Changes: physical or chemical?
In a physical change, the shape, size or state of a substance changes, but no new substance forms. Ice melting, sugar dissolving in tea and wax melting are physical changes. Dissolved sugar can be recovered, and melted wax can set again. In a chemical change, one or more new substances with new properties are made. Burning wood, a nail rusting, an egg cooking, milk turning into yoghurt, an apple turning brown and food rotting are chemical changes. Their results cannot easily be turned back. A good question to ask is: has a new substance formed? Changes of state are physical, so water stays H₂O when it freezes or boils. Chemical changes happen all around you: in a frying pan, in a battery, in your body. Burning and rusting are both reactions with oxygen, but burning is fast and rusting is very slow. A chemical change is also called a chemical reaction.
| Physical change | Chemical change |
|---|---|
| ice melting | wood burning |
| sugar dissolving | a nail rusting |
| wax melting | an egg cooking |
| water boiling | milk turning to yoghurt |
Sorting game. Write on small cards ten changes (water boiling, bread toasting, paper tearing, a candle burning, ice melting, a banana going brown, salt dissolving, iron rusting, juice freezing, milk going sour). Make two piles: physical and chemical. For each card say the question you asked. Check the piles with a grown-up. Never light anything yourself.