent: watersolute: salt
7
PSIs sugar dissolved in water a solution? Write yes or no.
8
PSIs sand mixed with water a solution? Write yes or no.
9
PSRuby says: “Salt is the solvent, because it is the bigger part.” Is she right? Write yes or no.
10
PSWorking scientifically. Plan a fair test. Name one liquid that could be a solvent, other than water. Write the word.
11
RExplain why salt water is called a solution, and not just a mixture. Write one sentence.
6.3

Recovering a solid from a solution

Year 5
M3: describe how to recover a substance from a solution, by evaporating the water
Video lesson: Recovering a solid from a solutionComing soon
Story. Ruby has a dish of salt water that has been left on a sunny windowsill for several days. “The water has gone, and there is white salt in the dish,” she says. “How did the salt come back?”

The salt can be recovered from the solution by evaporation. The water in the dish turns into water vapour, and it goes into the air. The salt does not evaporate, so it is left behind in the dish as crystals.

Evaporation happens faster in a warm, dry place, and on a dish with a large surface. It is safest to leave the dish somewhere warm and dry, rather than heating it.

water evaporates; salt is left in the dishwater vapour in the air
A dish of salt water. The water evaporates into the air, and salt crystals are left in the dish.

The table shows the mass of salt left in a dish, from a fair test. The dish was kept in the same warm, dry place each day.

Days in a warm, dry placeGrams of salt left in the dish
00
14
28

Key ideas

  • Evaporation turns the water in a solution into water vapour.
  • The dissolved solid does not evaporate, so it is left behind as crystals.
  • Evaporation is faster in a warm, dry place, and on a wide dish.
  • Recovering a solid from a solution is done by evaporating the solvent.
Worked example 6.3.1
Which substance is left in the dish when the water has evaporated? Write the word.

Method. Think about what evaporates.

  1. Think about the water. The water turns into vapour and goes into the air.
    water vapour
  2. Name the substance left. The salt does not evaporate, so the salt is left in the dish.
    salt
  3. Write the answer.
    salt

Check. The salt crystals are the recovered solid. ✓

Worked example 6.3.2
Look at the salt table in section 6.3. How many grams of salt were left after 2 days? Write the number.

Method. Find the row for 2 days.

  1. Find the row. The row for 2 days shows 8 grams of salt.
    2 days
  2. Read the mass. The mass of salt left after 2 days is 8 grams.
    8
  3. Write the answer.
    8

Check. The mass grows as the water leaves. ✓

Worked example 6.3.3
Why is it safer to leave the dish of salt water somewhere warm and dry, rather than heating it? Write one word for the reason: safe or fast.

Method. Think about why the dish is left out.

  1. Think about heating. Heating a dish needs a flame or a hot surface, which can be dangerous.
    dangerous
  2. Choose the word. Leaving it in a warm place is safer, so the reason is safe.
    safe
  3. Write the answer.
    safe

Check. Accept any answer that says it is safer. ✓

Watch out

Watch out: evaporation takes the water out of the solution, not the solid. The salt is still there, as crystals in the dish.

Try it (with your teacher). With your teacher, put one teaspoon of salt into a small dish of warm water and stir it until it dissolves. Leave the dish on a sunny windowsill or in a warm place for two or three days. Look at it each day and write what you see. Do not taste the salt or the water.

Exam tip

Grown-up tip. Ask: “Where has the water gone? Why is the salt still there?” Let your child predict what will happen, then check each day.

Exercise 6C  Recovering a solid from a solution

1
FWhat is the name for the process in which the water in a solution turns into a gas, and goes into the air? Write the word.
2
FAfter the water has evaporated from salt water, what is left in the dish? Write the word.
3
FLook at the salt table in section 6.3. How many grams of salt were left after 1 day? Write the number.
4
FLook at the salt table in section 6.3. How much more salt was left after 2 days than after 1 day? Write the number of grams.
5
PSLook at the salt table in section 6.3. At which day was there no salt left in the dish? Write the number of days.
6
FFill in the gap: evaporation takes the ___ out of the solution. Write one word.
7
FFill in the gap: the dissolved solid is recovered by ___. Write one word.
8
PSIs evaporation faster in a warm place or a cold place? Write the word: warm or cold.
9
PSAli says: “The salt evaporated along with the water.” Is Ali right? Write yes or no.
10
PSWorking scientifically. Predict the result. Predict what will be left in the dish if a sugar and water solution is left in a warm place. Write the word.
11
RExplain why the salt is still in the dish after the water has evaporated. Write one sentence.
6.4

