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Primary Science · Year 5Chapter 7 · Reversible and irreversible changes
Year 5 · Chapter 7

Reversible and irreversible changes

By the end of this chapter you will be able to

  • Say which changes are reversible, such as melting and freezing, and dissolving
  • Explain why a change that makes a new material is usually not reversible
  • Describe what happens when an acid, such as vinegar, reacts with bicarbonate of soda
  • Use evidence, such as a gas or a new colour, to decide whether a change has made a new material
  • Sort changes into reversible and irreversible, and give a reason

You should already know

  • Solids, liquids and gases, and changes of state in the water cycle (Year 4 Science)
  • Solutions, and dissolving solids in water (Year 5 Science, Chapter 6)

Where this leads

  • The Earth, Sun and Moon (Year 5 Science, Chapter 8)
  • Forces, gravity and resistance (Year 5 Science, Chapter 9)

National Curriculum for England, Key Stage 2 science, Year 5. Content: M5 demonstrate that dissolving, mixing and changes of state are reversible changes, M6 explain that some changes result in the formation of new materials, and that this kind of change is not usually reversible, including burning and the action of acid on bicarbonate of soda.

Exercise P  Do you remember?

1
FWhat is the name for the change when a solid turns into a liquid? Write the word.
2
FWhat is the name for the change when a liquid turns into a solid? Write the word.
3
FWrite the word for a change in which sugar disappears into water, but can be recovered by evaporating the water.
4
FWhat do we call a material that is left behind when water evaporates from salt water? Write the word.
5
FIs the change from water to water vapour a change of state? Write yes or no.
7.1

Reversible changes

Year 5
M5: demonstrate that dissolving, mixing and changes of state are reversible changes
Video lesson: Reversible changesComing soon
Story. Aisha leaves an ice cube on a plate on the kitchen table. In an hour it has turned into a puddle of water. “Can I make the ice again?” she asks. Her dad says: “Yes, put the water in the freezer, and it will change back.”

A reversible change is one that can be undone, so that the material can return to how it was at the start.

Three kinds of change are reversible:

icewatermeltsfreezesa reversible change: it can go back
Ice melts into water, and water freezes back into ice. This is a reversible change.
ChangeCan it be reversed to the start?
ice melts into wateryes
water freezes into iceyes
sugar dissolves in wateryes
salt water is left to evaporate, and the salt is recoveredyes
a candle burnsno
an egg is cookedno
vinegar and bicarbonate of soda fizz togetherno
a cake is bakedno

Key ideas

  • A reversible change can be undone, so the material can return to how it was.
  • Melting and freezing are reversible changes of state.
  • Dissolving is reversible, because the solid can be recovered by evaporating the water.
  • Mixing is reversible, because the parts of a mixture can be separated again.
Worked example 7.1.1
Look at the table in section 7.1. Is ice melting into water a reversible change? Write yes or no.

Method. Read the row for ice.

  1. Read the row. The row for ice melting into water says yes.
    yes
  2. Decide. So ice melting is a reversible change.
    yes
  3. Write the answer.
    yes

Check. Water can freeze back into ice. ✓

Worked example 7.1.2
Name a reversible change in which a liquid turns into a solid. Write the word.

Method. Think about a change of state.

  1. Think about the reversible list. Water freezing into ice is a reversible change.
    freezing
  2. Name it. The change is freezing.
    freezing
  3. Write the answer.
    freezing

Check. Melting is the reverse of freezing. ✓

Worked example 7.1.3
Is dissolving sugar in water a reversible change? Write yes or no.

Method. Think about how sugar can be recovered.

  1. Think about the sugar. Evaporating the water leaves the sugar behind.
    left behind
  2. Decide. So dissolving is a reversible change, and the answer is yes.
    yes
  3. Write the answer.
    yes

Check. The sugar can be recovered from the solution. ✓

Watch out

Watch out: a reversible change may need energy or time to be undone. Water needs to be cooled to freeze, and salt needs the water to evaporate.

Try it (with your teacher). Put an ice cube in a dish, and watch it melt. Then, with your teacher, put the water in the freezer and see whether it turns back into ice. Write down which change is reversible, and which way it went.

Exam tip

Grown-up tip. Ask: “What happened to the ice? Could we make it ice again? How?” Let your child explain the change in their own words.

