Experiment 8 — Physical or Chemical Change?

Topic: Chemistry · Classes 6–7 · Physical and chemical changes

Aim: Sort all six changes correctly. Ask yourself each time: has a new substance been formed, and can I get the original back?

Daily: Cooking is chemistry you can eat: a chapati browning is a chemical change, but melting butter is physical. Recognising the difference tells you what you can undo and what you cannot.

A: no new substance — the pieces are still paper

B: ash, smoke and gases are new substances, and you cannot get the paper back

C: wax changes state only; cool it and solid wax returns

D: iron combines with oxygen and water to form rust, a new substance

E: the sugar is still sugar — evaporate the water and it returns

F: new substances form and you cannot turn curd back into milk

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Question: Which of these can you reverse most easily?

Exp: Melting is a physical change of state: cool the wax and it solidifies again.

Question: A chemical change always…

Exp: New substances with new properties are the definition.

Question: Which is a physical change?

Exp: The salt can be recovered by evaporation.

Question: Cooking a chapati is a chemical change because…

Exp: Browning and flavour come from new substances.

Question: A gas being given off usually suggests…

Exp: Bubbles of a new gas are a classic clue.