Q.The alum which is used for the purification of water is

Aluminium is a strongly electropositive metal (unlike boron, it readily loses three electrons to form $Al^{3+}$, since its combined first-three ionisation enthalpy is much lower than boron's) and shows clear amphoteric character: it dissolves in dilute HCl liberating hydrogen ($2Al + 6HCl \rightarrow 2Al^{3+} + 6Cl^- + 3H_2$), and it also dissolves in hot aqueous NaOH, again liberating hydrogen and forming the soluble tetrahydroxoaluminate complex ($2Al + 2NaOH + 6H_2O \rightarrow 2Na^+[Al(OH)_4]^- + 3H_2$) — this is the basis of using an NaOH–aluminium mixture as a drain cleaner, since the reaction generates gas pressure and heat that dislodge blockages. Concentrated nitric acid, by contrast, makes aluminium passive by building a tough, self-limiting oxide film on its surface — this same protective oxide layer is why aluminium containers can safely transport concentrated $HNO_3$, and why aluminium metal generally resists corrosion in air. Aluminium chloride, in acidified aqueous solution, forms the octahedral, $sp^3d^2$-hybridised complex ion $[Al(H_2O)_6]^{3+}$, using al …