6.1 Physical and chemical changes
- Syllabus
- 0620–2026–2027
- Topic
- 6.1
- Level
- —
Learning objectives
Distinguish physical and chemical changes
| Question to ask | Physical change | Chemical change |
|---|---|---|
| Are new substances formed? | no | yes |
| What happens to the particles? | the same particles change arrangement, spacing or state | atoms are rearranged into different particles as bonds break and form |
| Typical examples | melting, freezing, boiling, condensing, dissolving | combustion, rusting, neutralisation, thermal decomposition |
| Can it be reversed? | often by a physical process | often difficult and may require another chemical reaction |
Classify a change by comparing the substances before and after it. If their chemical identities are unchanged, the change is physical. If one or more new substances with different compositions and properties form, the change is chemical.
Ice melting is physical: H₂O remains H₂O and only its state changes. Magnesium burning is chemical: magnesium and oxygen form a new substance, magnesium oxide. Sulfur melting is physical, but sulfur then burning to form sulfur dioxide is chemical.
A colour change, temperature change, gas, precipitate, or light may suggest a chemical change, but the defining test is formation of new substances. Reversibility is also not a perfect definition: some physical changes are difficult to reverse and some chemical changes can be reversed by another reaction.