When calcium metal reacts with sulphuric acid, the reaction stops after a short while. Why?

Correct — B, Formed calcium sulphate is insoluble in water and covers the calcium metal, preventing it from reacting any further. Calcium is high in the reactivity series, so it attacks dilute sulphuric acid at once and hydrogen bubbles off. The salt produced, calcium sulphate, is only sparingly soluble — around two grams in a litre of water — so instead of dissolving away it deposits as a crust on the metal surface. That crust is a physical barrier between the acid and the calcium underneath, and once it is continuous the reaction stops even though plenty of acid and plenty of calcium remain.

• (a)The reaction of calcium with sulphuric acid is not feasible at room temperature — Feasibility is not the issue. Calcium reacts with dilute sulphuric acid vigorously at room temperature; the stem itself says the reaction runs before it stops.
• (c)Formed calcium sulphate requires two molecules of water to crystallize as gypsum — Calcium sulphate does crystallise with two molecules of water as gypsum, CaSO4·2H2O, so the chemistry in this option is sound. It is simply not the reason the reaction halts — the barrier does that, not the water of crystallisation.
• (d)Formed calcium sulphate is unstable — Calcium sulphate is a stable salt. It is quarried as gypsum and used to make plaster of Paris and to regulate the setting of cement, which is not the behaviour of an unstable compound.

When a metal reacts with an acid, the reaction can only continue while fresh metal is exposed to fresh acid. If the salt formed is soluble it washes away and the surface stays clean. If the salt is insoluble it stays put and passivates the metal. Calcium with sulphuric acid is the classroom case; lead with dilute sulphuric or hydrochloric acid behaves the same way, and so does the oxide layer that protects aluminium in air.

Two of the four options are chemically true statements about calcium sulphate — that it crystallises with two waters as gypsum, and, by implication, that it forms at all. The question asks for a cause, not a fact, so the test is whether a candidate can tell a true statement from a relevant one. Only the coating explains a reaction that begins and then stops of its own accord, which is what the stem describes.

• Calcium sulphate is sparingly soluble in water, roughly 2 g per litre.
• The insoluble salt coats the metal and blocks further contact with the acid.
• Gypsum is calcium sulphate dihydrate, CaSO4·2H2O; plaster of Paris is the hemihydrate.
• Lead shows the same effect with dilute sulphuric and hydrochloric acids, whose lead salts are also insoluble.
• Calcium lies above hydrogen in the reactivity series, so it displaces hydrogen from dilute acids.

• Choosing the gypsum option because the chemistry in it is correct, without asking whether it explains the stopping.
• Assuming a reaction that stops must have run out of a reactant.
• Thinking calcium is too unreactive for dilute acid at room temperature.

A cause-and-effect item where more than one option is factually true; the examiner is testing whether the candidate can separate a correct statement from the operative reason.

• practice — not a real PYQ

  Lead reacts only briefly with dilute hydrochloric acid and then stops. The best explanation is that

  • (a)lead is below hydrogen in the reactivity series
  • (b)insoluble lead chloride coats the metal surface
  • (c)the acid is neutralised almost immediately
  • (d)lead chloride decomposes as fast as it forms

  Answer(b) insoluble lead chloride coats the metal surface — the same passivating effect as calcium sulphate on calcium.

• practice — not a real PYQ

  Gypsum is chemically

  • (a)CaSO4·2H2O
  • (b)CaSO4·½H2O
  • (c)CaCO3
  • (d)CaO

  Answer(a) CaSO4·2H2O — the hemihydrate CaSO4·½H2O is plaster of Paris.