Ncert Exercises & Solutions

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.

The given salt is alkaline to litmus. Therefore, X is a salt of a strong base and a weak acid. Also, when X is strongly heated, it swells to form substance Y. Therefore, X must be borax. When borax is heated, it loses water and swells to form sodium metaborate. When heating is continued, it solidifies to form a glassy material Y. Hence, Y must be a mixture of sodium metaborate and boric anhydride.

N a_{2} B_{4} O_{7} + 7 H_{2} O \overset{w a t e r}{\to} 2 N a O H + 4 H_{3} B O_{3}

B o r a x (X) S o d i u m h y d r o x i d e O r t h o b o r i c a c i d

N a_{2} B_{4} O_{7} . 10 H_{2} O \overset{∆}{\to} N a_{2} B_{4} O_{7} \overset{∆}{\to} B_{2} O_{3} + 2 N a B O_{2}

B o r a x (X) S o d i u m m e t a b o r a t e B o r i c a n h y d r i d e (G l a s s y m a t e r i a l)

When concentrated acid is added to borax, white crystals of orthoboric acid (Z) are formed.

N a_{2} B_{4} O_{7} . 10 H_{2} O + H_{2} S O_{4} \overset{∆}{\to} N a_{2} S O_{4} + 4 H_{3} B O_{3} + 5 H_{2} O

B o r a x (X) O r t h o b o r i c a c i d (Z)

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