Take a few zinc granules in a conical flask. Add dilute hydrochloric acid (HCl) or sulphuric acid (H2SO4) in the conical flask.
(a) What do you observe ?
(b) Touch the conical flask and observe any change in its temperature ?
Objective – A chemical reaction characterized by evolution of a gas and by evolving heat (exothermic reaction).
(a) When zinc granules react with dilute H2SO4 or HCl (acid), a gas (H2) is evolved briskly. It can be confirmed by bringing a lighted candle near the mouth of the flask, the gas evolved starts burning with a popping sound.
(b) This reaction liberates heat which can be observed when we touch the flask surface from outside, it feels hot.
Chemical equation –
Zinc + dil. HCl / H2SO4 —–> Zinc chloride / zinc sulphate + H2 (g) + Heat
(i) Reactions which produce heat are called exothermic reactions.
(ii) Evolution of a gas and liberation of heat characterize a chemical reaction.