CDS (I) 2024 General Knowledge · Question 33 of 120

Correct — D, Al2O3. An amphoteric oxide is one that reacts with acids and with bases alike to give a salt and water, and NCERT names aluminium oxide and zinc oxide as the standard examples: 'some metal oxides, such as aluminium oxide, zinc oxide show both acidic as well as basic behaviour'. Aluminium oxide proves it in two equations. With an acid it behaves as a base — Al2O3 + 6HCl gives 2AlCl3 + 3H2O. With an alkali it behaves as an acid — Al2O3 + 2NaOH gives 2NaAlO2 + H2O, the salt being sodium aluminate. No other oxide in this list will do both.

• (a)MgO — Magnesium oxide is a straightforwardly basic oxide. It neutralises acids and dissolves slightly in water to give an alkaline suspension, but it does not react with sodium hydroxide.
• (b)P4O10 — Phosphorus pentoxide is an acidic oxide, as non-metal oxides generally are; with water it gives phosphoric acid and it reacts with bases, but never as a base itself.
• (c)Na2O — Sodium oxide is the most strongly basic oxide on the list — with water it gives sodium hydroxide outright. There is no acidic side to it.

Oxides sort by their acid-base behaviour. Metal oxides are typically basic, non-metal oxides typically acidic, and a small group in between are amphoteric, reacting with both acids and bases to give a salt and water. The amphoteric set clusters around the metals near the metal-non-metal boundary of the periodic table — aluminium and zinc are the school examples, with lead and tin behaving similarly. A fourth category, neutral oxides such as carbon monoxide and nitrous oxide, reacts with neither.

The fastest route through this option list is to sort by element type before thinking about chemistry at all. Sodium and magnesium sit at the far left of the periodic table and give firmly basic oxides. Phosphorus is a non-metal, so its oxide is acidic. That leaves aluminium, the one element on the list that lies near the dividing line, and its oxide is the classic amphoteric case. The sodium aluminate reaction is worth remembering in its own right because it is the chemical basis of the Bayer process for purifying bauxite — alumina dissolves in hot caustic soda while the iron oxide impurities do not.

• An amphoteric oxide reacts with both acids and bases to produce a salt and water.
• NCERT names aluminium oxide and zinc oxide as oxides showing both acidic and basic behaviour.
• Al2O3 + 6HCl gives 2AlCl3 + 3H2O — the basic behaviour.
• Al2O3 + 2NaOH gives 2NaAlO2 + H2O, sodium aluminate — the acidic behaviour.
• Metal oxides are generally basic and non-metal oxides generally acidic; amphoteric oxides cluster near the metal-non-metal boundary.

• Assuming every metal oxide is basic; aluminium and zinc oxides break that rule.
• Choosing P4O10 because phosphorus forms several oxides — variety of oxides is not the same as amphoteric behaviour.
• Confusing an amphoteric oxide with a neutral one; a neutral oxide reacts with neither acid nor base.

Asked as a one-line pick-the-oxide item in which the option set deliberately mixes a strongly basic oxide, an acidic non-metal oxide and the amphoteric one.

• practice — not a real PYQ

  Which one of the following pairs consists of amphoteric oxides?

  • (a)Na2O and CaO
  • (b)Al2O3 and ZnO
  • (c)CO2 and SO2
  • (d)CO and N2O

  Answer(b) Al2O3 and ZnO — the two oxides NCERT names as showing both acidic and basic behaviour.

• practice — not a real PYQ

  Aluminium oxide reacting with sodium hydroxide produces

  • (a)sodium aluminate and water
  • (b)aluminium hydroxide and hydrogen
  • (c)aluminium chloride and water
  • (d)no reaction

  Answer(a) sodium aluminate and water — Al2O3 + 2NaOH gives 2NaAlO2 + H2O, which is the acidic half of amphoteric behaviour.