What is the difference between a physical change and a chemical change in the context of Cambridge IGCSE Coordinated Science?

In Cambridge IGCSE Coordinated Science, a physical change involves a change in the state or appearance of a substance without altering its chemical composition, such as melting ice into water. A chemical change, however, results in the formation of new substances with different chemical properties, such as rust forming on iron.

How can you identify a chemical change in a laboratory setting?

A chemical change can be identified by observing indicators such as a change in color, the formation of a precipitate, the evolution of gas, or a change in temperature. These changes suggest that a new substance has been formed, which is a key concept in the study of Atoms, Elements, and Compounds in the Cambridge IGCSE curriculum.

Why is the conservation of mass important in chemical changes?

The conservation of mass is a fundamental principle in chemical changes, stating that mass is neither created nor destroyed in a chemical reaction. This principle is crucial for balancing chemical equations, a skill emphasized in the Cambridge IGCSE Coordinated Science syllabus, ensuring that the number of atoms for each element is the same on both sides of the equation.

Can a physical change be easily reversed compared to a chemical change?

Yes, physical changes are generally easier to reverse than chemical changes. For example, freezing and melting are reversible physical changes. In contrast, chemical changes often result in new substances that cannot be easily reverted to their original form, such as burning wood turning into ash and smoke.

What role do atoms play in distinguishing between physical and chemical changes?

Atoms are the fundamental units of matter, and their arrangement determines whether a change is physical or chemical. In a physical change, the arrangement of atoms remains the same, while in a chemical change, atoms are rearranged to form new substances. Understanding this concept is essential in the study of Atoms, Elements, and Compounds in the Cambridge IGCSE Coordinated Science curriculum.

Why is "Classifying dissolving as a chemical change" a common mistake in Physical and Chemical Changes?

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 is "Saying that any colour change proves a chemical reaction has occurred" a common mistake in Physical and Chemical Changes?

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 is "Saying that all chemical changes are irreversible" a common mistake in Physical and Chemical Changes?

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.

Atoms, Elements and CompoundsCambridge IGCSE Coordinated Sciences 0654

Physical and Chemical Changes

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Physical and Chemical Changes — Cambridge IGCSE Coordinated Science 0654

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.

What you’ll learn

Mapped to the Cambridge IGCSE 0654 syllabus (2025-2027).

C3.1 — Distinguish between physical and chemical changes with examples.
C3.2 — Apply the law of conservation of mass to chemical reactions.

Physical vs Chemical Changes

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:

Gas produced (effervescence)
Precipitate formed (solid in solution)
Temperature change (exothermic or endothermic)
Colour change
New smell/odour
Light emitted

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:

Gas escapes (e.g. burning — mass decreases as CO₂ and H₂O vapour escape)
Gas is absorbed (e.g. magnesium burning — mass increases as O₂ from air is incorporated)
In a closed container, mass always stays the same.

Physical: no new substances, reversible. Chemical: new substances, usually irreversible.
Evidence of chemical change: gas, precipitate, temp change, colour change.
Conservation of mass: atoms rearranged, never created or destroyed.

How it’s examined

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.

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Step-by-step worked examples — Physical and Chemical Changes

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.

1Classifying dissolving salt as a physical change
Core
Question
State whether dissolving salt in water is a physical or chemical change, and explain your reasoning.
Step-by-step solution
1. Step 1
  Dissolving salt (NaCl) in water is a physical change.
2. Step 2
  No new substance is formed; NaCl dissociates into Na⁺ and Cl⁻ ions but no chemical reaction occurs.
3. 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.
2Evidence that iron rusting is a chemical change
Extended
Question
Iron rusting is a chemical change. Give TWO pieces of evidence that support this statement.
Step-by-step solution
1. Step 1
  A new substance is formed: iron oxide (rust, Fe₂O₃) has different properties from iron — it is brown/orange, brittle, and non-metallic.
2. 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.
3. 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.
3Identifying physical vs chemical changes from a list
Core
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
1. Step 1
  (a) Boiling water — physical change; water changes state (liquid → gas) but remains H₂O; reversible by condensation.
2. Step 2
  (b) Burning magnesium — chemical change; new substance MgO formed; bright white light; irreversible.
3. Step 3
  (c) Dissolving sugar — physical change; sugar molecules disperse in water but no new substance formed; reversible by evaporation.
4. 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.

Key Definitions and Keywords — Physical and Chemical Changes

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.

Physical change
Examiner keyword
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.
Chemical change (chemical reaction)
Examiner keyword
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.
Reversible change
A change that can be undone by a straightforward physical method, recovering the original substance.
Precipitate
Examiner keyword
An insoluble solid that forms when two solutions are mixed; its formation is evidence of a chemical change.

Common Mistakes and Misconceptions — Physical and Chemical Changes

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.

✕Classifying dissolving as a chemical change
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).
✕Saying that any colour change proves a chemical reaction has occurred
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.
✕Saying that all chemical changes are irreversible
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.

Practice questions

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Physical and Chemical Changes — frequently asked questions

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Physical and Chemical Changes

Short Study Notes in the form of Slides

Short Study Notes — Physical and Chemical Changes

Detailed Notes

What you’ll learn

How it’s examined

1Classifying dissolving salt as a physical change

2Evidence that iron rusting is a chemical change

3Identifying physical vs chemical changes from a list

Physical change

Chemical change (chemical reaction)

Reversible change

Precipitate

✕Classifying dissolving as a chemical change

✕Saying that any colour change proves a chemical reaction has occurred

✕Saying that all chemical changes are irreversible

Practice questions

Past paper style quiz

Assess your understanding

Physical and Chemical Changes — frequently asked questions