Question

Write two reactions to explain amphoteric nature of water.

Answer 1

Those compounds which can act as acid as well as base are called amphoteric compounds.

$1)$ Water acting as an acid:

When $H_{2}O$ reacts with $N{H}_3,$ it donates $H^{+}$ ions to $N{H}_3$ such that formation of ammonium ion $\left(N{H}_4^{+}\right)$ and hydroxide ion $\left(O{H}^{-}\right)$ takes place.

$H_{2}O\left(l\right)+N{H}_3\left(aq\right)\leftrightharpoons O{H}^{-}\left(aq\right)+N{H}_4^{+}\left(aq\right)$

According to Bronsted-Lowry theory, acid is a substance that donates protons$H^{+}$ .
Thus, in the above reaction $H_{2}O$ is acting as an acid.


(ii) Water acting as a base:

When $H_{2}O$ reacts with $H_{2}S,$ it accepts $H^{+}$ ions from $H_{2}S$ such that formation of hydronium ion $\left(H_3{O}^{+}\right)$ takes place.

$H_{2}O\left(l\right)+H_{2}S\left(aq\right)\leftrightharpoons H_3{O}^{+}\left(aq\right)+H{S}^{-}\left(aq\right)$

According to Bronsted-Lowry theory, base is a substance that accepts protons $H^{+}$ ions and forms its conjugate acid. Thus, in the above reaction $H_{2}O$ is acting as a base..