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A CHEMICAL REACTION IS CHARACTERIZED BY EVOLUTION OF A GAS AND BY EVOLVING HEAT (EXOTHERMIC REACTION).

Take a few zinc granules in a conical flask. Add dilute hydrochloric acid (HCl) or sulphuric acid (H2SO4) in the conical flask.

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(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).

Inference –
(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

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Conclusion –
(i) Reactions which produce heat are called exothermic reactions.
(ii) 
Evolution of a gas and liberation of heat characterize a chemical reaction.