Statement I : On mixing with water, Plaster of Paris hardens. Statement II : By combining with water, Plaster of Paris is converted into Gypsum.

Statement I is true as Plaster of Paris (POP) is a white powder that, when mixed with water, sets into a hard solid mass [c1, t4]. Statement II is also true and provides the correct chemical explanation for this hardening: POP, which is calcium sulfate hemihydrate (CaSO4·0.5H2O), reacts with water to undergo a hydration process [t1, t7]. This reaction converts the hemihydrate back into calcium sulfate dihydrate (CaSO4·2H2O), commonly known as gypsum [c1, t3]. The chemical equation CaSO4·0.5H2O + 1.5H2O → CaSO4·2H2O illustrates this transformation [c1, t6]. The formation of an interlocking network of gypsum crystals during this rehydration is what causes the material to solidify and harden, making Statement II the direct reason for the observation in Statement I [t1, t10].

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 2: Acids, Bases and Salts > Plaster of Paris > p. 33
2. [2] https://www.sciencedirect.com/science/article/abs/pii/S0950061821014744
3. [3] https://www.sciencedirect.com/science/article/abs/pii/S0960897407000034