Sieving a mixture

Year 5
M4: decide how a mixture of solids might be separated, using a sieve
Video lesson: Sieving a mixtureComing soon
Story. Tom has a box of sand and pebbles from the beach. “I want to get the sand out, but I don’t want to pick every pebble out by hand,” he says. His teacher says: “Use a sieve. Think about the size of the pieces.”

A mixture is two or more substances mixed together, without a chemical change. Some mixtures can be separated by their particle size.

A sieve has a mesh with holes. Small particles, such as sand, fall through the holes. Big particles, such as pebbles, stay on the top of the mesh.

sand has fallen into the tray; pebbles stay on the sieve
A sieve over a tray: sand falls through the mesh, and pebbles stay on the top.

The results of a sieving test are in the table.

ParticleWhat happens in the sieve
sandpasses through the sieve
pebblesstays on top of the sieve

Key ideas

  • A sieve separates a mixture of solids with different particle sizes.
  • Small particles pass through the holes of the sieve.
  • Large particles stay on the top of the sieve.
  • Sieving does not need water or heat.
Worked example 6.4.1
In a sieve, which particles pass through the holes: sand, or pebbles? Write the word.

Method. Read the sieve results.

  1. Read the results. The sand passes through the sieve, but the pebbles stay on top.
    sand passes
  2. Name the particle. So the particles that pass through are the sand.
    sand
  3. Write the answer.
    sand

Check. Small particles fall through the holes. ✓

Worked example 6.4.2
Why does sieving separate sand from pebbles? Write one word: size or colour.

Method. Think about why the sieve works.

  1. Think about the particles. The sand particles are smaller than the holes, but the pebbles are bigger.
    smaller
  2. Choose the word. The separation depends on the size of the particles.
    size
  3. Write the answer.
    size

Check. The sieve separates by size, not colour. ✓

Worked example 6.4.3
Does sieving need water to separate sand and pebbles? Write yes or no.

Method. Think about the method.

  1. Think about the method. Sieving separates by particle size, so water is not needed.
    not needed
  2. Decide. So the answer is no.
    no
  3. Write the answer.
    no

Check. Sieving is a dry method. ✓

Watch out

Watch out: a sieve separates by size, not by what the material is made of. A sieve will not separate salt from water, because salt dissolves.

Try it (with your teacher). With your teacher, put a small handful of dry sand and small pebbles (or dried peas and rice) into a sieve. Shake the sieve over a tray. Write which particles went through the holes, and which stayed on top.

Exam tip

Grown-up tip. Ask: “Which pieces went through? Which stayed on top? Why do you think that happened?” Let your child use the words size and particle.

Exercise 6D  Sieving a mixture

1
FWhich mixture is separated by a sieve: a sand and pebble mixture, or salt water? Write the words.
2
FWhat is a sieve made of, which lets small particles fall through? Write the word.
3
FWhich particles stay on top of the sieve? Write the word.
4
FWhich particles fall through the holes of the sieve? Write the word.
5
FFill in the gap: a sieve separates particles of different ___. Write one word.
6
FDoes a sieve need water to separate a dry mixture? Write yes or no.
7
PSWhich method would separate a mixture of salt and water: sieving, or evaporating? Write the word.
8
PSWhich method would separate a mixture of sand and pebbles, if the pebbles are much larger than the sand? Write the word.
9
PSLook at the table in section 6.4. Write the word for the particles that stay on the sieve.
10
PSWorking scientifically. Plan a fair sieving test. Name one thing that must stay the same in a fair test of sieving. Write one thing.
11
RExplain why a sieve can separate sand from pebbles, but not salt from water. Write one sentence.
6.5

Filtering, and choosing a method

Year 5
M4: decide how mixtures might be separated, including filtering and evaporating
Video lesson: Filtering, and choosing a methodComing soon
Story. Mr Patel has three mixtures in the lab: sand and water, salt and water, and sand and pebbles. “Which method would you use for each one?” he asks. “Give me a reason, based on what you know about solids and liquids.”

