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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. $H _{2} SO _{4}$ 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 $Z$
Solution
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 heatin is continued, it solidifies to form a glassy material $Y$. Hence, $Y$ must be a mixture of sodium metaborate and boric anhydride.
$\underset{Borax\,(X)}{\mathop{N{{a}_{2}}{{B}_{4}}{{O}_{7}}}}\,+7{{H}_{2}}O\xrightarrow{water}\underset{Sodiumhydroxide}{\mathop{2NaOH}}\,+\underset{Orthoboricacid}{\mathop{4{{H}_{3}}B{{O}_{3}}}}\,$
$\underset{Borax\,(X)}{\mathop{N{{a}_{2}}{{B}_{4}}{{O}_{7}}}}\,+10{{H}_{2}}O\xrightarrow{\Delta }\underset{Sodium\,metaborate}{\mathop{N{{a}_{2}}{{B}_{4}}{{O}_{7}}}}\,\xrightarrow{\Delta }\underset{\begin{smallmatrix}
Boric\,anhydride \\
(Glassy\,material)
\end{smallmatrix}}{\mathop{{{B}_{2}}{{O}_{3}}}}\,+2NaB{{O}_{2}}$
When concentrated acid is added to borax, white crystals of orthoboric acid ( $Z$ ) are formed.
$\underset{Borax\,(X)}{\mathop{N{{a}_{2}}{{B}_{4}}{{O}_{7}}}}\,+10{{H}_{2}}O+{{H}_{2}}S{{O}_{4}}\xrightarrow{\Delta }N{{a}_{2}}S{{O}_{4}}+\underset{Orthoboric\,acid\,(Z)}{\mathop{4{{H}_{3}}B{{O}_{3}}}}\,+5{{H}_{2}}O$
