IGCSE Chemistry: Physical and Chemical Changes — Practice Questions

These are original questions written for Marlbridge, in the style and at the standard of the examination. They are not reproduced past-paper questions — examination boards hold copyright in their own papers. Use these alongside the official past papers available free from your board.

Related: Physical and Chemical Changes revision notes

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Questions

1. State the key difference between a physical change and a chemical change. [1]

2. List three pieces of evidence that would suggest a chemical change has occurred. [3]

3. Explain why melting is a physical change but burning is a chemical change. [2]

4. A student says: “If a change is hard to reverse, it must be a chemical change.” Explain why this statement, on its own, is not a reliable test. [2]

5. For each of the following, state whether it is a physical or chemical change, and give one reason: (a) dissolving sugar in water; (b) an iron nail rusting; (c) water boiling. [6]

6. Aqueous silver nitrate is added to aqueous sodium chloride, and a white solid appears immediately.

(a) State whether this is a physical or a chemical change. [1] (b) Give two pieces of evidence from the description that support your answer. [2]

7. Dilute hydrochloric acid is added to calcium carbonate, and vigorous fizzing is observed.

(a) State whether this is a physical or a chemical change. [1] (b) Give two pieces of evidence from the description that support your answer. [2]

8. Solid iodine is gently heated and turns directly into a purple gas without melting (sublimation), and the gas condenses back into solid iodine on a cool surface.

(a) State whether sublimation is a physical or a chemical change. [1] (b) Explain why the appearance of a gas here is not, on its own, reliable evidence of a chemical change. [2]

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Answers

1. A chemical change produces at least one new substance; a physical change does not — the same substance is present throughout, just in a different form [1].

2. Any three of: a new substance forms (e.g. a gas or precipitate appears); a permanent colour change; a temperature change caused by the reaction itself; a precipitate forms; effervescence [3]. (Reversibility is not reliable evidence on its own — some chemical changes are readily reversible and some physical changes are not, so it should not be credited here.)

3. Melting does not create a new substance — it is still the same substance (e.g. ice to water is still H₂O), just in a different state [1]. Burning produces new substances with different properties (e.g. magnesium oxide from magnesium), so it is chemical [1].

4. Reversibility is supporting evidence, not the definition [1]. Some chemical changes (e.g. reversible reactions) are easily reversed even though new substances are genuinely formed in both directions, so “hard to reverse” alone can wrongly classify a genuine chemical change as physical, or vice versa [1].

5. (a) Physical [1] — the sugar is still chemically the same substance, just dispersed among the water particles, and can be recovered unchanged by evaporation [1]. (b) Chemical [1] — a new substance (hydrated iron(III) oxide) is formed, with different properties from the original iron [1]. (c) Physical [1] — water remains H₂O throughout; only its state changes [1].

6. (a) Chemical [1]. (b) A new substance (the white solid/precipitate) has formed [1], and it appears immediately/is not easily reversed by simple physical means [1].

7. (a) Chemical [1]. (b) Effervescence (fizzing/gas produced) [1], and this indicates new substances have formed (a salt, water and carbon dioxide) that were not present in the original acid and carbonate [1].

8. (a) Physical [1]. (b) The iodine is still iodine throughout — gas, solid, or in between — with no new substance formed [1]; the process is also easily reversed (the gas condenses straight back to the original solid), which shows a gas appearing is only useful evidence of a chemical change when it is accompanied by an actual change in chemical identity, not simply a change of state [1].

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Where marks are usually lost

• Treating “a gas is produced” as the definition of a chemical change, rather than one piece of supporting evidence.
• Using “hard to reverse” as the test itself, ignoring reversible reactions that are still chemical changes.
• Saying dissolving is a chemical change because the solid “disappears”.
• Forgetting that a temperature change only counts as evidence if it comes from the reaction itself, not external heating or cooling.
• Classifying sublimation as a chemical change simply because a gas is produced — the substance’s chemical identity never changes, so it remains physical, however dramatic the state change looks.
• Forgetting that the underlying test is always “has a new substance been formed?” — every other piece of evidence (colour, effervescence, temperature, difficulty of reversal) is only useful because it usually, but not always, points to that answer.

The one question that decides every case

Every example in this topic, however varied the surface description, reduces to the same underlying question: has a chemically new substance been formed? Effervescence, a precipitate, a permanent colour change or a self-generated temperature change are all worth citing as evidence in an answer, but none of them is the definition itself — they are useful precisely because a new substance is usually, though not always, what produces them. Iodine subliming (question 8) shows why this distinction matters: a dramatic-looking state change (solid straight to gas) can still be entirely physical, because the substance itself never stops being iodine. For the full evidence table and further worked examples, see the Physical and Chemical Changes study guide.