Question
What is the difference between a physical change and a chemical change? How can we identify whether a change is physical or chemical? Give five examples of each.
Solution — Step by Step
In a physical change, the substance changes its form, size, shape, or state — but its chemical composition stays the same. The change is usually reversible.
Examples: ice melting to water, tearing paper, dissolving sugar in water, breaking a glass, boiling water.
In a chemical change, the substance transforms into a completely new substance with different properties. Usually irreversible (or very hard to reverse).
Examples: burning wood (produces ash and CO₂), rusting of iron, cooking food, curdling of milk, digestion of food.
Look for these signs of a chemical change:
- Change in colour (iron turning reddish-brown — rust)
- Release of gas (bubbles when vinegar meets baking soda)
- Change in temperature (burning feels hot)
- Change in smell (cooking produces new odours)
- Formation of a precipitate (insoluble solid forms)
If none of these signs appear and the original substance can be recovered, it is likely a physical change.
flowchart TD
A[Is it a change?] --> B{New substance formed?}
B -->|Yes| C[Chemical Change]
B -->|No| D[Physical Change]
C --> C1[Colour change?]
C --> C2[Gas released?]
C --> C3[Heat/Light produced?]
C --> C4[Precipitate formed?]
D --> D1[Change in state/shape/size]
D --> D2[Usually reversible]Why This Works
The key criterion is composition. If the atoms rearrange to form new molecules, it is chemical. If the molecules stay the same but just change arrangement (solid to liquid) or mix (dissolving), it is physical.
Common Mistake
Dissolving salt in water looks like a chemical change (the salt “disappears”), but it is a physical change — the salt molecules separate and disperse in water. You can recover the salt by evaporating the water. However, dissolving a reactive metal in acid IS a chemical change (new compounds form).
Reversibility is a useful clue but not a perfect rule. Burning is chemical and irreversible. But some chemical changes CAN be reversed (e.g., electrolysis of water reverses its formation). Focus on whether a new substance forms.