Physical and Chemical Change
Physical vs chemical change for IGCSE Chemistry 0620: exam definitions, the evidence examiners accept, reversibility and worked exam answers.
The IGCSE Chemistry Specialist Team · founded by Rig
Written to the Cambridge IGCSE Chemistry (0620) syllabus and mark-scheme conventions. Last updated 2026-06-11.
This looks like the easiest subtopic in Section 6, which is exactly why its marks get thrown away: candidates answer from everyday intuition instead of the one test the mark scheme applies: is a new substance formed? Paper 1 and 2 MCQs dress the question in unfamiliar contexts (heating copper sulfate crystals, mixing two solutions) and the intuitive answer is wrong more than you would expect.
The one-question test (Core)
| Physical change | Chemical change | |
|---|---|---|
| New substance formed? | No | Yes |
| Reversibility | Usually easily reversed | Usually difficult to reverse |
| Examples | Melting, boiling, freezing, dissolving, grinding | Combustion, rusting, neutralisation, thermal decomposition |
Everything else (colour, fizzing, heat) is evidence, not definition. The definition mark always belongs to “a new substance is formed” (chemical) or “no new substance is formed” (physical). State changes are the cleanest physical examples: ice, water and steam are all H2O, so melting and boiling rearrange nothing chemically. The particle picture behind state changes belongs to States of Matter.
The evidence examiners accept
When a question asks “what would you observe that shows a chemical reaction has occurred”, four observations score:
- A colour change: grey iron and yellow sulfur heating to black iron sulfide.
- Effervescence: bubbles of gas given off, as when magnesium meets dilute acid.
- A precipitate: a solid forming when two solutions mix.
- An energy change: the mixture becomes hotter or colder without external heating.
Each observation must be observable. “A new substance forms” is the definition, not an observation; “the solution turned from blue to colourless” is an observation. Keeping those two registers separate (define vs observe) is a habit that pays across the whole of Paper 6, where identifying changes overlaps with qualitative analysis ion and gas tests.
The borderline cases MCQs love
Dissolving is physical: evaporate the water and the original salt returns. Heating hydrated copper(II) sulfate is chemical by 0620 convention: blue CuSO4·5H2O loses its water of crystallisation to form white anhydrous CuSO4, a different substance, although adding water reverses it. The exam treats it as a (reversible) chemical reaction, and it doubles as the test for water.
Reversibility is a guide, not a rule. Most chemical changes are hard to reverse, but reversible chemical reactions exist and have their own Supplement page: Reversible Reactions and Equilibrium. The reliable criterion remains new substance or not.
Burning vs boiling a fuel: boiling ethanol is physical (still ethanol, now gaseous); burning ethanol is chemical (carbon dioxide and water form). Same starting material, opposite classifications, a regular MCQ pairing.
Energy changes happen in both
A chemical change is usually accompanied by an energy transfer: that is why “the test tube got hot” counts as evidence. But melting also takes in energy without anything chemical happening. So an energy change supports a chemical change only alongside other evidence. The direction and vocabulary of those energy transfers (exothermic, endothermic) are covered in Chemical Energetics, and the parent overview for this section is the Chemical Reactions pillar.
Worked exam question
A student heats blue copper(II) sulfate crystals in a test tube. The crystals turn white and drops of a colourless liquid collect at the mouth of the tube. (a) State two observations that suggest a chemical change has taken place. [2] (b) The student adds the colourless liquid to the white solid. The solid turns blue and the tube becomes warm. What does this suggest about the reaction? [2] (c) Name a physical change that occurs in this experiment. [1]
Model answer: (a) The colour changes from blue to white (1); a new substance (the liquid) is produced / collects at the mouth of the tube (1). (b) The reaction is reversible (1); the reverse reaction is exothermic, shown by the tube warming (1). (c) The water vapour condensing to liquid at the cool mouth of the tube (1).
Mark-by-mark: (a) wants observations, so quote what is seen: colour change and the liquid appearing. (b) carries the thinking marks: blue returning means the change can be undone (reversible), and the warmth identifies the rehydration as exothermic. (c) is the trap unpicked: within this chemical experiment, condensation of water is purely physical.
The mistakes that cost marks
- Defining a chemical change by difficulty of reversal. Reversibility is typical, not definitional: the defining feature is a new substance.
- Calling dissolving a chemical change “because the salt disappears”. It has not gone; it is recoverable unchanged.
- Writing “a reaction happens” as an observation. Observations are things seen, heard or felt: colour change, bubbles, precipitate, temperature change.
- Assuming heat always means chemical. Melting and boiling absorb energy with no new substance formed.
How examiners want it phrased
| Student wording | Mark-scheme wording |
|---|---|
| ”It changed into something else" | "A new substance is formed" |
| "It fizzed" | "Effervescence: bubbles of gas are given off" |
| "A solid appeared in the liquid" | "A precipitate forms" |
| "You can’t undo it" | "The change is difficult to reverse, and a new substance has formed” |
The discipline here transfers everywhere: answer the definition question with the definition, the observation question with observations. Students who blur the two lose single marks across an entire paper without noticing, which is why we audit exactly this habit in a free 1-hour trial lesson with a Chemistry specialist.
Test yourself
Apply the new-substance test to each one before clicking the answer.
Q1 (2 marks). Classify each change as physical or chemical, giving a reason: (a) boiling ethanol; (b) iron rusting.
Show answer
• (a) physical: no new substance forms; the gas is still ethanol [1] • (b) chemical: a new substance (hydrated iron(III) oxide / rust) is formed [1]
Q2 (2 marks). A student mixes two colourless solutions. A white solid appears and the mixture becomes warm. State two observations that suggest a chemical change has occurred.
Show answer
• a precipitate (white solid) forms [1] • the temperature increases, an energy change without external heating [1]
Q3 (2 marks). Explain why dissolving sugar in water is a physical change.
Show answer
• no new substance is formed [1] • the sugar can be recovered unchanged by evaporating the water / the change is easily reversed [1]
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Frequently asked questions
What is the difference between a physical and a chemical change?
A chemical change forms one or more new substances and is usually difficult to reverse. A physical change forms no new substance (only the state or appearance changes) and is usually easy to reverse, like melting or dissolving.
Is dissolving salt in water a chemical change?
No. No new substance forms. The salt can be recovered unchanged by evaporating the water. Dissolving, melting, boiling and freezing are all physical changes.
What evidence shows a chemical change has happened?
A new substance forms. Observable signs include a colour change, a gas given off (effervescence), a precipitate forming, or an energy change such as the mixture getting hotter or colder.