Filtering separates a solid that does not dissolve from a liquid. The liquid passes through the filter paper, and the solid is left in the paper.

Evaporating separates a dissolved solid from its solvent. The solvent turns into vapour and leaves the solid behind.

Sieving separates solids of different particle sizes.

clear water in the beakersand stays in the filter paper
A funnel with filter paper. Sand stays in the paper, and clear water runs into a beaker.
MixtureMethod to separate itWhy this method works
sand and waterfilteringthe sand is a solid that does not dissolve, so filter paper holds it back
salt and waterevaporatingthe salt dissolves, so it must be left behind when the water evaporates
sand and pebblessievingthe particles are different sizes, so a sieve keeps the big pieces
sugar and waterevaporatingthe sugar dissolves, and the water evaporates to leave the sugar

Key ideas

  • Filtering separates an insoluble solid from a liquid, using filter paper.
  • Evaporating separates a dissolved solid from its solvent.
  • Sieving separates solids of different particle sizes.
  • Choose the method by thinking about whether the solid dissolves, and whether the particles differ in size.
Worked example 6.5.1
Which method would separate sand from water? Write the word.

Method. Think about whether the sand dissolves.

  1. Think about the sand. Sand does not dissolve in water, so it can be held back by filter paper.
    does not dissolve
  2. Choose the method. So the method is filtering.
    filtering
  3. Write the answer.
    filtering

Check. Filter paper holds back the sand, and the water runs through. ✓

Worked example 6.5.2
Which method would separate salt from water? Write the word.

Method. Think about whether the salt dissolves.

  1. Think about the salt. Salt dissolves, so filter paper cannot separate it.
    dissolves
  2. Choose the method. So the method is evaporating.
    evaporating
  3. Write the answer.
    evaporating

Check. Evaporating leaves the salt behind. ✓

Worked example 6.5.3
Which method would separate sand and pebbles? Write the word.

Method. Think about the sizes of the particles.

  1. Think about the sizes. Sand and pebbles are different sizes.
    different sizes
  2. Choose the method. So the method is sieving.
    sieving
  3. Write the answer.
    sieving

Check. The sieve holds back the pebbles. ✓

Watch out

Watch out: filtering does not separate salt from water, because the salt dissolves and passes through the filter paper with the water.

Try it (with your teacher). With your teacher, pour a little sand and water through filter paper in a funnel. Watch the water run through, and look at the paper. Then explain which method you would use to get the salt back from salt water.

Exam tip

Grown-up tip. Ask: “Does the solid dissolve? If it does not, what can you use? If it does, what happens to the water?” Let your child explain each choice.

Exercise 6E  Filtering, and choosing a method

1
FWhich method separates a solid that does not dissolve from a liquid? Write the word.
2
FWhich method separates a dissolved solid from its solvent? Write the word.
3
FWhich method separates solids of different particle sizes? Write the word.
4
FLook at the method table in section 6.5. Which method separates sugar and water? Write the word.
5
FLook at the method table in section 6.5. Which mixture is separated by filtering? Write the words.
6
FLook at the method table in section 6.5. How many of the four mixtures are separated by evaporating? Write the number.
7
PSWhich method separates a mixture of sand and water? Write the word: sieving, filtering or evaporating.
8
PSWhich method is best for getting the salt back from salt water? Write the word.
9
PSMo says: “I can separate salt from water by filtering.” Is Mo right? Write yes or no.
10
PSWorking scientifically. Choose a method and justify it. A mixture of sand and pebbles needs separating. Write the method you would use.
11
RExplain why filter paper can separate sand from water, but cannot separate salt from water. Write one sentence.
12
RExplain how you would choose a method for a mixture of solids and a liquid. Write one sentence.