Exercise 7A  Reversible changes

1
FIs melting ice a reversible change? Write yes or no.
2
FWhat is the name of the change when water turns back into ice? Write the word.
3
FIs dissolving sugar in water a reversible change? Write yes or no.
4
FIs mixing sand and pebbles a reversible change? Write yes or no.
5
FLook at the table in section 7.1. How many of the eight changes are reversible? Write the number.
6
PSLook at the table in section 7.1. Which change in the table is about salt water? Write the words.
7
PSLook at the picture in section 7.1. What is the change from ice to water? Write the word.
8
PSTrue or false: mixing is an irreversible change. Write true or false.
9
PSWorking scientifically. Predict and check. Predict whether a puddle of melted ice can be made into ice again in a freezer. Write yes or no.
10
RExplain why a change of state from ice to water is called reversible. Write one sentence.
11
RExplain how you could recover the sugar from a sugar and water solution. Write one sentence.
7.2

Changes of state and mixing

Year 5
M5: demonstrate that changes of state and mixing are reversible changes, using melting and freezing
Video lesson: Changes of state and mixingComing soon
Story. Mo puts a bar of chocolate in a bowl over warm water. It goes soft and shiny. Later, he puts the bowl in the fridge, and the chocolate goes hard again. “It has changed back,” he says. “Is that another reversible change?”

A change of state happens when a material changes between solid, liquid and gas. Melting, freezing, evaporating and condensing are changes of state.

When a solid melts, it becomes a liquid. When the liquid cools, it can freeze back into the solid. In both directions the material is still the same kind of material, so the change is reversible.

Mixing is also reversible. A mixture of sand and pebbles can be separated again, by sieving. The parts of the mixture are still the same materials after they are mixed.

icewatermeltsfreezesa reversible change: it can go back
Ice melts into water, and water freezes back into ice. Both changes are reversible.

Key ideas

  • Melting changes a solid into a liquid.
  • Freezing changes a liquid back into a solid.
  • A reversible change of state can be undone, and the material returns to its start.
  • Mixing is reversible, because the parts can be separated again.
Worked example 7.2.1
When chocolate melts and then cools, does it change back into a solid? Write yes or no.

Method. Think about the change of state.

  1. Think about the change. When the liquid cools, it can freeze back into the solid.
    freeze back
  2. Decide. So chocolate can change back, and the answer is yes.
    yes
  3. Write the answer.
    yes

Check. Melting and cooling are reversible changes of state. ✓

Worked example 7.2.2
What change of state turns a liquid into a gas? Write the word.

Method. Think about evaporation.

  1. Think about the change. Water turns into water vapour, which is a gas, by evaporation.
    water vapour
  2. Name the change. The change of state is evaporation.
    evaporation
  3. Write the answer.
    evaporation

Check. Evaporation can be reversed by condensing the vapour. ✓

Worked example 7.2.3
Can sand and pebbles be separated again after they have been mixed? Write yes or no.

Method. Think about mixing.

  1. Think about the mixture. The sand and pebbles are still the same materials after mixing.
    same materials
  2. Decide. So they can be separated again, by sieving. The answer is yes.
    yes
  3. Write the answer.
    yes

Check. Mixing is a reversible change. ✓

Watch out

Watch out: a change that is reversible is not always easy to reverse. Melting chocolate needs warmth, and cooling it needs a fridge. The change can still be undone.

Try it (with your teacher). Put a few small pieces of chocolate in a dish, and watch them melt in a warm place (with your teacher). Then let the dish cool, and see whether the chocolate becomes hard again. Write one sentence about whether the change is reversible.

Exam tip

Grown-up tip. Ask: “What happened when the chocolate melted? What happened when it cooled? Did it change back?” Keep the heating very gentle, with a grown-up.

