Al2O3 is amphoteric: dissolves in acid (Al2O3 + 6HCl → 2AlCl3 + 3H2O) and base (Al2O3 + 2NaOH → 2NaAlO2 + H2O). AlCl3 is covalent and dimerises to Al2Cl6 in vapour phase — each Al achieves octet through coordinate bonds from bridging Cl atoms. Thermite reaction: 2Al + Fe2O3 → Al2O3 + 2Fe ($Delta_H$ = -850 kJ/mol, temp reaches ~2500°C). Used for welding railway tracks. Al is extracted by Hall-Heroult process: electrolysis of Al2O3 dissolved in cryolite (Na3AlF6) at ~950°C. Aluminium shows only +3 state (no inert pair effect — 3$s^2$ 3$p^1$, all three readily removed).