State what is a chemical reaction. A chemical reaction is often accompanied by external indications or characteristics which includes 1.colour change 2.effervesence or gas evolved 3. Evolution or absorption of heat 4. Formation of precipitate. With reference to each of the above indications, State the external indications seen during - 1. Addition of dilute acid to an active metal 2. Addition of dilute hydrochloric acid to silver nitrate. 3. Addition of water to quicklime. 4. Thermal decomposition of mercury(ll) oxide