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

When calcium metal reacts with sulphuric acid, the reaction initially produces calcium sulphate and hydrogen gas. However, the reaction stops shortly after it begins because the formed calcium sulphate is insoluble in water and dilute acid [2]. This insoluble salt precipitates and forms a dense, protective coating or layer on the surface of the unreacted calcium metal [2]. This physical barrier prevents the sulphuric acid from coming into contact with the underlying metal, effectively passivating the surface and halting further chemical activity. While calcium sulphate can exist as gypsum (dihydrate) or anhydrite depending on conditions [1], its primary role in this context is acting as an insoluble shield. This behavior contrasts with reactions involving nitric or hydrochloric acid, where the resulting calcium salts are highly soluble, allowing the reaction to proceed until the reactants are consumed [2].

Sources

1. [2] https://nvlpubs.nist.gov/nistpubs/jres/27/jresv27n2p191_A1b.pdf
2. [1] https://homepages.see.leeds.ac.uk/~earlgb/Publications/Vam%20Driessche%20et%20al%20New%20Perspectives%20on%20Mineral%20Nucleation%20and%20Growth%20Springer%202016_Chapter12.pdf