Q.What happens when

1. $\text{Na}_2\text{B}_4\text{O}_7\cdot 10\text{H}_2\text{O} \xrightarrow{\Delta} \text{Na}_2\text{B}_4\text{O}_7 \xrightarrow{\Delta} 2\text{NaBO}_2 + \text{B}_2\text{O}_3$ (transparent glassy bead).
2. $\text{B(OH)}_3 + 2\text{H}_2\text{O} \rightarrow [\text{B(OH)}_4]^- + \text{H}_3\text{O}^+$ (boric acid is monobasic and acts as a Lewis acid, not a Brønsted acid). …

1. (a) Borax heated strongly. Borax first loses its water of crystallisation on heating, swelling up ("puffs up"/intumescence) into a fluffy, porous mass of anhydrous borax. Further strong heating melts it, and on continued heating it decomposes into a transparent glassy bead — a mixture of sodium metaborate and boric oxide (this is the basis of the borax-bead test):

2. (b) Boric acid added to water. Boric acid is a weak, monobasic acid, but it does not ionise by donating a proton itself. Instead it behaves as a Lewis acid: it accepts a hydroxide ion from a water molecule, releasing a proton in the process:

3. (c) Aluminium treated with dilute NaOH. Aluminium is amphoteric and dissolves in alkali (as well as in acid), liberating hydrogen gas and forming soluble sodium aluminate: $2\text{Al} + 2\text{NaOH} + 2\text{H}_2\text{O} \rightarrow 2\text{NaAlO}_2 + 3\text{H}_2\uparrow$ …

Treat each sub-part as a short reaction-prediction question grounded in the known chemical behaviour of boron/aluminium compounds: borax's stepwise thermal dehydration/decomposition, boric acid's Lewis-acid mode of "acidity," aluminium's amphoteric reaction with alk …

• Writing boric acid as ionising directly like a normal Brønsted acid ($\text{H}_3\text{BO}_3 \to \text{H}^+ + \text{H}_2\text{BO}_3^-$) instead of via its Lewis-acid hydrolysis mechanism. …