Question

A certain salt $X$, gives the following results.

(i) Its aqueous solution is alkaline to litmus.

(ii) It swells up to a glassy material $Y$ on strong heating.

(iii) When conc. $H_{2}S{O}_4$ is added to a hot solution of $X$, white crystal of an acid $Z$ separates out.

Write equations for all the above reactions and identify $X,Y$ and $Z$.

Answer 1

The compounds X, Y and Z are borax, sodium metaborate + boric anhydride and boric acid respectively.

When borax is heated, it first swells and then forms a transparent glass like bead of sodium meta borate and boric anhydride.

$\underset{\text{Borax}}{N{a}_2{B}_4{O}_7}\stackrel{heat}{\to }\underset{\text{sodium metaborate}}{2NaB{O}_2}+\underset{\text{Boric anhydride}}{B_2{O}_3}+10H_{2}O$

Aqueous solution of borax is alkaline due to formation of strong base NaOH.

Hence, it turns red litmus blue.

$N{a}_2{B}_4{O}_7+7H_{2}O\to 4H_{3}B{O}_3+2NaOH$

Borax reacts with sulphuric acid to form boric acid and sodium sulphate.

$\underset{\text{Borax}}{N{a}_2{B}_4{O}_7}+H_{2}S{O}_4+5H_{2}O\to \underset{\text{Boric acid}}{4H_{3}B{O}_3}+\underset{\text{Sodium sulphate}}{N{a}_{2}S{O}_4}$