Question

What are the characteristics of a chemical reaction ? A chemical reaction is characterised by an irreversible change in which a new substance is formed.

Answer 1

Characteristics of a chemical reaction:

1. Evolution of a gas: A gas may evolve during a chemical reaction. This is shown by writing an arrow pointing upwards (↑) besides the gas. Some gases have characteristic smell also.
   e.g. 2Na + 2HCl → 2NaCl + H2 ↑
2. Change in colour: Change in colour takes place in some chemical reactions e.g.
   
3. Formation of precipitate: Some chemical reactions involve the formation of a precipitate which is characterised by an arrow pointing downwards (↓) beside the precipitate.
   
4. Change of physical state: In some chemical reactions, a change of state takes place from reactants to products, e.g.