Cut paper, melt butter or dissolve sugar, and you still have paper, butter and sugar. Burn paper or bake a cake, and you get something new.
Key idea
A physical change changes size, shape or state, but it is still the same substance. A chemical change makes a new substance.
| Physical changes | Chemical changes |
|---|---|
| Cutting, tearing, folding | Burning wood or a candle wick |
| Melting, freezing, boiling | Baking a cake |
| Dissolving sugar | An iron nail rusting |
| Crushing a can | Baking soda fizzing in vinegar |
Signs of a chemical change: bubbles of a new gas, a new color, light or heat given off, or a new smell. (Bubbles alone are not proof: boiling water bubbles too.)
Worked example: Which kind?
An apple slice turns brown after an hour on the counter. Physical or chemical?
- The brown color is new.
- It formed because the apple reacted with oxygen in the air.
Answer: A chemical change.
Common mistake
Calling melting a chemical change. Melted ice is still water and can freeze again. A burnt piece of toast cannot be turned back into bread.
In any change, the total mass stays the same if nothing escapes. When baking soda fizzes in vinegar in an open cup, the cup gets lighter because the gas floats away.
Try it at home
- Do this in a sink or on a tray, with a grown-up.
- Put a spoonful of baking soda in a cup and add a little vinegar.
- What signs of a chemical change do you see? (The bubbles are a new gas, carbon dioxide.)
Going further. Try mixing safely on screen in the Mixing Lab. Michael Faraday's The Chemical History of a Candle is a classic for older readers.