Chemical changes

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 accompanied by:
  • Heat, light or any other radiation (for example - UV) being given off or absorbed.
  • Sound 
  • 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.