We know a physical reaction took place if...
- no new substance is produced
- the change can be reversed
Water freezing into ice is an example of a physical reaction.
We know a chemical reaction took place if...
- heat is produced
- a new substance is produced with a new chemical formula
- a odour is produced
- the change is permanent
Mixing yeast with hydrogen peroxide creates heat, visible reaction, an odour, and a new gas (CO2)