Q. Explain the theory of chlorination?

Chlorine is an element, having the symbol Cl with an atomic weight of 35.45 . Gaseous chlorine is greenish yellow in colour and is approximately 2.5 times heavier than air. Under pressure, it is a liquid with an amber colour and oily nature approximately 15 times as heavy as water. Liquefaction of chlorine gas is accomplished by drying, cleaning and compressing the gas to 35 $\mathrm{kg} / \mathrm{cm}^{2}$ .

When chlorine is dissolved in water at temperature between $49^{\circ} \mathrm{F}$ and $212^{\circ} \mathrm{F}$ , it reacts with water to form hypochlorus acid $(\mathrm{HOCl})$ and hydrochloric acid (HCl) within few seconds according to the equation

$\mathrm{Cl}_{2}+\mathrm{H}_{2} \mathrm{O} \rightleftharpoons \mathrm{HOCl}+\mathrm{HCl}_{(\mathrm{Hydrolysis})}$

This hydrolysis reaction is reversible. The HOCl ionizes or dissociates into hydrogen ion ($H^{+}$ ) and hypochlorite ions $\left(\mathrm{OCl}^{-}\right)$ according to the equation,

$\mathrm{HOCl} \rightleftharpoons \mathrm{H}^{+}+\mathrm{OCl}^{-}$ (lonization)

This reaction is also reversible. It is the hypochlorus acid and hypochlorite ions which accomplish disinfection.