Questions: Reversible and Irreversible Changes

Question 1

You melt a stick of butter in a pan, then put the melted butter in the refrigerator. What kind of change is this?

• A. Irreversible — the butter has been permanently changed by heat
• B. Reversible — the butter melted into a liquid and will harden back into a solid when cooled
• C. Chemical — the butter turned into a new substance
• D. No change happened — butter is always soft

Answer: (B)

Melting butter is a reversible change. The butter changes from solid to liquid when heated, and changes back from liquid to solid when cooled. No new substance is created — it is still butter. The only thing that changed was its state.

Question 2

Dissolving sugar in water is irreversible because the sugar disappears.

• T. True
• F. False

Answer: False

False. Dissolving sugar in water is reversible. The sugar seems to disappear but is still there — you can taste it. If you evaporate the water, the sugar returns as solid crystals. The sugar never stopped being sugar.

Question 3

Give one example of a reversible change and one example of an irreversible change.

Model answer: Reversible: melting ice into water (you can freeze it back). Irreversible: burning a piece of paper (you cannot turn ash back into paper).

Reversible changes can be undone — the original material can be recovered. Irreversible changes create new substances that cannot be turned back into the starting materials through simple means. Many other examples work: freezing/melting, dissolving/evaporating (reversible), cooking, rusting, burning (irreversible).