Exercise 7B  Changes of state and mixing

1
FIs melting chocolate a reversible change? Write yes or no.
2
FWhat is the name of the change when a liquid cools and becomes a solid? Write the word.
3
FFill in the gap: when a solid melts, it becomes a ___. Write one word.
4
FWhat is the change of state that turns water into a gas? Write the word.
5
FTrue or false: mixing is an irreversible change. Write true or false.
6
FCan sand and pebbles be separated again after mixing? Write yes or no.
7
PSMo says: “If it has melted, it can never be the same again.” Is Mo right? Write yes or no.
8
PSLook at the picture in section 7.2. Is the change from water to ice reversible? Write yes or no.
9
PSWorking scientifically. Plan a test. Name one thing that must stay the same in a test of how long ice takes to melt. Write one thing.
10
PSLook at the melting table in section 7.2. Work out the mean time for the ice to melt. Write the number of minutes.
TrialMinutes for the ice to melt
Trial 112
Trial 214
Trial 313
11
RExplain why the water from a melted ice cube can be frozen back into ice. Write one sentence.
12
RExplain why mixing sand and pebbles is a reversible change. Write one sentence.
7.3

Irreversible changes and new materials

Year 5
M6: explain that some changes result in the formation of new materials, and that this kind of change is not usually reversible, including burning
Video lesson: Irreversible changes and new materialsComing soon
Story. Ruby watches a candle burn down. “The wax has gone, and there is black soot on the glass,” she says. “Can we get the wax back?” Mr Patel says: “No. Burning makes new materials, and they cannot easily be changed back.”

Some changes make new materials. The new materials have different properties from the original ones. These changes are usually irreversible, which means they cannot be undone.

Examples include:

candleburnsnew materialsburning makes new materials; it cannot be undone
A candle burns, and new materials are made. The change cannot be undone.
raw eggcooked eggcookingan irreversible change: it cannot be undoneno arrow goes back
A raw egg becomes a cooked egg. There is only one arrow, so the change cannot be undone.

Key ideas

  • Some changes make new materials, with different properties.
  • Burning makes new materials, such as ash and gases.
  • Cooking an egg is a change that cannot be undone.
  • New materials usually cannot be changed back to the original materials.
Worked example 7.3.1
Is burning a candle a reversible change? Write yes or no.

Method. Think about whether the wax can be recovered.

  1. Think about the wax. The wax is changed into new materials, such as soot and gases.
    new materials
  2. Decide. So burning is not reversible, and the answer is no.
    no
  3. Write the answer.
    no

Check. New materials cannot easily be changed back. ✓

Worked example 7.3.2
Does an egg become raw again after it has been cooked? Write yes or no.

Method. Think about the change.

  1. Think about the egg. Cooking changes the egg, so it cannot become raw again.
    cannot become raw
  2. Decide. So the answer is no.
    no
  3. Write the answer.
    no

Check. Cooking is an irreversible change. ✓

Worked example 7.3.3
Which new material does a burning candle leave, that is black and sooty? Write the word.

Method. Read the picture and the story.

  1. Read the story. Ruby sees black soot on the glass after the candle has burned.
    black soot
  2. Name the material. The new material is soot.
    soot
  3. Write the answer.
    soot

Check. Soot is a new material made in burning. ✓

Watch out

Watch out: a change that makes a new material is usually irreversible, even if it looks like a small change. Do not say that burning or cooking can be undone.

Try it (with your teacher). Look at a cooked food and a raw food, with your teacher. Describe the colour, smell and texture of each. Then write whether the change from raw to cooked can be undone. Burning is never done by pupils. Watch any burning only as a demonstration, from a safe distance.

Exam tip

Grown-up tip. Ask: “Can the egg be raw again? Why not?” Keep this at a calm and factual level, and avoid any unsafe demonstration at home.

Exercise 7C  Irreversible changes and new materials

1
FIs burning a candle a reversible change? Write yes or no.
2
FIs cooking an egg a reversible change? Write yes or no.
3
FWhat new material is left on the glass when a candle burns? Write the word.
4
FFill in the gap: a change that makes new materials is usually ___. Write one word.
5
FFill in the gap: a change that cannot be undone is called ___. Write one word.
6
PSTrue or false: cooking an egg is a reversible change. Write true or false.
7
PSLook at the picture of the raw egg in section 7.3. How many arrows are there from the cooked egg back to the raw egg? Write the number.
8
PSWhich change in section 7.3 makes ash and gases? Write the word.
9
PSBaking a cake makes new materials. Is baking a reversible change? Write yes or no.
10
PSWorking scientifically. Observe the evidence. After a candle burns, what new material is left on the glass? Write the word.
11
RExplain why the wax from a candle cannot easily be recovered after it burns. Write one sentence.
12
RExplain why we say cooking an egg is an irreversible change. Write one sentence.
7.4

Acid and bicarbonate of soda

Year 5
M6: explain the action of acid on bicarbonate of soda, which forms new materials and a gas, and is not usually reversible
Video lesson: Acid and bicarbonate of sodaComing soon
Story. Priya pours vinegar onto a spoonful of bicarbonate of soda in a cup. It fizzes and bubbles up. “Where is all that gas coming from?” she asks. Her teacher says: “The vinegar is an acid. Acids react with bicarbonate of soda and make new materials, including a gas.”

