1. State what is a chemical reaction. A chemical reaction is often accompanied by external indications or characteristics which include - al Colour change b] Effervescence for 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.
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Explanation:
A chemical reaction is a chemical change where substances react with each other to form a new substance.
(i) addition of dilute acid to an active metal - The test tube becomes hot and hydrogen gas is evolved, which means heat is produced.
(ii) Addition of dilute hydrochloric acid to silver nitrate - A white coloured precipitate is formed which is AgCl
(iii) Addition of water to quicklime - The solution boils and heat is produced accompanied by a hissing sound.
(iv) Thermal decomposition of mercury oxide - The colour of mercury changes from red to silvery, that is metallic.