Question
What is the difference between a physical change and a chemical change? Give 5 examples of each.
Solution — Step by Step
A physical change is a change in the physical properties of a substance (shape, size, state, colour) without any change in its chemical composition. The substance remains the same at the molecular level — no new substances are formed.
Physical changes are generally reversible (you can get back the original substance).
A chemical change (also called a chemical reaction) produces one or more new substances with entirely different chemical properties. Chemical bonds are broken and new bonds are formed. The original substance cannot be recovered by simple physical means.
Chemical changes are generally irreversible.
| Feature | Physical Change | Chemical Change |
|---|---|---|
| New substance formed? | No | Yes |
| Chemical composition | Same | Different |
| Reversibility | Usually reversible | Usually irreversible |
| Energy change | Small or no energy change | Significant energy change (heat, light) |
| Signs | Change in state/shape | Change in colour, gas evolved, precipitate formed |
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Melting of ice: Ice (solid water) → Water (liquid). Chemical formula stays H₂O. Reversible by cooling.
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Cutting of paper: Paper’s size and shape change, but it is still paper (cellulose). No new substance formed.
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Dissolving salt in water: Salt particles disperse in water. If water is evaporated, salt is recovered unchanged (NaCl remains NaCl).
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Breaking of glass: A glass bottle broken into pieces — same glass, just smaller pieces. No new substance.
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Stretching a rubber band: Rubber’s shape changes temporarily. No change in composition — returns to original shape when released.
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Burning of wood: Wood + oxygen → carbon dioxide + water + ash. The original wood cannot be recovered. Energy released as heat and light.
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Rusting of iron: 4Fe + 3O₂ + 6H₂O → 4Fe(OH)₃ → Iron(III) oxide (rust). New substance formed; reversible only by industrial processes, not by simple physical means.
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Cooking an egg: Heat causes proteins to denature and coagulate irreversibly. The raw egg cannot be “uncooled” to get the original white and yolk back.
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Souring of milk: Bacteria convert lactose (milk sugar) into lactic acid. The milk composition changes permanently — you cannot convert soured milk back to fresh milk.
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Burning of a candle: Wax + O₂ → CO₂ + H₂O. Wax is consumed; new gases are produced. The light and heat are signs of a chemical change.
Why This Works
The key question to distinguish the two is: “Are new substances with different chemical properties formed?”
If ice melts to water, both are H₂O — same substance, different state. Physical change. If wood burns to carbon dioxide, CO₂ has completely different properties from wood — new substance. Chemical change.
Signs that strongly suggest a chemical change (not proof, but helpful indicators): change in colour, gas produced, precipitate formed, heat or light evolved, smell produced.
Alternative Method
Test reversibility: if the original substance can be easily recovered by physical means (filtering, evaporation, cooling), it’s likely a physical change. If you need a completely different chemical process to recover the original, it’s a chemical change.
A common exam question asks: “Is dissolving of sugar a physical or chemical change?” It’s physical — sugar can be recovered by evaporating the water, and the sugar retains its original properties. Compare with burning sugar (chemical — new substances like carbon and water are formed).
Common Mistake
Students often classify “dissolving” as a chemical change because it “looks different.” This is wrong. When salt or sugar dissolves in water, the ions/molecules just disperse — no new chemical bonds form between the solute and water (beyond hydration). Evaporate the water and you get the original substance back. It’s a physical change.