UPSC NDA II 2015 General Ability Paper

Concept:Boric acid acts as a Lewis acid by accepting a hydroxide ion from water.Explanation:In water, $\mathrm{B(OH)_3}$ does not donate a proton directly.It accepts $\mathrm{OH^-}$ from $\mathrm{H_2O}$.This forms the tetrahydroxyborate ion $\mathrm{[B(OH)_4]^-}$ and releases a proton $\mathrm{H^+}$.The reaction is: $\mathrm{B(OH)_3 + H_2O \rightleftharpoons [B(OH)_4]^- + H^+}$.The released $\mathrm{H^+}$ makes the solution acidic.Thus, boric acid is acidic because it accepts $\mathrm{OH^-}$, not because it gives up a proton directly.Answer:Accepts $\mathrm{OH^-}$ from water releasing a proton (Option 1).