When an aqueous solution of borax is acidified with hydrochloric acid,a white crystalline solid is formed which is soapy to touch. Is this solid acidic or basic in nature? Explain.

(N/A) An aqueous solution of borax is acidified with $HCl$ to produce boric acid $(H_{3}BO_{3})$,which appears as a white crystalline solid and is soapy to the touch.
The chemical reaction is:
$Na_{2}B_{4}O_{7} + 2HCl + 5H_{2}O \rightarrow 2NaCl + 4H_{3}BO_{3}$
This solid is acidic in nature. Boric acid is a weak monobasic Lewis acid. It does not act as a protonic acid (it does not release $H^{+}$ ions directly); instead,it acts as a Lewis acid by accepting a lone pair of electrons from a hydroxyl ion $(OH^{-})$ from water:
$B(OH)_{3} + 2H_{2}O \rightarrow [B(OH)_{4}]^{-} + H_{3}O^{+}$