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Physical and Chemical Changes hero

Physical and Chemical Changes

~6 min read

In 30 seconds
  • Physical: No new substance formed. Original can usually be recovered. Examples: melting, dissolving, breaking glass.
  • Chemical: New substance(s) formed. Composition changes. Usually irreversible. Examples: burning, rusting, cooking, digestion.
  • Signs of chemical change: Colour change, gas evolved, precipitate formed, heat absorbed or released, smell.

Knowing whether a change is physical or chemical is foundational. NDA tests both with everyday examples.

Physical Changes

  • No new substance is formed.
  • Composition stays the same; only physical properties change.
  • Usually reversible.
  • Examples:
    • Melting of ice (still H₂O).
    • Boiling of water (steam is still H₂O).
    • Dissolving sugar in water (sugar molecules intact).
    • Breaking glass.
    • Stretching rubber band.
    • Magnetisation of iron.
    • Sublimation of camphor.

Chemical Changes

  • New substance(s) formed with different properties.
  • Atomic composition changes.
  • Usually irreversible by simple means.
  • Examples:
    • Burning of paper (carbon, water vapour, ash — not paper any more).
    • Rusting of iron (iron → iron oxide).
    • Cooking of food.
    • Souring of milk.
    • Digestion.
    • Photosynthesis.
    • Mixing baking soda with vinegar (fizz = CO₂ released).

Signs of a Chemical Change

  1. Colour change (e.g., milk souring; copper turning green).
  2. Gas evolved (effervescence on mixing acid with marble).
  3. Precipitate formed (insoluble solid appearing in solution).
  4. Temperature change (exothermic: heat released; endothermic: absorbed).
  5. Odour change.
  6. Energy change (light, sound, electrical).

NDA PYQ Examples

Q: Which is a physical change?

(a) Burning of wood (b) Rusting of iron (c) Melting of ice (d) Cooking of egg

Answer: (c) Melting of ice — still H₂O.

Q: Rusting of iron is:

(a) Physical change only (b) Chemical change (c) Reversible (d) Endothermic

Answer: (b) Chemical change — iron → iron oxide.

Q: Souring of milk is due to:

(a) Physical change (b) Action of bacteria forming lactic acid (c) Evaporation (d) Freezing

Answer: (b) Bacteria convert lactose to lactic acid — chemical change.

Drill Physical and Chemical Changes for NDA

NDA-pattern items on Physical and Chemical Changes with answer keys and explanations.

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Frequently Asked Questions

How do I tell if a change is physical or chemical?

Ask: is a new substance formed? Physical change preserves identity (water still water as ice/steam). Chemical change produces something new (rusted iron is no longer pure iron — it's iron oxide).

Is dissolving sugar a chemical change?

Physical. Sugar molecules disperse in water but remain sugar molecules. Evaporate the water and you'll get sugar back.

Is burning a chemical change?

Yes — typically combustion combines fuel with oxygen to produce CO₂, H₂O, heat, and light. New substances are formed.

Are all chemical changes irreversible?

Most are practically irreversible by simple means (you can't 'un-burn' wood). But some chemical changes are reversible — e.g., colour change of CuSO₄·5H₂O (blue) ↔ CuSO₄ (white) by heating/adding water.

Why does a candle burning involve both physical and chemical changes?

Wax melting near the wick → physical (just changing state). Wax vapor combusting at flame → chemical (producing CO₂, water, light, heat). A candle demonstrates both simultaneously.