Question

A certain salt $X$ gives the following test :
(i) Its aqueous solution is alkaline to litmus.
(ii) On strongly heating, it swells to give glassy bead.
(iii) When concentrated $H_2{SO}_4$ is added to a solution of $X$ white crystals of a weak acid. separate out.

Identify $X$ and write down all reaction.

• A. $NaHC{O}_3$
• B. ${Na}_2{B}_4{O}_7.10H_{2}O$
• C. $CaS{O}_4.2H_{2}O$
• D. $CaC{O}_3$

Answer 1

The correct option is B ${Na}_2{B}_4{O}_7.10H_{2}O$

$X$ is ${Na}_2{B}_4{O}_7.10H_{2}O$

(i) ${Na}_2{B}_4{O}_7+7H_{2}O⟶2NaOH+4H_3{BO}_3$
Due to presence of $NaOH$ the aqueous solution is alkaline to litmus.

(ii) ${Na}_2{B}_4{O}_7.10H_2\underset{}{\overset{\Delta }{\to }}{Na}_2{B}_4{O}_7⟶(2Na{BO}_2+B_2{O}_3)$ (Glassy bead)

(iii) ${Na}_2{B}_4{O}_7+H_2{SO}_4+5H_{2}O⟶{Na}_2{SO}_4+4H_3{BO}_3$ (white crystalline weak acid)