With the help of an appropriate example, justify that some of the chemical reactions are determined by

(a) Change in temperature,

(b) Evolution of a gas, and

(c) Change in colour

Give chemical equation for the reaction involved in each case.

Chemical reactions can be determined by various methods. Here are some examples:

(a) Change in temperature: The reaction between quick lime (calcium oxide) and water is an example of a reaction that is determined by a change in temperature. The reaction is exothermic, meaning it releases heat, and the temperature of the mixture increases.

CaO(s) + H2O(l) $\rightarrow$ Ca(OH)2(aq) + Heat

(b) Evolution of a gas: The reaction between zinc and dilute hydrochloric acid is an example of a reaction that is determined by the evolution of a gas. The reaction produces hydrogen gas, which can be seen as bubbles in the reaction mixture.

Zn(s) + 2HCl(aq) $\rightarrow$ ZnCl2(aq) + H2(g)

(c) Change in colour: The reaction between silver chloride and light is an example of a reaction that is determined by a change in colour. The reaction causes the silver chloride to decompose into silver and chlorine, resulting in a change in colour from white to grey.

2AgCl(s) $\rightarrow$ 2Ag(s) + Cl2(g)