NDA Chemistry · Chemical Reactions
Physical vs Chemical Changes
A physical change alters only the form or state of a substance with no new substance formed; a chemical change rearranges atoms into a genuinely new substance — that is what a chemical reaction is.
Why this matters
The foundation of the whole chapter — a chemical reaction IS a chemical change, so everything later builds on this one distinction. The bank asks it directly as 'which of the following is a chemical change?', testing whether you can tell burning, rusting and neutralisation apart from melting, dissolving and evaporating.
Concept 1 of 1
Telling a physical change from a chemical change
Intuition
Definition
The test and the markers:
- Physical change — no new substance; usually reversible; only state, shape or size changes. Examples: melting ice, boiling water, dissolving sugar, breaking glass, magnetising iron.
- Chemical change — a new substance forms; usually irreversible; often shows heat/light, colour change, gas evolution or a precipitate. Examples: burning, rusting, cooking, neutralisation, digestion, photosynthesis.
- Neutralisation (acid + base, e.g. NaOH + HCl → NaCl + H₂O) is a chemical change — a new salt forms.
- Burning magnesium (2Mg + O₂ → 2MgO) is a chemical change — the bright white light and the new white solid MgO are the giveaways.
| Process | Change type | Why |
|---|---|---|
| Melting of ice | Physical | Still water, only state changes — reversible |
| Boiling / evaporation of water | Physical | Water vapour is still water |
| Dissolving sugar in water | Physical | Sugar can be recovered by evaporation |
| Mixing NaOH and HCl | Chemical | Neutralisation — new salt (NaCl) + water form Mixing an acid and a base is a chemical change, not just mixing — a new substance (the salt) is made. |
| Burning of magnesium ribbon | Chemical | New substance MgO forms with light and heat Burning is always a chemical change — a new oxide forms. |
| Rusting of iron | Chemical | New substance (hydrated iron oxide) forms |
Practice this concept4 quick reps
From the bank · past-year question
[Q120 · Sep · 2024]
Dissolving and boiling are physical, not chemical
Mixing an acid and a base is a chemical change
Burning is always chemical
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
Reference tables (1)
Telling a physical change from a chemical change6 rows
| Process | Change type | Why |
|---|---|---|
| Melting of ice | Physical | Still water, only state changes — reversible |
| Boiling / evaporation of water | Physical | Water vapour is still water |
| Dissolving sugar in water | Physical | Sugar can be recovered by evaporation |
| Mixing NaOH and HCl | Chemical | Neutralisation — new salt (NaCl) + water form Mixing an acid and a base is a chemical change, not just mixing — a new substance (the salt) is made. |
| Burning of magnesium ribbon | Chemical | New substance MgO forms with light and heat Burning is always a chemical change — a new oxide forms. |
| Rusting of iron | Chemical | New substance (hydrated iron oxide) forms |
Watch out for (3)
- Dissolving and boiling are physical, not chemical→ Telling a physical change from a chemical change
- Mixing an acid and a base is a chemical change→ Telling a physical change from a chemical change
- Burning is always chemical→ Telling a physical change from a chemical change
Drill every past-year question on this subtopic
2 questions from the bank — paginated, with cart and Word-export support.