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Byju's Answer

Standard XII

Chemistry

Electrolysis and Electrolytes

A certain sal...

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 material.
(iii) When concentrated sulphuric acid is added to a hot concentrated solution of (X) , white crystals of a weak acid separate out
Write equations for all the above reactions and identify X, Y, Z.

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Solution

The salt (X) is borax $N a_{2} B_{4} O_{7} .10 H_{2} O$
(i) In the aqueous solution it undergoes hydrolysis to form a mixture of boric acid and sodium hydroxide.
Hence, the aqueous solution is alkaline in nature.
$N a_{2} B_{4} O_{7} + 7 H_{2} O ⟶ 4 H_{3} B O_{3} + 2 N a O H o r N a [B (O H)_{4}]$
(ii) On strong heating, borax swels to a glassy material $2 N a B O_{2} + B_{2} O_{3}$
$N a_{2} B_{4} O_{7} △ ⟶ 2 N a B O_{2} + B_{2} O_{3}      g l a s s y$
(iii) When borax reacts with hot concentrated sulphuric acid solution, boric acid crystallizes out.
$N a_{2} B_{4} O_{7} . 10 H_{2} O + H_{2} S O_{4} ⟶ N a_{2} S O_{4} + 4 H_{3} B O_{3} + 5 H_{2} O$

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