First slide

Borax

Question

A certain salt (X) gives the following tests
(i) Its aqueous solution is alkaline to litmus.
(ii) On strongly heating it swells to give a glassy bead.
(iii) When concentrated sulphuric acid is added to a hot concentrated solution of (X), crystals of $H_{3} {BO}_{3}$ separate out. Identify the colour of these crystals.

Difficult

A

Violet
B

Blue
C

Brown
D

White

Solution

$(X) is borax, {Na}_{2} B_{4} O_{7} . 10 H_{2} O (i) {Na}_{2} B_{4} O_{7} + 7 H_{2} O ⇌ \underset{(Strong Base)}{2 NaOH} + \underset{(Weak acid)}{4 H_{3} {BO}_{3}}$
Due to presence of NaOH, the aqueous solution is alkaline to litmus.
$(ii) {Na}_{2} B_{4} O_{7} . 10 H_{2} O \to_{(swells)}^{Heat} {Na}_{2} B_{4} O_{7} + 10 H_{2} O \to 2 {NaBO}_{2} + \underset{(Glassy bead)}{B_{2} O_{3}} (iii) {Na}_{2} B_{4} O_{7} + H_{2} {SO}_{4} + 5 H_{2} O \to {Na}_{2} {SO}_{4} + \underset{White crystals}{4 H_{3} {BO}_{3}}$

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