Vinegar is an acid. When an acid is mixed with bicarbonate of soda, the two materials react together. The reaction makes new materials, including a gas called carbon dioxide. The bubbles and the fizzing are the gas escaping.

This is an irreversible change. The bicarbonate of soda and the vinegar cannot be recovered from the new materials.

gas risesvinegar and bicarbonate of soda: new materials and a gas
A cup in which vinegar and bicarbonate of soda fizz, with bubbles of gas rising.

In a fair test, three equal amounts of bicarbonate of soda were added to vinegar, and the bubbles were counted in 30 seconds.

TrialBubbles counted in 30 seconds
Trial 114
Trial 216
Trial 315

Key ideas

  • An acid reacts with bicarbonate of soda to make new materials.
  • The reaction makes a gas, carbon dioxide, which makes the bubbles and the fizzing.
  • This change is irreversible, because new materials are made.
  • Evidence of a new material includes a gas, a new colour, or a new smell.
Worked example 7.4.1
What does the vinegar do when it is mixed with bicarbonate of soda? Write the word: acid, or gas.

Method. Think about what kind of material vinegar is.

  1. Think about vinegar. Vinegar is an acid, and it reacts with bicarbonate of soda.
    acid
  2. Choose the word. So the word is acid, which describes the vinegar.
    acid
  3. Write the answer.
    acid

Check. Vinegar is an acid. ✓

Worked example 7.4.2
In the fizz test, how many bubbles were counted in trial 2? Write the number.

Method. Read the table.

  1. Read the row for trial 2. The count for trial 2 is 16 bubbles.
    trial 2
  2. Write the number. So there were 16 bubbles in trial 2.
    16
  3. Write the answer.
    16

Check. Trial 2 was the largest count. ✓

Worked example 7.4.3
Is the fizzing reaction between vinegar and bicarbonate of soda reversible? Write yes or no.

Method. Think about the new materials.

  1. Think about the new materials. The reaction makes new materials, including a gas.
    new materials
  2. Decide. So the change cannot easily be undone, and the answer is no.
    no
  3. Write the answer.
    no

Check. New materials are made, so the change is irreversible. ✓

Watch out

Watch out: the bubbles are a gas. They are not the bicarbonate of soda coming back out of the cup, and the change cannot be undone.

Try it (with your teacher). With your teacher, put a spoonful of bicarbonate of soda into a clean cup. Add a small amount of vinegar, and watch the fizz. Count the bubbles for 30 seconds. Repeat the test two more times, using the same amounts each time, and find the mean. Do not taste anything, and wash your hands afterwards.

Exam tip

Grown-up tip. Ask: “What is the gas? Could the vinegar and bicarbonate of soda be recovered from the fizzing mixture?” Keep the amounts small and the test fair.

Exercise 7D  Acid and bicarbonate of soda

1
FIs vinegar an acid? Write yes or no.
2
FWhat gas is made when an acid reacts with bicarbonate of soda? Write the words.
3
FWhat do the bubbles and the fizzing show? Write one word.
4
FLook at the fizz test table in section 7.4. How many bubbles were counted in trial 1? Write the number.
TrialBubbles counted in 30 seconds
Trial 114
Trial 216
Trial 315
5
FLook at the fizz test table in section 7.4. Work out the mean number of bubbles. Write the number.
TrialBubbles counted in 30 seconds
Trial 114
Trial 216
Trial 315
6
PSLook at the fizz test table in section 7.4. Which trial had the most bubbles? Write the trial name.
TrialBubbles counted in 30 seconds
Trial 114
Trial 216
Trial 315
7
PSLook at the fizz test table in section 7.4. How many more bubbles were there in trial 2 than in trial 1? Write the number.
TrialBubbles counted in 30 seconds
Trial 114
Trial 216
Trial 315
8
PSIs the change between vinegar and bicarbonate of soda reversible? Write yes or no.
9
PSWorking scientifically. Compare the evidence. Name one piece of evidence that a new material has formed in the fizz test. Write one word.
10
RExplain why the fizzing mixture cannot easily be turned back into vinegar and bicarbonate of soda. Write one sentence.
11
RExplain why the bubbles in the fizz test are evidence that a new material has been made. Write one sentence.
7.5

