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Reversible and Irreversible Changes — Answers

Answer sheet with model answers and success criteria for the Year 4 states of matter & materials worksheet.

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Success TutoringWorksheetsAnswer sheetYear 4 · Science · States of Matter & Materials

Reversible and Irreversible Changes — Answers

Part A: Can it be undone?

Read the passage. Then circle the best answer.

Materials can change when we heat them, cool them, mix them or change their shape.

A reversible change can be undone. Melting, freezing, dissolving and changing shape are usually reversible. Ice can melt into water, and the water can be frozen into ice again.

An irreversible change cannot be undone. A new material is made. Cooking, baking and burning are irreversible. A baked cake can never turn back into flour, eggs and milk.

Signs of an irreversible change include a new colour, a new smell, bubbles of gas, smoke or ash.

1.Which change is reversible?
  1. Aburning paper
  2. Bmelting butter
  3. Cbaking bread
  4. Dfrying an egg

Answer: melting butter

Melted butter can be cooled to become solid again.

2.Which change is irreversible?
  1. Afreezing juice
  2. Bmelting ice
  3. Ctoasting bread
  4. Ddissolving salt in water

Answer: toasting bread

Toast can never be turned back into soft bread. A new material has formed.

3.How could you get salt back after dissolving it in water?
  1. AFreeze it.
  2. BLet the water evaporate.
  3. CStir it more.
  4. DAdd more water.

Answer: Let the water evaporate.

When the water evaporates, the salt is left behind. Dissolving is reversible.

4.Which is a sign that a new material has been made?
  1. AThe material changes shape.
  2. BThe material melts.
  3. CSmoke, ash or a new smell is made.
  4. DThe material gets colder.

Answer: Smoke, ash or a new smell is made.

Burning makes smoke and ash, which are new materials. This shows an irreversible change.

5.Chocolate is melted, poured into a mould and cooled. What kind of change is this?
  1. Airreversible, because it is a new material
  2. Breversible, because it is still chocolate
  3. Cirreversible, because it was heated
  4. Dnot a change at all

Answer: reversible, because it is still chocolate

Melting and freezing (setting) only change the state. It is still chocolate and can be melted again.

Part B: Sort and explain

6.Sort each change: reversible or irreversible?
  • freezing water into ice
  • burning a log
  • squashing play dough
  • boiling an egg
  • irreversible
  • reversible
  • reversible
  • irreversible

Answer: freezing water into ice → reversible; burning a log → irreversible; squashing play dough → reversible; boiling an egg → irreversible

7.A change that can be undone is called a (answer space) change.

Answer: reversible

8.When wood burns, new materials such as smoke and (answer space) are made.

Answer: ash

9.Why is baking a cake an irreversible change?

Answer: Heating the mixture makes a new material (cake). You cannot get the flour, eggs and milk back.

Part C: True or false?

Write True or False.

10.Melting is usually a reversible change.

Answer: True

Cooling the liquid turns it back into a solid.

11.A burnt match can be turned back into a new match.

Answer: False

Burning is irreversible; new materials have formed.

12.Every change caused by heating is irreversible.

Answer: False

Heating ice melts it, which can be reversed. Some heating changes are reversible.

13.Dissolving sugar in water can be reversed by evaporating the water.

Answer: True

The sugar is left behind when the water evaporates.

Part D: Thinking scientifically: explain your claim

14.Lily says: "Melting a candle and burning a candle are the same kind of change." Do you agree? Explain.

Model answer: I disagree. Melting candle wax is reversible: the liquid wax cools and becomes solid wax again. Burning the wick and wax is irreversible: it makes new materials such as smoke and gases, so the wax is used up and cannot come back.

Success criteria:

  • States disagree
  • Explains melting is reversible with a reason
  • Explains burning is irreversible because new materials form
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