Which one of the following oxides reacts with both acid and base?

Correct — A, Aluminium oxide. An amphoteric oxide reacts with acids and with bases, and Al₂O₃ is the standard school example. With acid it behaves as a base — Al₂O₃ + 6HCl gives 2AlCl₃ + 3H₂O. With alkali it behaves as an acid — Al₂O₃ + 2NaOH + 3H₂O gives 2Na[Al(OH)₄], sodium tetrahydroxoaluminate. The dual behaviour follows from aluminium's position on the metal-nonmetal borderline of the periodic table, where the oxide is neither firmly basic nor firmly acidic.

• (b)Calcium oxide — CaO is a strongly basic oxide. It reacts with acids and with water to give slaked lime, but alkali leaves it untouched.
• (c)Sodium oxide — Na₂O is one of the most basic oxides there is — it dissolves in water to give sodium hydroxide and does nothing with a base.
• (d)Potassium oxide — K₂O is basic for the same reason as Na₂O, and even more vigorously so, since basic character increases down Group 1.

Oxides sort themselves by the character of the element bonded to oxygen. Metals on the left of the periodic table give basic oxides, non-metals on the right give acidic oxides, and the elements on the diagonal borderline — aluminium, zinc, lead, tin, beryllium — give oxides that answer to both, which is what amphoteric means.

Three of the four options are Group 1 or Group 2 metals, so a candidate who knows only that 'metal oxides are basic' will have nothing to separate them and will guess. The discriminator is that aluminium sits in Group 13 on the metal-nonmetal boundary. Zinc oxide and lead oxide belong to the same short list and are the usual substitutes when the question is reset.

• Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O shows the basic side; Al₂O₃ + 2NaOH + 3H₂O → 2Na[Al(OH)₄] shows the acidic side.
• The commonly examined amphoteric oxides are Al₂O₃, ZnO, PbO and SnO; BeO belongs to the same family.
• Basic character of oxides increases down a group and decreases across a period from left to right.
• Amphoteric is not the same as neutral — CO, NO and N₂O are neutral oxides, reacting with neither acid nor base.

• Assuming every metal oxide must be basic, which leaves the four options indistinguishable.
• Treating amphoteric as a synonym for neutral.
• Forgetting zinc oxide and lead oxide, which are the substitutes when this item reappears.

Asked directly as 'which oxide is amphoteric', or indirectly through a metal that dissolves in both hydrochloric acid and sodium hydroxide.

• practice — not a real PYQ

  Which one of the following metals dissolves both in dilute hydrochloric acid and in hot sodium hydroxide solution?

  • (a)Copper
  • (b)Zinc
  • (c)Silver
  • (d)Magnesium

  Answer(b) Zinc — zinc and its oxide are amphoteric, so zinc gives zinc chloride with acid and sodium zincate with alkali; magnesium reacts only with the acid.

• practice — not a real PYQ

  Carbon monoxide is described as a neutral oxide because it

  • (a)reacts with both acids and bases
  • (b)reacts with acids only
  • (c)reacts with bases only
  • (d)reacts with neither acids nor bases

  Answer(d) reacts with neither acids nor bases — that is the definition of a neutral oxide, and it is what separates neutral from amphoteric.