Chapter summary

SkillMethod
DissolvingSome solids, such as salt and sugar, dissolve in water. Sand and pebbles do not dissolve.
SolutionsA solution is a mixture in which a solid has dissolved. The solute dissolves into the solvent.
Recovering a solidEvaporate the water from the solution, and the dissolved solid is left behind as crystals.
SievingA sieve separates solids of different particle sizes. Small particles pass through the holes.
FilteringFilter paper holds back a solid that does not dissolve, and the liquid runs through.
Choosing a methodThink about whether the solid dissolves, and whether the particles differ in size.

Can you…?

  • I can say which solids dissolve in water to form a solution
  • I can name the solute, the solvent and the solution in a mixture
  • I can describe how to recover a dissolved solid, by evaporating the water
  • I can say how to separate a mixture using a sieve, filter paper or evaporation
  • I can choose a method for a mixture, and give a reason

Key terms

solution
a mixture in which a solid has dissolved in a liquid
dissolve
when a solid spreads out through a liquid so that its particles cannot be seen
solute
the solid that dissolves in a liquid
solvent
the liquid that a solid dissolves into
evaporation
the change of a liquid into a gas, which then goes into the air
mixture
two or more substances mixed together, which can often be separated again
sieving
separating solids of different sizes, using a sieve with holes
filtering
separating a solid that does not dissolve from a liquid, using filter paper
particle
a very small piece of a substance

Exercise 6Z  Check-up

1
Which solid in the table in section 6.1 does not dissolve in water? Write the word.
Solid stirred into waterDoes it dissolve?
saltyes
sugaryes
sandno
pebblesno
2
In a salt and water solution, what is the solvent? Write the word.
3
What is the name for a mixture in which a solid has dissolved in a liquid? Write the word.
4
Look at the salt table in section 6.3. How many grams of salt were left after 2 days? Write the number.
Days in a warm, dry placeGrams of salt left in the dish
00
14
28
5
Which process turns water into water vapour, so that a dissolved solid is left behind? Write the word.
6
Look at the sieve picture in section 6.4. Which particles fall through the holes? Write the word.
sand has fallen into the tray; pebbles stay on the sieve
7
Which method would separate a mixture of sand and water? Write the word.
8
Which method would separate a mixture of salt and water? Write the word.
9
Which method would separate a mixture of sand and pebbles? Write the word.
10
Explain why the salt is still in the dish after the water has evaporated. Write one sentence.

Total 18 marks. Allow about 18 minutes.

Exercise 6X  Practice test questions

1
FWrite the word for a mixture in which a solid has dissolved in a liquid.[1 mark]
2
FIn a sugar and water solution, which substance is the solute? Write the word.[1 mark]
3
FWhich method separates a dissolved solid from its solvent? Write the word.[1 mark]
4
FLook at the salt table in section 6.3. How many grams of salt were left after 1 day? Write the number.
Days in a warm, dry placeGrams of salt left in the dish
00
14
28
[1 mark]
5
FLook at the salt table in section 6.3. How much more salt was left after 2 days than after 1 day? Write the number of grams.
Days in a warm, dry placeGrams of salt left in the dish
00
14
28
[2 marks]
6
RWhich method would separate sand from water? Write the word, and give one reason in one sentence.[2 marks]
7
RExplain why a sieve can separate sand from pebbles, but cannot separate salt from water. Write one sentence.[3 marks]
8
RA dish of salt water is left in a warm, dry place for two days. Explain what happens to the water, and what is left in the dish. Write two sentences.[3 marks]
9
PSMo says: “Sugar is the solvent in sugar and water.” Is Mo right? Write no, and give the correct word for the sugar.[2 marks]
10
RLook at the method table in section 6.5. Write the method for the mixture of sugar and water, and the reason, in one sentence.
MixtureMethod to separate itWhy this method works
sand and waterfilteringthe sand is a solid that does not dissolve, so filter paper holds it back
salt and waterevaporatingthe salt dissolves, so it must be left behind when the water evaporates
sand and pebblessievingthe particles are different sizes, so a sieve keeps the big pieces
sugar and waterevaporatingthe sugar dissolves, and the water evaporates to leave the sugar
[2 marks]
← Chapter 5: Properties and uses of materialsChapter 7: Reversible and irreversible changes →

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