Question

Why is sodium chloride less soluble in heavy water than in ordinary water?

Answer 1

Inorder for a ionic compound to solubilise in solvent it has to first get in between the solvent molecules, for this in case of water molecules it has to break the O----H Hydrogen bond.
However in case of Heavy water the O----D bond is slightly stronger and hence cannot be broken as easily as the H bond. hence the Na and Cl ions are less solvated making them less soluble in heavy water.