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.