NDA Chemistry · Matter and Its States
Physical vs Chemical Changes
A physical change alters only the form or state of a substance and is usually reversible; a chemical change makes a brand-new substance with new properties and is usually irreversible.
Why this matters
Four PYQs, all EASY, all the same two shapes: 'which one is a chemical change?' and 'which statement is NOT correct?'. The entire subtopic rests on one test — is a new substance formed? — plus knowing which everyday examples sit on each side. Souring of milk, rusting and burning are chemical; melting, boiling and dissolving are physical.
Concept 1 of 2: The test: is a new substance formed?
Definition
The distinguishing features:
- Physical change — only the physical form or state changes; no new substance is formed; the chemical composition of the molecules stays the same; usually reversible. Examples: melting, boiling, freezing, dissolving, cutting, magnetising.
- Chemical change — a new substance with new properties is formed; the chemical composition changes; usually irreversible and often releases or absorbs energy. Examples: rusting, burning, cooking, souring of milk.
- Some events involve both: a burning candle melts wax (physical) and burns the vapour (chemical) at the same time.
- The interconversion of states of matter (ice ⇌ water ⇌ steam) is a physical change — same H₂O molecules throughout.
| Feature | Physical change | Chemical change |
|---|---|---|
| New substance? | No | Yes |
| Molecular composition | Unchanged | Changed |
| Reversible? | Usually yes | Usually no |
| Examples | Melting, boiling, dissolving | Rusting, burning, souring Burning a candle is BOTH: wax melting is physical, vapour burning is chemical. |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
Example 1 · Matter and Its States · Physical vs Chemical Changes
Physical change keeps the composition
Concept 2 of 2: Examples: which side is each change on?
Definition
Sort the common bank examples:
- Chemical changes (new substance formed): rusting of iron, burning of coal/candle, souring of milk (lactose → lactic acid by bacteria), natural greying of hair (loss of pigment by a chemical process), cooking food, reaction of acid with base, digestion.
- Physical changes (no new substance): melting of ice, boiling of water, dissolving salt/sugar in water, cutting paper, magnetising iron, glowing of a bulb filament.
| Event | Change type | Why |
|---|---|---|
| Rusting of iron | Chemical | Iron → iron oxide (new substance) |
| Burning of coal | Chemical | Carbon → CO₂ + ash |
| Souring of milk | Chemical | Bacteria turn lactose into lactic acid Souring of milk LOOKS physical but is chemical — a new acid is formed. |
| Greying of hair (natural) | Chemical | Pigment chemically lost — irreversible Natural greying of hair is a chemical change, not a physical one. |
| Melting of ice | Physical | Still H₂O, just a state change |
| Reaction of acid with base | Chemical | Forms salt + water |
Practice this concept4 quick reps
The same idea in a real exam question:
Example 2 · Matter and Its States · Physical vs Chemical Changes
Souring and greying are 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 (2)
The test: is a new substance formed?4 rows
| Feature | Physical change | Chemical change |
|---|---|---|
| New substance? | No | Yes |
| Molecular composition | Unchanged | Changed |
| Reversible? | Usually yes | Usually no |
| Examples | Melting, boiling, dissolving | Rusting, burning, souring Burning a candle is BOTH: wax melting is physical, vapour burning is chemical. |
Examples: which side is each change on?6 rows
| Event | Change type | Why |
|---|---|---|
| Rusting of iron | Chemical | Iron → iron oxide (new substance) |
| Burning of coal | Chemical | Carbon → CO₂ + ash |
| Souring of milk | Chemical | Bacteria turn lactose into lactic acid Souring of milk LOOKS physical but is chemical — a new acid is formed. |
| Greying of hair (natural) | Chemical | Pigment chemically lost — irreversible Natural greying of hair is a chemical change, not a physical one. |
| Melting of ice | Physical | Still H₂O, just a state change |
| Reaction of acid with base | Chemical | Forms salt + water |
Watch out for (2)
- Physical change keeps the composition→ The test: is a new substance formed?
- Souring and greying are chemical→ Examples: which side is each change on?
Test yourself on Matter and Its States
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.