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
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
The same idea in a real exam question:
Example 1 · Chemical Reactions · Physical vs Chemical Changes
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
Test yourself on Chemical Reactions
15 past NDA questions from this chapter, timed at 15 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.