Deciding whether a change is reversible

Year 5
M5 and M6: decide whether a change is reversible or irreversible, using evidence such as a new colour, a gas, a new smell, or whether it can be undone
Video lesson: Deciding whether a change is reversibleComing soon
Story. Tom is sorting the changes in a science quiz. “Is melting ice the same kind of change as burning?” he asks. Mrs Ali says: “Look for the evidence. Has a new material been made? Can the change be undone?”

To decide whether a change is reversible, ask two questions:

If a new material has been made, the change is usually irreversible. If it can be undone, the change is reversible.

EvidenceWhat you would see
a new colourthe mixture changes to a different colour
a gasbubbles form, or the mixture fizzes
a new smella new smell is produced
it cannot be undonethe original materials cannot be recovered
ChangeCan it be reversed to the start?
ice melts into wateryes
water freezes into iceyes
sugar dissolves in wateryes
salt water is left to evaporate, and the salt is recoveredyes
a candle burnsno
an egg is cookedno
vinegar and bicarbonate of soda fizz togetherno
a cake is bakedno

Key ideas

  • Look for evidence of a new material: a new colour, a gas, a new smell, or a change that cannot be undone.
  • If a new material is made, the change is usually irreversible.
  • If the material can be returned to how it was, the change is reversible.
  • Melting, freezing, dissolving and mixing are reversible changes.
Worked example 7.5.1
Look at the change table in section 7.5. Is a candle burning reversible? Write yes or no.

Method. Find the row for the candle.

  1. Find the row. The row for the candle burning says no.
    no
  2. Decide. So a candle burning is not reversible.
    no
  3. Write the answer.
    no

Check. Burning makes new materials. ✓

Worked example 7.5.2
Look at the evidence table in section 7.5. Which evidence shows that a gas has been made? Write the words.

Method. Find the row about a gas.

  1. Find the row. The row for a gas says bubbles form, or the mixture fizzes.
    bubbles
  2. Name the evidence. So the evidence is a gas, which shows bubbles or fizzing.
    a gas
  3. Write the answer.
    a gas

Check. Fizzing shows that a gas is escaping. ✓

Worked example 7.5.3
Is ice melting a change that makes a new material? Write yes or no.

Method. Think about whether a new material is made.

  1. Think about the ice. Ice melts into water, which is the same material.
    same material
  2. Decide. So no new material is made, and the answer is no.
    no
  3. Write the answer.
    no

Check. Melting is a reversible change of state. ✓

Watch out

Watch out: a change that cannot be undone is not always a new material. Look at the evidence before you decide. Ask whether a new material has been made.

Try it (with your teacher). With your teacher, look at four changes at home or school, such as an ice cube melting, a biscuit baking, a lump of sugar dissolving, and a piece of paper torn up. For each, write whether it is reversible, and one piece of evidence that supports your answer.

Exam tip

Grown-up tip. Ask: “Can you undo it? Has something new been made?” Let your child sort changes into two groups and explain each choice.

Exercise 7E  Deciding whether a change is reversible

1
FLook at the change table in section 7.5. Is salt recovered by evaporating salt water reversible? Write yes or no.
2
FLook at the change table in section 7.5. Is an egg being cooked reversible? Write yes or no.
3
FLook at the change table in section 7.5. How many of the eight changes are irreversible? Write the number.
4
FLook at the evidence table in section 7.5. Which evidence means that the change cannot be undone? Write the words.
5
FWhich evidence shows a new colour has formed? Write the word, using the evidence table: colour or smell.
6
FFill in the gap: if a new material is made, the change is usually ___. Write one word.
7
PSIs dissolving sugar in water reversible? Write yes or no.
8
PSAli says: “Vinegar and bicarbonate of soda fizz, so the change is reversible.” Is Ali right? Write yes or no.
9
PSMo says: “A cake that has been baked can be turned back into the mixture.” Is Mo right? Write yes or no.
10
PSWorking scientifically. Predict, then test. Predict whether a lump of sugar dissolved in water can be recovered by evaporation. Write yes or no.
11
RExplain how you decide whether a change is reversible. Write one sentence.
12
RExplain why melting ice is reversible, but burning wood is not. Write one sentence.

