Physical and Chemical Changes: Revision Notes

Condensed for revision. For the full explanation, use the Physical and Chemical Changes study guide, then test yourself with the practice questions.

Tier note: 6.1 has a single learning outcome, and it sits in the Core column of the 0620 syllabus. There is no Supplement content in 6.1, so nothing on this page is marked (0620 Extended, 5070 required). 5070 has no tiers, and its wording for 6.1 is identical to 0620.

Syllabus: Cambridge IGCSE Chemistry 0620 and Cambridge O Level Chemistry 5070, 2026–2028, subtopic 6.1 Physical and chemical changes: identify physical and chemical changes, and describe the differences between them.

The two definitions (6.1)

	Physical change	Chemical change
New substance formed?	No: same substance, same formula	Yes: at least one new substance
What changes	state, shape, or how the substance is spread out (e.g. dissolved)	the atoms are rearranged into new substances with new chemical properties
Equation	the same formula on both sides; only the state symbol changes (e.g. H₂O(s) → H₂O(l))	a chemical equation with different formulae on each side
Energy change	energy may be taken in or given out (e.g. melting, boiling), so an energy change on its own does not prove a chemical change	often a noticeable energy change (heat or light given out or taken in)
Reversing it	usually easy, by physical means (cooling, heating, evaporating)	usually difficult; some reactions are reversible, but they are still chemical
Mass	total mass unchanged	total mass unchanged (atoms are conserved), though a gas may escape from an open container

The deciding question is always: has a new substance been made?

Evidence of a chemical change

Any of these observations suggests that a new substance has formed:

• Effervescence (fizzing): a gas is given off.
• A precipitate (an insoluble solid) forms when two solutions are mixed.
• A permanent colour change of the substance itself.
• A temperature change produced by the reaction itself (not by an outside heater).
• Light or a flame is given out (combustion).

Examples of physical changes: melting, freezing, boiling, evaporating, condensing, sublimation, dissolving a solid in water, mixing sand with salt, and separating a mixture by filtration or distillation.

Worked classification

Change	Type	Evidence
Ice melting	physical	still H₂O; refreezes on cooling
Iodine subliming	physical	still I₂; the vapour turns back to solid on cooling
Salt dissolving in water	physical	salt is recovered unchanged by evaporating the water
Magnesium burning in air	chemical	bright white light; a white powder, a new substance, forms
Copper(II) carbonate heated	chemical	green solid turns black; carbon dioxide is given off
Calcium carbonate added to hydrochloric acid	chemical	effervescence; new substances form
Iron rusting	chemical	a new orange-brown substance forms on the surface
Water added to anhydrous copper(II) sulfate	chemical	white solid turns blue; the reaction can be reversed by heating, but it is still chemical

Equations for the chemical changes above:

2Mg + O₂ → 2MgO
CuCO₃ → CuO + CO₂
CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂
CuSO₄ + 5H₂O ⇌ CuSO₄·5H₂O

Writing the answer

When a question asks you to identify a change and explain:

1. Name the type: physical or chemical.
2. Say whether a new substance is formed.
3. Quote the evidence from the description (gas, precipitate, colour change, temperature change), or, for a physical change, say how the original substance can be recovered.

Exam traps

• Dissolving is physical. The solid seems to disappear, but it is the same substance and is recovered by evaporating the solvent. Some salts cause a temperature change when they dissolve; at this level dissolving is still classed as physical.
• A gas is not proof on its own. Boiling water and subliming iodine also produce a gas, but no new substance forms.
• “Hard to reverse” is not the definition. Hydrating anhydrous copper(II) sulfate is easily reversed by heating, but it is a chemical change because a new substance forms each way.
• Outside heating is not evidence. A beaker getting hot over a Bunsen burner tells you nothing; a temperature change counts only when the reaction itself produces it.
• Diluting a coloured solution makes it paler, but the solute is unchanged: that is not a chemical colour change.
• Always give evidence, not just the label “chemical” or “physical”.

Self-test

1. State the key difference between a physical change and a chemical change.
2. Give three observations that show a chemical change has taken place.
3. Classify the melting of wax, and explain your answer.
4. Sugar dissolves in water and the sugar is recovered by evaporation. Which type of change is the dissolving? Give a reason.
5. A green powder is heated and turns black, and a gas that turns limewater milky is given off. Which type of change is this? Give two pieces of evidence.
6. Two colourless solutions are mixed and a white solid appears. What is the solid called, and what does it show?
7. Explain why the reaction of water with anhydrous copper(II) sulfate is a chemical change even though it can be reversed.
8. Why is the formation of steam from boiling water not evidence of a chemical change, even though a gas forms?
9. Write the balanced equation for magnesium burning in oxygen.

Answers:

1. A chemical change forms at least one new substance; a physical change forms no new substance.
2. Any three of: effervescence (gas given off); a precipitate forms; a permanent colour change; a temperature change caused by the reaction; light or a flame given out.
3. Physical: the wax is the same substance in a different state, and it solidifies again on cooling.
4. Physical: no new substance is formed, and the sugar is recovered unchanged.
5. Chemical: a permanent colour change (green to black) and a new gas (carbon dioxide) is given off.
6. A precipitate; it shows that a new, insoluble substance has formed, so a chemical change has taken place.
7. A new substance forms in each direction (blue hydrated copper(II) sulfate from white anhydrous copper(II) sulfate, and back again on heating). Forming a new substance is what makes a change chemical, whether or not it can be reversed.
8. Steam is still water (H₂O); only the state has changed, so no new substance has formed.
9. 2Mg + O₂ → 2MgO

These are original notes written for revision. Check the full syllabus wording in the official 0620 syllabus and official 5070 syllabus.