Year 5 Science: Deciding whether a change is reversible
Part of the Year 5 Science reversible and irreversible changes chapter. Chapter overview · whole chapter on one page
Deciding whether a change is reversible
To decide whether a change is reversible, ask two questions:
- Has a new material been made? Look for evidence, such as a new colour, a gas, a new smell, or a material that does not go back.
- Can the change be undone, so that the material returns to how it was at the start?
If a new material has been made, the change is usually irreversible. If it can be undone, the change is reversible.
| Evidence | What you would see |
|---|---|
| a new colour | the mixture changes to a different colour |
| a gas | bubbles form, or the mixture fizzes |
| a new smell | a new smell is produced |
| it cannot be undone | the original materials cannot be recovered |
| 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
- 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.
Method. Find the row for the candle.
- Find the row. The row for the candle burning says no.no
- Decide. So a candle burning is not reversible.no
- Write the answer. no
Check. Burning makes new materials. ✓
Method. Find the row about a gas.
- Find the row. The row for a gas says bubbles form, or the mixture fizzes.bubbles
- Name the evidence. So the evidence is a gas, which shows bubbles or fizzing.a gas
- Write the answer. a gas
Check. Fizzing shows that a gas is escaping. ✓
Method. Think about whether a new material is made.
- Think about the ice. Ice melts into water, which is the same material.same material
- Decide. So no new material is made, and the answer is no.no
- 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
Method. Find the salt row.
- Find the row. The salt water row says yes.yes
- Decide. So the change is reversible.yes
- Write the answer. yes
Check. Evaporation gives back the salt. ✓
Method. Find the egg row.
- Find the row. The egg row says no.no
- Decide. So cooking an egg is not reversible.no
- Write the answer. no
Check. A cooked egg cannot become raw again. ✓
Method. Count the no rows.
- Count the no rows. The no rows are burning, cooking, fizzing and baking.four no rows
- Write the number. So four changes are irreversible.4
- Write the answer. 4
Check. The other four are reversible. ✓
Method. Read the evidence table.
- Read the table. The row that says it cannot be undone describes the original materials not being recovered.cannot be undone
- Name the evidence. So the evidence is that it cannot be undone.it cannot be undone
- Write the answer. it cannot be undone
Check. Irreversible changes cannot be undone. ✓
Method. Read the first row of the evidence table.
- Read the row. The first row says a new colour.new colour
- Choose the word. So the word is colour.colour
- Write the answer. colour
Check. A new colour is evidence of a new material. ✓
Method. Read the key ideas in section 7.5.
- Read the key idea. If a new material is made, the change is usually irreversible.usually irreversible
- Fill the gap. So the missing word is irreversible.irreversible
- Write the answer. irreversible
Check. New materials cannot usually be changed back. ✓
Method. Read the change table.
- Read the row. The sugar dissolving row says yes.yes
- Decide. So dissolving is reversible.yes
- Write the answer. yes
Check. The sugar can be recovered by evaporation. ✓
Method. Look at the change table.
- Read the row. The row for vinegar and bicarbonate of soda says no.no
- Decide. Fizzing makes a gas and new materials, so Ali is not right.not right
- Write the answer. no
Check. The fizzing is evidence of a new material. ✓
Method. Look at the change table.
- Read the row. The cake row says no, so baking is not reversible.no
- Decide. So Mo is not right.no
- Write the answer. no
Check. Baking makes new materials. ✓
Method. Think about evaporation.
- Think about the sugar. Evaporating the water leaves the sugar behind.left behind
- Make the prediction. So the sugar can be recovered, and the answer is yes.yes
- Write the answer. yes
Check. Test it with your teacher, and compare the result. ✓
Method. Think about the two questions.
- Think about the material. Has a new material been made?new material
- Think about the undoing. Can the change be undone?undone
- Write the answer. Accept any sentence that says you look for a new material, such as a gas or a new colour, and ask whether the change can be undone.
Check. Two questions decide it. ✓
Method. Think about the materials after each change.
- Think about the ice. Melted ice is still water, the same material.same material
- Think about the wood. Burning wood makes new materials, which cannot easily be changed back.new materials
- Write the answer. Accept any sentence that says melting ice makes the same material, which can freeze back, but burning wood makes new materials that cannot easily be changed back.
Check. The evidence of a new material decides it. ✓