When calcium metal reacts with sulphuric acid, the reaction stops after a short while. Why?
The reaction of calcium with sulphuric acid is not feasible at room temperature
Formed calcium sulphate is insoluble in water and covers the metal, preventing it from reacting any further
Formed calcium sulphate requires two molecules of water to crystallize as gypsum
Formed calcium sulphate is unstable
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UPSC CAPF – 2021
Calcium is a relatively reactive metal that reacts with acids like sulfuric acid to produce hydrogen gas and a salt: Ca(s) + H₂SO₄(aq) → CaSO₄(s) + H₂(g). However, calcium sulfate (CaSO₄) is sparingly soluble in water, and its solubility is even lower in concentrated sulfuric acid. As the reaction proceeds, a layer of solid calcium sulfate forms on the surface of the calcium metal. This layer acts as a barrier, preventing the sulfuric acid from coming into contact with the underlying calcium metal, effectively stopping or significantly slowing down the reaction after a short while. This phenomenon is known as passivation.