Question

What happens when $NaB{H}_4$ is dissolved in water?
(A) It decomposes with the evolution of $H_2$
(B) $N{a}^{+}$ and ${B{H}_4}^{-}$ are formed, which are stable
(C) $B{H_4}^{-}$ ions formed initially, decompose to produce $O{H}^{-}$ ions, which prevent further decomposition
(D) $NaH$ and $B_2{H}_6$ are produced

Answer 1

Hint: $NaB{H}_4$ is soluble in water but it also reacts with water. On reaction with water, it forms Hydrogen as a by-product but it does not evolve Hydrogen when it is dissolved in water

Complete step by step answer
The common name of $NaB{H}_4$ is sodium borohydride. This is a powder like white solid. $NaB{H}_4$ is a well-known reducing agent. It can reduce a large number of compounds including organic carbonyls in specific conditions. It has vast use in organic chemistry. It is commonly used in Chemistry labs to convert aldehydes or ketones to alcohols.
Sodium borohydride can react with water as well as moist air. When it reacts with water, it generates Hydrogen as a reaction by-product. Hydrogen gas is extremely flammable and also explosive. It also reacts with strong acids. Here, strong acids mean strong inorganic acids like Hydrochloric acid, Nitric acid, Sulphuric acid etc. Organic acids are way weaker than inorganic acids. On reaction with strong acids, sodium borohydride forms Diborane gas which is poisonous. Reaction of Sodium borohydride with water is as follows:
 $NaB{H}_4+2H_{2}O\to NaB{O}_2+4H_{2}O$
This is an exothermic reaction. Using this reaction, sodium borohydride is largely used now-a-days in the prototypes of the direct borohydride fuel cell.
When Sodium borohydride gets dissolved in water, it forms sodium hydroxide and Hydrogen by decomposition. Further decomposition does not happen because formation of Sodium Hydroxide prevents that.
Hence, option (C) is the correct answer.

Note
Sodium borohydride reacts with water and also dissolves in it. Sodium borohydride is a common reducing agent used in organic reactions. So different ions are released after dissolving in water.