Which one of the following reactions does not result in the evolution of hydrogen gas ?
- A.Reaction of zinc metal with dilute sulphuric acid solution
- B.Mixing water to Plaster of Paris
- C.Heating zinc metal with sodium hydroxide solution
- D.Reaction of potassium metal with water
Correct answer
Explanation
The correct answer is B, Mixing water to Plaster of Paris. Here water only combines with calcium sulphate hemihydrate to form gypsum; no gas is given off and the paste simply sets hard.
Hydrogen is released when a reactive metal displaces it from an acid, from water or, for amphoteric metals, from a strong alkali. The setting of plaster of Paris is a hydration: CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O. It gives out a little heat but no hydrogen.
A is wrong because zinc displaces hydrogen from dilute sulphuric acid: Zn + H₂SO₄ → ZnSO₄ + H₂. This is the usual laboratory method of making hydrogen.
C is wrong because zinc is amphoteric and reacts with hot sodium hydroxide to form sodium zincate and hydrogen: Zn + 2NaOH → Na₂ZnO₂ + H₂.
D is wrong because potassium reacts violently with cold water to form potassium hydroxide and hydrogen, which often catches fire.
Exam tip: metals with acids, very reactive metals with water, and zinc or aluminium with NaOH all give hydrogen.
Practice Questions
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