Which one of the following metal oxides does not usually react with sodium hydroxide?

Correct — D, Calcium oxide. Sodium hydroxide attacks amphoteric oxides, the ones that behave as an acid towards a base and as a base towards an acid. Aluminium oxide is the standard example: Al2O3 + 2NaOH → 2NaAlO2 + H2O, giving soluble sodium aluminate. Calcium oxide has no such character. It is the oxide of an alkaline-earth metal and is purely basic, so it has nothing to offer a base — indeed lime is used in industry to make caustic soda from soda ash, not to consume it.

• (a)Copper oxide — Copper(II) oxide is a transition-metal oxide with a weak amphoteric streak. It dissolves in hot concentrated alkali to give cuprate(II), so of the three basic oxides on this list it is the one with a real claim to reacting.
• (b)Aluminium oxide — Aluminium oxide is the amphoteric oxide of the set and does react with sodium hydroxide, forming sodium aluminate. It is the option the question is built to exclude.
• (c)Potassium oxide — Potassium oxide is the most strongly basic oxide here, and it will not react with a base. Dropped into aqueous sodium hydroxide it is destroyed at once by the water present, K2O + H2O → 2KOH, so it does not simply stand aside either.

Metal oxides sort into three classes. Basic oxides such as Na2O, K2O, CaO and MgO react with acids to give a salt and water. Acidic oxides such as CO2 and SO2 react with bases. Amphoteric oxides — aluminium oxide and zinc oxide are the two named in school textbooks — react with both, which is why aluminium and zinc metal both dissolve in caustic soda solution with hydrogen evolved.

Work from the option that certainly reacts rather than from the three that might not. Aluminium oxide is the amphoteric member and therefore the one the negative stem is asking you to leave out, and once it is set aside the intended answer is whichever remaining oxide is furthest from amphoteric. That is the alkaline-earth oxide, calcium oxide. The item is worth flagging as loosely set: on a strict reading potassium oxide does not react with sodium hydroxide either, and a cleaner version of this question would have asked which oxide does react.

• Amphoteric oxides react with both acids and bases; aluminium oxide and zinc oxide are the standard examples.
• Al2O3 + 2NaOH → 2NaAlO2 + H2O, giving soluble sodium aluminate.
• Basic oxides are those of the alkali and alkaline-earth metals — Na2O, K2O, CaO, MgO.
• Aluminium and zinc metal both dissolve in hot sodium hydroxide solution with hydrogen released.
• Sodium hydroxide is manufactured industrially by the chloralkali process, and historically from soda ash and lime.

• Assuming every metal oxide reacts with a strong alkali.
• Missing that zinc oxide behaves like aluminium oxide, which is often the answer in the positively phrased version.
• Reading a negative stem as a positive one and marking the oxide that does react.

Set as a negative single-answer item on amphoterism; sibling papers ask the same idea positively, either by naming the amphoteric oxide or by asking which reaction with alkali releases hydrogen.

• practice — not a real PYQ

  Which one of the following oxides reacts with both hydrochloric acid and sodium hydroxide?

  • (a)Magnesium oxide
  • (b)Zinc oxide
  • (c)Sodium oxide
  • (d)Calcium oxide

  Answer(b) Zinc oxide — it is amphoteric, giving zinc chloride with the acid and sodium zincate with the alkali.

• practice — not a real PYQ

  Aluminium metal dissolves in hot sodium hydroxide solution and a gas is released. That gas is

  • (a)oxygen
  • (b)hydrogen
  • (c)carbon dioxide
  • (d)nitrogen

  Answer(b) hydrogen — sodium aluminate is formed and hydrogen escapes.