Physical and chemical changes: use particle identity
Look for formation of new substances rather than treating reversibility, bubbles or colour as a universal rule.
Exam scope and assessment notes
Core and Extended · Chemistry 0971, 2026–2028. Cambridge states that IGCSE 0971 has the same subject content as IGCSE 0620, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Describe changes of state using particles.
- Distinguish a compound from a mixture.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What happens to water molecules when ice melts?
What you will learn
- Distinguish changes of state or arrangement from formation of new substances.
- Use evidence carefully when classifying a physical or chemical change.
Ask whether new substances have formed
A physical change changes a substance's state, shape or arrangement without producing new substances. Melting ice gives liquid water while the molecular identity remains H₂O. A chemical change produces different substances: iron reacting with oxygen to form iron oxide changes chemical identity. State the before-and-after substances rather than relying only on a visible change.
Dissolving sodium chloride in water and recovering it by evaporating the water is treated as a physical change in this syllabus context: the salt's ions disperse in the water and can form the salt crystal again. It is different from sodium reacting with chlorine to form a new compound. A mixture's component separation and a chemical reaction that changes substances are different processes.
Observations are evidence, not automatic proof
Bubbling can occur because a liquid boils, a dissolved gas escapes or a chemical reaction forms gas. Colour can change because coloured solutions mix or because a reaction produces a new species. Support a classification with what happened to chemical identity and any specific reaction evidence, rather than declaring every bubble or colour change chemical.
Reversibility alone is not a reliable dividing rule. Many physical changes can be reversed, but some chemical reactions are reversible too. Burning a fuel is chemical because different substances form; melting wax is physical because the same substance changes state. A candle can show both processes at once, so identify which part of the event is being discussed.
Pause and explain
Which is the strongest basis for classifying a chemical change?
Worked example
A candle's wax first melts, then wax vapour burns. Classify the two processes and explain why the whole observation should not receive one label without specifying the process.
Show the worked solution
- Melting changes wax from solid to liquid without identifying new substances, so that part is physical.
- Burning reacts fuel with oxygen and produces different substances, so that part is chemical.
- The same object can undergo both processes; the explanation must name the particular change.
Answer Melting wax is physical; burning wax vapour is chemical.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Compare melting iron with iron reacting with oxygen to form iron oxide.
Give me a hint
Compare the substances present before and after each change.
Compare my reasoning
- Molten iron is still iron, so melting is a physical change.
- Iron oxide is a different substance from iron and oxygen, so its formation is chemical.
- The classification depends on chemical identity, not simply on whether heating occurred.
Melting: physical; oxide formation: chemical.
Look for these in your work
- The before-and-after identities are explicit.
- Heating is not used as the deciding rule.
Classify dissolving NaCl in water and recovering its crystals by evaporation in the stated syllabus context. Explain what changes at particle level.
Give me a hint
Track dispersion and rearrangement of salt ions rather than inventing a new reaction product.
Compare my reasoning
- Dissolved salt ions disperse among water particles.
- Removing water allows the ions to form salt crystals again.
- This dissolution-and-recovery sequence is treated as physical in the stated context.
Physical: salt ions disperse and are recovered as the salt, without a new substance being identified.
Look for these in your work
- The context and particle change are described.
- Recovery supports the explanation without becoming a universal reversibility rule.
A report labels an event chemical only because a brown solution becomes lighter when water is added. Explain why the evidence is insufficient and give an alternative interpretation.
Give me a hint
Dilution can change observed colour intensity without producing a new substance.
Compare my reasoning
- Adding water spreads the coloured species through a larger volume.
- Lower concentration can make the colour look paler without changing the species' chemical identity.
- Further evidence of a new substance is needed before classifying that event as a chemical reaction.
Dilution is a plausible physical explanation; a paler colour alone does not prove a chemical reaction.
Look for these in your work
- A concrete alternative is supplied.
- Observation is distinguished from the conclusion it can support.
Common mistakes
- Saying every reversible change is physical.
- Using bubbles or a colour change as the only proof of a new substance.
- Giving one label to several different processes without distinguishing them.
What can you explain now?
Is boiling pure water a chemical change merely because bubbles appear? Explain.
Compare with the explanation
No; it is a physical change of state.
The vapour and liquid are both H₂O; bubbles of water vapour do not establish formation of a new substance.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Explain one physical and one chemical change using particle identity. Give an observation that can occur in either type, and explain why it cannot decide the classification by itself.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.