Statement I: Enthalpy changes are positive when .10, .5 and salt like , , etc. is dissolved in water. But enthalpy changes are negative when anhydrous salts capable of forming hydrates are dissolved i

Question

Statement I: Enthalpy changes are positive when $Na_2SO_4$.10$H_2O$, $CuSO_4$.5$H_2O$ and salt like $NaCl$, $KCl$, etc. is dissolved in water. But enthalpy changes are negative when anhydrous salts capable of forming hydrates are dissolved in water.

Statement II: The difference in the behaviour is due to a large difference in the molecular masses of hydrated and anhydrous salts.

Prashant Kotian · Accepted Answer

Correct answer: C. Anhydrous salts capable of forming hydrates undergo hydration upon dissolution, which is highly exothermic (ΔH < 0). Hydrated salts (which already have water of crystallization) do not undergo hydration upon dissolution, and the breakdown of their crystal lattice absorbs heat (ΔH > 0). This is due to hydration enthalpy, not molecular mass. Statement I is correct, Statement II is incorrect.

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