Properties of aluminium
Aluminium is an active metal, but in air its surface is covered by a thin, dense Al₂O₃ film that protects the metal from further oxidation and from reacting with water. If the film is removed, Al burns in oxygen: 4Al + 3O₂ → 2Al₂O₃, and reacts with sulfur 2Al + 3S → Al₂S₃ and with halogens. With dilute acids it releases hydrogen: 2Al + 6HCl → 2AlCl₃ + 3H₂↑; in cold concentrated HNO₃ and H₂SO₄ the film is strengthened and the metal is passivated. Aluminium is amphoteric: an alkali solution dissolves both the oxide film and the metal: 2Al + 2NaOH + 6H₂O → 2Na[Al(OH)₄] + 3H₂↑. Being a strong reducing agent, aluminium displaces less active metals from their oxides (aluminothermy): 8Al + 3Fe₃O₄ → 4Al₂O₃ + 9Fe; a mixture of Al and Fe₂O₃ is called thermite and is used to weld rails.
Fill in the table: “Reagent” (O₂, S, HCl, NaOH solution, H₂O), “Does it react?”, “Products”. The thermite reaction gives a very high temperature; only specialists carry it out under special conditions and it must never be tried alone at home or at school.