Physical and Chemical Changes

This guide covers Topic 6.1, Physical and chemical changes, for Cambridge IGCSE Chemistry 0620 and Cambridge O Level Chemistry 5070, 2026–2028 series. It’s the shortest topic in the syllabus — one outcome, Core only — but it’s foundational: every later topic (rates of reaction, redox, salt preparation) assumes you can already tell a physical change from a chemical one.

Syllabus coverage

CAMBRIDGE IGCSE CHEMISTRY 0620 / O LEVEL CHEMISTRY 5070

Core

• Identify physical and chemical changes, and describe the differences between them (6.1)

There is no Supplement content for this subtopic — it’s Core-only, and identical for 0620 Extended, 0620 Core and O Level 5070 candidates.

The difference

A physical change alters the form or state of a substance, but no new substance is made — the same particles, the same chemical formula, throughout.

A chemical change produces at least one new substance with different chemical properties from what you started with — a new arrangement of atoms, usually shown by a chemical equation with different formulae either side of the arrow.

physical change:   ice → water            (same substance, H2O throughout)
chemical change:   magnesium + oxygen → magnesium oxide   (new substance, MgO)

Evidence — what actually counts

This is the part examiners test directly: given a description of what happened, decide which type of change it is, and justify it with evidence.

Evidence of a chemical change (any one is usually enough to justify it):

• A new substance is formed with different properties (e.g. a gas escapes that wasn’t there before, or a solid appears where there was none)
• A permanent colour change
• A temperature change that isn’t simply due to heating or cooling from outside (i.e. the reaction itself is exothermic or endothermic)
• A precipitate forms (an insoluble solid appearing from two clear solutions)
• Effervescence (bubbles of a gas being produced)

Physical changes, by contrast, involve no new substance:

• Changes of state — melting, freezing, boiling, condensing, subliming
• Dissolving (the solute is still chemically the same substance, just dispersed in the solvent — this is why it can be recovered by evaporation)
• Simple mixing without reaction (e.g. mixing sand and salt)
• Most physical changes are easily reversed by physical means (cooling a liquid to re-freeze it, evaporating a solvent to recover a dissolved solid)

The reversibility trap

Reversibility is useful evidence but not the definition — don’t rely on it alone.

• Most physical changes are easily reversible: this is genuinely useful supporting evidence.
• Most chemical changes are difficult to reverse by simple means (you can’t “un-burn” magnesium back into magnesium ribbon and oxygen gas just by cooling it).
• But some chemical changes are readily reversible — reversible reactions and equilibrium (Topic 6.3) are still chemical changes, because new substances are genuinely formed in both directions. Reversibility on its own is a clue, not a proof.

The safest test is always: has a new substance been formed? If yes, it’s chemical, however easily it reverses. If no — same substance, different form — it’s physical, however hard it looks to undo.

Worked examples

Change	Type	Why
Melting ice	Physical	Still H₂O; easily re-frozen
Burning magnesium ribbon	Chemical	New substance (MgO), light/heat given out, cannot be reversed by cooling
Dissolving salt in water	Physical	Salt is still NaCl, chemically unchanged; recoverable by evaporation
Rusting iron	Chemical	New substance (hydrated iron(III) oxide) forms slowly; different properties from iron
Boiling water	Physical	Same substance, different state
Adding acid to a carbonate (fizzing)	Chemical	Gas evolved (CO₂), new substances formed (salt + water + carbon dioxide)

Common mistakes

• Treating “produces a gas” as the definition rather than one piece of evidence among several — some physical changes also involve gases (e.g. sublimation), so always check whether a genuinely new substance has formed, not just whether a gas appeared.
• Using “hard to reverse” as the definition of chemical change. It’s supporting evidence, not the test itself — see the reversible-reactions exception above.
• Assuming dissolving is a chemical change because the solid “disappears.” It hasn’t changed chemically — it’s still the same substance, dispersed among the solvent particles, and can be recovered unchanged by evaporating the solvent.
• Forgetting temperature change needs to come from the reaction itself. Warming a beaker with a Bunsen burner is not evidence of a chemical change happening inside it — the heat is coming from outside, not from the reaction.

Quick revision checklist

• Definition: chemical change = new substance formed; physical change = no new substance, same particles throughout
• The five pieces of evidence for a chemical change: new substance/colour change/unexplained temperature change/precipitate/effervescence
• Why reversibility is useful evidence but not the definition
• Being able to classify an unfamiliar described change and justify your answer with the correct type of evidence

Related resources

• Rates of Reaction and Reversible Reactions — reversible reactions are chemical changes despite being reversible
• Redox Reactions — a named class of chemical change with its own evidence and vocabulary
• Cambridge IGCSE Chemistry hub · Cambridge O Level Chemistry hub

Written against Cambridge IGCSE Chemistry 0620 and Cambridge O Level Chemistry 5070, 2026–2028 series. Always check the current syllabus for your examination year.