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