Chapter summary

SkillMethod
Reversible changesMelting and freezing, dissolving (with evaporation) and mixing can be undone, and the material returns.
Irreversible changesChanges that make new materials, such as burning and cooking, are usually not reversible.
Acid and bicarbonate of sodaAn acid, such as vinegar, reacts with bicarbonate of soda to make new materials and carbon dioxide gas.
EvidenceA new colour, a gas, a new smell, or a change that cannot be undone shows a new material has been made.
DecidingAsk whether a new material has been made, and whether the change can be undone.

Can you…?

  • I can say which changes are reversible, such as melting, freezing and dissolving
  • I can explain why a change that makes a new material is usually not reversible
  • I can describe what happens when an acid reacts with bicarbonate of soda
  • I can use evidence to decide whether a change has made a new material
  • I can sort changes into reversible and irreversible, and give a reason

Key terms

reversible change
a change that can be undone, so that the material returns to how it was
irreversible change
a change that cannot easily be undone, usually because a new material is made
change of state
a change between solid, liquid and gas, such as melting or freezing
new material
a material with different properties, made by a change that cannot easily be undone
acid
a substance such as vinegar that reacts with bicarbonate of soda
bicarbonate of soda
a white powder that reacts with an acid to make carbon dioxide gas
carbon dioxide
a gas that makes bubbles and fizzing when an acid reacts with bicarbonate of soda
evidence
something you can see or measure that shows whether a change has happened

Exercise 7Z  Check-up

1
Is melting ice a reversible change? Write yes or no.
2
Is burning a candle a reversible change? Write yes or no.
3
What gas is made when vinegar reacts with bicarbonate of soda? Write the words.
4
Fill in the gap: when a solid melts, it becomes a ___. Write one word.
5
Is dissolving sugar in water reversible? Write yes or no.
6
Look at the fizz table in section 7.4. Work out the mean number of bubbles. Write the number.
TrialBubbles counted in 30 seconds
Trial 114
Trial 216
Trial 315
7
Look at the picture of the raw egg in section 7.3. Is the change from cooked back to raw shown? Write yes or no.
raw eggcooked eggcookingan irreversible change: it cannot be undoneno arrow goes back
8
Which new material does a burning candle leave on glass? Write the word.
9
True or false: mixing sand and pebbles is an irreversible change. Write true or false.
10
Explain why burning wood is an irreversible change. Write one sentence.

Total 19 marks. Allow about 19 minutes.

Exercise 7X  Practice test questions

1
FWrite the word for a change in which a solid turns into a liquid.[1 mark]
2
FIs dissolving salt in water a reversible change? Write yes or no.[1 mark]
3
FIs cooking an egg a reversible change? Write yes or no.[1 mark]
4
FWhat is the gas that bubbles out when vinegar reacts with bicarbonate of soda? Write the words.[1 mark]
5
FLook at the fizz table in section 7.4. Work out the mean number of bubbles in 30 seconds. Write the number.
TrialBubbles counted in 30 seconds
Trial 114
Trial 216
Trial 315
[2 marks]
6
FLook at the melting table in section 7.2. Work out the mean time for the ice to melt. Write the number of minutes.
TrialMinutes for the ice to melt
Trial 112
Trial 214
Trial 313
[2 marks]
7
FName the change that makes new materials and cannot easily be undone, from burning, dissolving, melting and freezing. Write the word.[2 marks]
8
RExplain why a candle burning is not a reversible change. Give one piece of evidence in your answer. Write one sentence.[3 marks]
9
RExplain why vinegar and bicarbonate of soda cannot be recovered from the fizzing mixture, and name the evidence. Write two sentences.[3 marks]
10
RAli says: “If I melt the ice, then freeze it, the water is a different material.” Is Ali right? Write no, and explain in one sentence.[3 marks]
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