The world around us is constantly changing. Ice melts, iron rusts, candles burn, paper tears. Not all changes are the same — some are easy to reverse, while others produce entirely new substances. Scientists classify changes as physical or chemical.
Physical Changes
A physical change is a change in which the form or appearance of a substance changes but no new substance is formed. The change is usually reversible.
- Examples of physical changes:
- Melting of ice → water (reversible: freeze water to get ice back)
- Dissolving sugar in water (reversible: evaporate water to get sugar back)
- Cutting paper into pieces
- Stretching a rubber band
- Boiling water to steam
- Key features:
- No new substance formed
- Usually reversible
- Same chemical composition before and after
Chemical Changes
A chemical change is a change in which one or more new substances are formed with different properties from the original substances. Chemical changes are generally irreversible.
- Examples of chemical changes:
- Burning of wood (produces ash, CO2, water vapour — cannot get wood back)
- Rusting of iron (iron + oxygen + water → iron oxide)
- Souring of milk (proteins break down — cannot get fresh milk back)
- Cooking an egg
- Digestion of food in the body
- Key features:
- New substance(s) formed
- Usually irreversible
- Change in colour, smell, temperature, gas evolution, or formation of a precipitate
Signs (Indicators) of a Chemical Change
- 1.Change in colour – e.g., iron turns reddish-brown when it rusts.
- 2.Evolution of gas – e.g., fizzing when baking soda meets vinegar.
- 3.Change in temperature – Exothermic (releases heat) or endothermic (absorbs heat).
- 4.Formation of a precipitate – A solid forms in a liquid solution.
- 5.Change in smell – New odour indicates new substance.
- 6.Change in state that involves new substance formation.
Reversible and Irreversible Changes
- Reversible – Can be undone to get the original substance back (mostly physical changes): melting wax, dissolving salt.
- Irreversible – Cannot be undone (mostly chemical changes): burning, rusting, cooking.
Note: Some physical changes (tearing paper) are also irreversible, but they are still physical because no new substance forms.
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Ice melts to form water when heated. This is a physical change — both ice and water are H2O. Cooling the water again gives ice back. No new substance is formed.
When iron is left in moist air for a few weeks, it develops a reddish-brown coating of rust (iron oxide, Fe2O3). This is a chemical change — a new substance (rust) forms and the process cannot easily be reversed.
Mixing baking soda (NaHCO3) with vinegar (acetic acid) produces CO2 gas, water, and sodium acetate. The fizzing (gas evolution) and formation of new substances confirm a chemical change.
Dissolving common salt in water is a physical change — the salt can be recovered by evaporating the water. The chemical composition of salt (NaCl) does not change.
Burning a candle involves both physical and chemical changes. The wax melting is physical (reversible); the wax burning and producing CO2 and water vapour is chemical (irreversible).
Cooking an egg changes the white (albumin protein) permanently from translucent to white and solid. This is a chemical change — the proteins denature and form new bonds. The change is irreversible.
The souring of milk into curd is a chemical change caused by bacteria converting lactose to lactic acid, producing a substance (curd) with completely different properties from fresh milk.
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Quick Comparison Table
| Feature | Physical Change | Chemical Change |
|---------|----------------|-----------------|
| New substance formed | No | Yes |
| Reversibility | Usually reversible | Usually irreversible |
| Chemical composition | Same | Different |
| Example | Melting ice | Burning wood |
Common mistakes
- Thinking that all irreversible changes are chemical: tearing paper is irreversible but still a physical change (no new substance forms).
- Assuming dissolving is a chemical change: dissolving sugar or salt in water is a physical change because the substance can be recovered and its composition has not changed.
Summary
Physical changes alter the form, shape, or state of a substance without forming new substances and are usually reversible. Chemical changes produce new substances with different properties and are usually irreversible. Signs of chemical change include colour change, gas evolution, temperature change, precipitate formation, and change in smell.