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Detailed notes on Atoms, Elements and Compounds for Cambridge IGCSE Coordinated Science, covering key concepts, explanations, examples, and exam-focused revision points.
Physical changes are reversible and do not produce new substances. Chemical changes produce new substances and are usually irreversible. Cambridge tests distinguishing them with evidence.
Mapped to the Cambridge IGCSE 0654 syllabus (2025-2027).
Physical changes are reversible with no new substances. Chemical changes produce new substances and are usually irreversible.
| Feature | Physical Change | Chemical Change |
|---|---|---|
| New substances? | No | Yes |
| Reversibility | Usually reversible | Usually irreversible |
| Energy change | Small | Often large |
| Examples | Melting, dissolving, cutting, boiling | Burning, rusting, electrolysis, neutralisation |
Evidence a chemical reaction has occurred:
Law of conservation of mass: In any chemical reaction, the total mass of reactants = total mass of products. Atoms are rearranged but not created or destroyed.
Why mass sometimes appears to change in an open container:
Verbatim phrases and definitions Cambridge mark schemes credit.
Paper 4: 'State TWO observations that suggest a chemical reaction has occurred' (2 marks). 'Explain why the mass of a reaction mixture in an open container decreases when zinc reacts with acid' (2 marks — hydrogen gas escapes). MCQ tests classification of changes as physical or chemical.
Sources: Cambridge IGCSE Coordinated Sciences 0654 syllabus 2025-2027 (C3); 0654 Examiner Reports 2022-2024. Last reviewed 2026-05-14.
Step-by-step solutions to past-paper-style questions on physical and chemical changes, written exactly the way a tutor would explain them at the board.
Question
State whether dissolving salt in water is a physical or chemical change, and explain your reasoning.
Step-by-step solution
Step 1
Dissolving salt (NaCl) in water is a physical change.
Step 2
No new substance is formed; NaCl dissociates into Na⁺ and Cl⁻ ions but no chemical reaction occurs.
Step 3
The change is easily reversed by evaporating the water, recovering the solid salt.
Answer
Physical change — no new substance is formed, and the salt can be recovered by evaporation.
Question
Iron rusting is a chemical change. Give TWO pieces of evidence that support this statement.
Step-by-step solution
Step 1
A new substance is formed: iron oxide (rust, Fe₂O₃) has different properties from iron — it is brown/orange, brittle, and non-metallic.
Step 2
The mass of iron increases as oxygen from the air is incorporated into the rust, confirming a new substance with a different composition has formed.
Step 3
The reaction is not easily reversible; you cannot convert rust back to iron by a simple physical process.
Answer
Two pieces of evidence: (1) a new substance (iron oxide) with different properties is formed; (2) the mass increases as oxygen is incorporated — the change is irreversible.
Examiner tip
Cambridge mark schemes accept any two of: new substance formed, colour change (shiny → orange/brown), irreversible, energy change, mass change.
Question
Classify each of the following as a physical or chemical change: (a) boiling water; (b) burning magnesium; (c) dissolving sugar; (d) the electrolysis of copper sulfate solution.
Step-by-step solution
Step 1
(a) Boiling water — physical change; water changes state (liquid → gas) but remains H₂O; reversible by condensation.
Step 2
(b) Burning magnesium — chemical change; new substance MgO formed; bright white light; irreversible.
Step 3
(c) Dissolving sugar — physical change; sugar molecules disperse in water but no new substance formed; reversible by evaporation.
Step 4
(d) Electrolysis of copper sulfate — chemical change; copper deposited at cathode, oxygen at anode; new substances formed from ionic decomposition.
Answer
(a) Physical; (b) Chemical; (c) Physical; (d) Chemical.
Definitions to memorise and the exact keywords mark schemes credit for physical and chemical changes answers — sharpened from recent examiner reports for the 2026 0654 sitting.
A change in which no new substance is formed; the chemical composition of the material remains the same. Usually reversible.
Example
Melting ice, dissolving salt, boiling water.
A change in which new substances are formed with different chemical compositions. Usually accompanied by an energy change and is often irreversible.
Example
Burning fuel, rusting iron, acid–base neutralisation.
A change that can be undone by a straightforward physical method, recovering the original substance.
An insoluble solid that forms when two solutions are mixed; its formation is evidence of a chemical change.
The traps other students keep falling into on physical and chemical changes questions — taken from recent Cambridge IGCSE 0654 examiner reports and mark schemes — and how to avoid them.
Why it happens
The substance 'disappears' when dissolved, so students think it has been chemically altered.
How to avoid it
Dissolving is physical — the dissolved substance can be recovered unchanged (e.g. evaporate water to get salt back).
Why it happens
Colour change is evidence for a chemical change, but some physical changes also show colour changes (e.g. hot metals glow).
How to avoid it
Look for multiple lines of evidence: new smell, gas produced, precipitate, irreversibility, energy change alongside the colour change.
Why it happens
Students over-generalise the reversibility criterion.
How to avoid it
Some chemical changes can be reversed by further chemical reactions (e.g. electrolysis reverses certain reactions). However, they cannot be reversed by simple physical methods.
The things students keep getting wrong in this sub-topic, answered.