Which calcium compound is not appreciably more soluble in 0.1 m hydrochloric acid than it is in pure water? (a) limestone, caco3 (b) slaked lime, ca(oh)2 (c) gypsum, caso4 • 2 h2o (d) hydroxyapatite, ca5(oh)(po4)3?

The answer is "c", "Gypsum, CaSO₄ • 2 H₂O".
Because in option A,B and D there is a component which acts as a base and reacts with the acid like CO₃²⁻ in limestone, CaCO₃, OH⁻ in slaked lime, Ca(OH)₂ and OH⁻ in hydroxyapatite, Ca₅(OH)(PO₄)₃.
However in Gypsum, CaSO₄ • 2 H₂O,  SO₄²⁻ are weak bases as they are the conjugate bases of strong acid H₂SO₄ .
Thus option C shows minimum tendency of reaction with HCl and hence minimum solubility. 

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