Q.A certain salt X, gives the following results.

1. $\text{Na}_2\text{B}_4\text{O}_7 + 7\text{H}_2\text{O} \rightarrow 2\text{NaOH} + 4\text{H}_3\text{BO}_3$ — excess NaOH (strong base) over weak boric acid makes the solution alkaline.
2. $\text{Na}_2\text{B}_4\text{O}_7\cdot 10\text{H}_2\text{O} \xrightarrow{\Delta} \text{Na}_2\text{B}_4\text{O}_7 \xrightarrow{\Delta} 2\text{NaBO}_2 + \text{B}_2\text{O}_3$ (Y = glassy bead). …

1. Identifying X from clue (i): alkaline aqueous solution. Borax, $\text{Na}_2\text{B}_4\text{O}_7\cdot 10\text{H}_2\text{O}$, is the salt of a strong base (NaOH) and a weak acid (boric acid). In water it hydrolyses:

Because the base produced (NaOH) is strong while the acid (H$_3$BO$_3$) is weak, the resulting solution is alkaline to litmus — matching clue (i), so X = borax.

2. Clue (ii): swelling to a glassy material Y on strong heating. Borax first loses its water of crystallisation and swells up (intumesces), then on further strong heating melts and decomposes into a transparent glassy solid — a mixture of sodium metaborate and boric oxide (the basis of the "borax bead test"):

This glassy bead is Y. …

Match each of the three given clues to the known, characteristic behaviour of borax: its hydrolysis giving an alkaline solution, its stepwise dehydration/fusion to a glassy bead on strong heating, and its reaction with a stronger, less volatile acid (conc. H$_2$SO$_4$) to precipitate …

• Identifying X as a simple borate salt without recognising the specific clue combination (alkaline solution + intumescence + boric acid liberation) that uniquely fits borax.
• Forgetting that Y is a mixture (NaBO$_2$ + B$_2$O$_3$), not a single pure compound. …