CDS (II) 2020 General Knowledge · Question 99 of 120

Correct — A, Zinc oxide. An oxide that reacts with acids and with bases alike, giving a salt and water in both cases, is called amphoteric, and zinc oxide is one of the two examples every school chemistry course names — the other being aluminium oxide. With hydrochloric acid it behaves as a base: ZnO + 2HCl gives zinc chloride and water. With sodium hydroxide it behaves as an acid: ZnO + 2NaOH gives sodium zincate and water. The other three oxides on the list are oxides of metals that sit well above zinc in reactivity and are basic only — they neutralise acids and do nothing with alkalis. The textbook statement is explicit that aluminium oxide and zinc oxide show both acidic and basic behaviour, which is the sentence this stem is built on.

• (b)Copper oxide — Copper(II) oxide is a basic oxide. It dissolves in dilute acids to give copper salts and water — the familiar reaction with dilute sulphuric acid that produces copper sulphate — but it does not react with sodium hydroxide under ordinary laboratory conditions.
• (c)Magnesium oxide — A strongly basic oxide. It reacts with water to give magnesium hydroxide, an alkali mild enough to be sold as milk of magnesia for indigestion, and neutralises acids. There is no acidic side to it.
• (d)Calcium oxide — Quicklime, the most emphatically basic oxide on the list. Adding water gives slaked lime, calcium hydroxide, with a vigorous release of heat. It reacts with acids and not with alkalis.

Oxides sort into four classes by how they behave towards acids and bases. Basic oxides are metal oxides that neutralise acids — sodium oxide, calcium oxide, magnesium oxide, copper oxide. Acidic oxides are usually non-metal oxides that neutralise bases — carbon dioxide, sulphur dioxide, phosphorus pentoxide. Amphoteric oxides react with both, and the standard pair is aluminium oxide and zinc oxide. Neutral oxides such as carbon monoxide and nitrous oxide react with neither.

Three of the four options are oxides of metals high in the reactivity series — calcium, magnesium and, less so, copper — and metals high in the series give firmly basic oxides. Zinc sits lower, closer to the metal-non-metal borderline of the periodic table, and that borderline position is what produces amphoteric behaviour. If you remember only one thing here, remember the pair aluminium oxide and zinc oxide; it answers most amphoteric-oxide questions in this exam. The product names are worth carrying too, because they are what the follow-up questions ask for: zinc oxide with alkali gives a zincate, aluminium oxide with alkali gives an aluminate.

• An amphoteric oxide reacts with both acids and bases to give a salt and water; aluminium oxide and zinc oxide are the standard examples.
• With hydrochloric acid, ZnO + 2HCl gives ZnCl2 and water; with sodium hydroxide, ZnO + 2NaOH gives sodium zincate and water.
• Calcium oxide, magnesium oxide and copper oxide are basic oxides — they react with acids only.
• Carbon dioxide, sulphur dioxide and phosphorus pentoxide are acidic oxides; carbon monoxide and nitrous oxide are neutral.
• Amphoteric behaviour goes with a position near the metal to non-metal borderline of the periodic table.

• Assuming every metal oxide is basic — zinc and aluminium oxides are the exceptions that get asked about.
• Confusing amphoteric with neutral; a neutral oxide reacts with neither, an amphoteric one with both.
• Testing an oxide with acid only, finding a reaction, and concluding it is amphoteric.

As a one-line 'which oxide is amphoteric' item, or in reverse as 'which of the following is a basic oxide', or as a statement item on reactions with NaOH and HCl.

• practice — not a real PYQ

  Which one of the following is an acidic oxide?

  • (a)Sodium oxide
  • (b)Sulphur dioxide
  • (c)Zinc oxide
  • (d)Calcium oxide

  Answer(b) Sulphur dioxide — a non-metal oxide that dissolves in water to give sulphurous acid and reacts with alkalis to give sulphites.

• practice — not a real PYQ

  When zinc oxide reacts with sodium hydroxide, the salt formed is

  • (a)sodium zincate
  • (b)zinc hydroxide
  • (c)sodium zinc carbonate
  • (d)zinc chloride

  Answer(a) sodium zincate — the reaction in which zinc oxide behaves as an acidic oxide, giving the zincate salt and water.