Year 5 Science: Reversible changes
Part of the Year 5 Science reversible and irreversible changes chapter. Chapter overview · whole chapter on one page
Reversible changes
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:
- changes of state, such as melting ice into water and freezing water back into ice;
- dissolving, such as sugar dissolving in water, which can be undone by evaporating the water;
- mixing, such as sand and pebbles, which can be separated again.
| Change | Can it be reversed to the start? |
|---|---|
| ice melts into water | yes |
| water freezes into ice | yes |
| sugar dissolves in water | yes |
| salt water is left to evaporate, and the salt is recovered | yes |
| a candle burns | no |
| an egg is cooked | no |
| vinegar and bicarbonate of soda fizz together | no |
| a cake is baked | no |
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.
Method. Read the row for ice.
- Read the row. The row for ice melting into water says yes.yes
- Decide. So ice melting is a reversible change.yes
- Write the answer. yes
Check. Water can freeze back into ice. ✓
Method. Think about a change of state.
- Think about the reversible list. Water freezing into ice is a reversible change.freezing
- Name it. The change is freezing.freezing
- Write the answer. freezing
Check. Melting is the reverse of freezing. ✓
Method. Think about how sugar can be recovered.
- Think about the sugar. Evaporating the water leaves the sugar behind.left behind
- Decide. So dissolving is a reversible change, and the answer is yes.yes
- 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
Method. Read the reversible list in section 7.1.
- Read the list. Melting and freezing are reversible changes of state.reversible changes of state
- Decide. So melting ice is a reversible change.yes
- Write the answer. yes
Check. Water freezes back into ice. ✓
Method. Read the picture in section 7.1.
- Read the picture. The arrow from water back to ice is labelled freezes.freezes
- Name the change. The change is freezing.freezing
- Write the answer. freezing
Check. Freezing is the reverse of melting. ✓
Method. Read the key ideas in section 7.1.
- Read the key idea. Dissolving is reversible, because the solid can be recovered by evaporating the water.recovered
- Decide. So the answer is yes.yes
- Write the answer. yes
Check. Evaporation gives back the sugar. ✓
Method. Read the key ideas in section 7.1.
- Read the key idea. Mixing is reversible, because the parts of a mixture can be separated again.separated again
- Decide. So the answer is yes.yes
- Write the answer. yes
Check. Sieving can separate the sand and pebbles again. ✓
Method. Count the yes rows.
- Read the column. The yes rows are ice melting, water freezing, sugar dissolving and salt recovered by evaporation.four yes
- Count them. So four changes are reversible.4
- Write the answer. 4
Check. The other four changes are not reversible. ✓
Method. Read the change column.
- Read the column. The row about salt water is salt water left to evaporate, and the salt is recovered.salt water
- Name the change. That change is reversible, so it is listed as yes.yes
- Write the answer. salt water is left to evaporate, and the salt is recovered
Check. Salt is recovered by evaporating the water. ✓
Method. Read the picture.
- Read the arrows. The arrow from ice to water is labelled melts.melts
- Name the change. So the change from ice to water is melting.melting
- Write the answer. melting
Check. Melting is a change from solid to liquid. ✓
Method. Read the reversible list in section 7.1.
- Read the list. Mixing is listed as a reversible change.reversible
- Decide. So the statement is false.false
- Write the answer. false
Check. Mixtures can usually be separated again. ✓
Method. Think about the reversible change of freezing.
- Think about freezing. Water freezes into ice when it is cold enough.freezes
- Make the prediction. So the puddle can be frozen back into ice.yes
- Write the answer. yes
Check. Test it with your teacher, and check the result. ✓
Method. Think about what can be undone.
- Think about the change. Ice melts into water, and water freezes back into ice.back into ice
- Write the reason. So the material can return to its original state.original state
- Write the answer. Accept any sentence that says the water can freeze back into ice, so the material can return to how it was.
Check. A reversible change can be undone. ✓
Method. Think about the method.
- Think about the water. Evaporating the water removes the liquid.removes the liquid
- Write the method. The sugar is left behind as crystals.left behind
- Write the answer. Accept any sentence that says you evaporate the water, and the sugar is left behind as crystals.
Check. Evaporation recovers the dissolved sugar. ✓