A change in which one or more new substances are formed is a chemical change.
- It is a permanent change.
- It cannot be reversed .
It is often
- Heat, light or any other radiation (for example - UV) being given off or absorbed.
- A change in smell / a new smell
- A colour change
- Formation of a gas .
Hence, Digestion, Photosynthesis, and even simpler phenomena such as burning, spoiling of food, neutralisation, etc are all chemical changes.