(a) When powdered borax is heated strongly on the bunsen burner, it forms colourless transparent glassy (glass like) bead made of sodium metaborate and boric anhydride. $\mathrm{Na}_2\mathrm{B}_4\mathrm{O}_7 \cdot 10\mathrm{H}_2\mathrm{O}$ $\xrightarrow{Heat}$ $\mathrm{Na}_2\mathrm{B}_4\mathrm{O}_7$ $\xrightarrow{Heat}$ $\underset{\text{Transparent glassy bead}}{2\mathrm{NaBO}_2+\mathrm{B}_2\mathrm{O}_3}$ (b) It dissolves in water behaving as Lewis acid. It forms $[\mathrm{B}(\mathrm{OH})_4]^{-}$ by accepting a pair of electrons from OH–. $\mathrm{B}(\mathrm{OH})_3 + \mathrm{H}-\mathrm{OH}$ → $[\mathrm{B}(\mathrm{OH})_4]^{-} + \mathrm{H}^{+}$. (c) Aluminium dissolves in NaOH solution to form a soluble complex and liberates hydrogen gas. $2\mathrm{Al}_{(s)} + 2\mathrm{NaOH}_{(aq)} + 6\mathrm{H}_2\mathrm{O}_{(l)}$ → $\underset{\text{Soluble complex}}{2\mathrm{Na}[\mathrm{Al}(\mathrm{OH})_4]_{(aq)}} + 3\mathrm{H}_{2(g)}$ (d) $\mathrm{BF}_3$ (Lewis acid in nature) forms an addition compound with $\mathrm{NH}_3$ (Lewis base in nature). $\underset{\text{Lewis base (Acceptor)}}{\mathrm{F}-\overset{\overset{\mathrm{F}}{\vert}}{\underset{\underset{\mathrm{F}}{\vert}}{\mathrm{B}}}}$ + $\underset{\text{Lewis base (Donor)}}{:\overset{\overset{\mathrm{H}}{\vert}}{\underset{\underset{\mathrm{H}}{\vert}}{\mathrm{N}}}-\mathrm{H}}$ → $\underset{\text{Addition compound}}{\left[\mathrm{F}-\overset{\overset{\mathrm{F}}{\vert}}{\underset{\underset{\mathrm{F}}{\vert}}{\mathrm{B}}} \leftarrow :\overset{\overset{\mathrm{H}}{\vert}}{\underset{\underset{\mathrm{H}}{\vert}}{\mathrm{N}}}-\mathrm{H}\right]}$