State what is a chemical reaction. A chemical reaction is often accompanied by external indications or characteristics which include

(a) Colour change

(b) Effervescence or gas evolved

(c) Evolution or absorption of heat

(d) Formation of a precipitate

With reference to each of the above indications, state the external indication seen during

(i) Addition of dilute acid to an active metal

(ii) Addition of dilute hydrochloric acid to silver nitrate

(iii) Addition of water to quicklime

(iv) Thermal decomposition of mercury [II] oxide

A chemical reaction is a chemical change in which matter changes into a new substance or substances.

(i) Gas is evolved

(ii) Formation of a precipitate

(iii) Evolution of heat

(iv) Colour change — Red mercury [II] oxide changes to silver coloured mercury.