Question

A certain salt X, gives the following results.

1. Its aqueous solution is alkaline to litmus.
2. It swells up to a glassy material Y on strong heating.
3. When conc. H₂SO₄ 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 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.

\[\ce{\underset{Borax (X)}{Na2B4O7} + 7H2O ->[water] \underset{Sodium hydroxide}{2NaOH} + \underset{Orthoboric acid}{4H3BO3}}\]

\[\ce{\underset{Borax (X)}{Na2B4O7 * 10H2O} ->[\Delta] \underset{Sodium metaborate}{Na2B4O7} ->[\Delta] \underset{(Glassy material)}{\underset{Boric anhydric}{B2O3}} + 2NaBO2}\]

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

\[\ce{\underset{Borax (X)}{Na2B4O7 * 10H2O} +  H2SO4 ->[\Delta] Na2SO4 +\underset{Orthoboric acid}{4H3BO3 + 5H2O}}\]