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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.

All Matter and Its States notesNDA Chemistry strategy

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?

Intuition

Only one question separates the two kinds of change: after the change, do you still have the same substance (just in a new form), or a completely new substance? Same substance = physical. New substance = chemical.

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.
FeaturePhysical changeChemical change
New substance?NoYes
Molecular compositionUnchangedChanged
Reversible?Usually yesUsually no
ExamplesMelting, boiling, dissolvingRusting, burning, souring
Burning a candle is BOTH: wax melting is physical, vapour burning is chemical.
The whole distinction is one question: is a new substance formed?
Practice this conceptself-check · 4 quick reps

Try it yourself

Which statement is NOT correct? (a) Interconversion of states is a physical change (b) A burning candle shows both physical and chemical change (c) During a physical change, the chemical composition of molecules changes (d) Rusting of iron is a chemical change.

Practice — Level 1 (4 reps)

Quick reps to lock in the method. Try each, then check.

  1. 1.
    What is the single test that distinguishes a chemical change from a physical change?
  2. 2.
    Is melting of ice a physical or chemical change?
  3. 3.
    Does the molecular composition change in a physical change?
  4. 4.
    Burning of a candle involves which kind(s) of change?

From the bank · past-year question

Example 1Matter and Its StatesEASY
Which one of the following statements is NOT correct ?

[Q101 · Sep · 2025]

Physical change keeps the composition

A common false statement is 'during a physical change, the chemical composition of molecules changes' — that is NOT correct. In a physical change the molecules are unchanged; only the form/state differs.

Concept 2 of 2

Examples: which side is each change on?

Intuition

The bank simply lists four everyday events and asks which one is the chemical (or the only non-chemical) change. The trick events are the ones that look physical but make a new substance — souring of milk and the natural greying of hair.

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.
EventChange typeWhy
Rusting of ironChemicalIron → iron oxide (new substance)
Burning of coalChemicalCarbon → CO₂ + ash
Souring of milkChemicalBacteria turn lactose into lactic acid
Souring of milk LOOKS physical but is chemical — a new acid is formed.
Greying of hair (natural)ChemicalPigment chemically lost — irreversible
Natural greying of hair is a chemical change, not a physical one.
Melting of icePhysicalStill H₂O, just a state change
Reaction of acid with baseChemicalForms salt + water
Practice this concept4 quick reps

Practice — Level 1 (4 reps)

Quick reps to lock in the method. Try each, then check.

  1. 1.
    Is souring of milk a physical or chemical change?
  2. 2.
    Is the natural greying of hair a physical or chemical change?
  3. 3.
    Of burning coal, rusting metal, melting ice and acid–base reaction, which is NOT a chemical change?
  4. 4.
    Is dissolving sugar in water a physical or chemical change?

From the bank · past-year question

Example 2Matter and Its StatesEASY
Which one of the following is NOT a chemical change ?

[Q100 · Sep · 2025]

Souring and greying are chemical

Souring of milk and natural greying of hair both look harmless and physical, but each forms a new substance — they are chemical changes. Melting, boiling and dissolving are the physical ones.
Drill 2 more on examples: which side is each change on?

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
FeaturePhysical changeChemical change
New substance?NoYes
Molecular compositionUnchangedChanged
Reversible?Usually yesUsually no
ExamplesMelting, boiling, dissolvingRusting, burning, souring
Burning a candle is BOTH: wax melting is physical, vapour burning is chemical.
The whole distinction is one question: is a new substance formed?
Examples: which side is each change on?6 rows
EventChange typeWhy
Rusting of ironChemicalIron → iron oxide (new substance)
Burning of coalChemicalCarbon → CO₂ + ash
Souring of milkChemicalBacteria turn lactose into lactic acid
Souring of milk LOOKS physical but is chemical — a new acid is formed.
Greying of hair (natural)ChemicalPigment chemically lost — irreversible
Natural greying of hair is a chemical change, not a physical one.
Melting of icePhysicalStill H₂O, just a state change
Reaction of acid with baseChemicalForms salt + water

Watch out for (2)

Mastery check — 2 interleaved questions

Try each one before clicking. Questions are interleaved across the concepts above, not grouped — interleaving sharpens transfer.

Example 1Matter and Its StatesEASY
Which one of the following is a chemical change?

[Q59 · Apr · 2022]

Example 2Matter and Its StatesEASY
Which one of the following is a chemical change?

[Q118 · Sep · 2017]

Drill every past-year question on this subtopic

4 questions from the bank — paginated, with cart and Word-export support.

Drill the 4 questions