Question

$NaCl$ is less soluble in $D_{2}O$ than $H_{2}O$. why?

Answer 1

In order for a ionic compound to be soluble in a solvent it has to be first get in between the solvent molecules, for this in case of water molecules it has to break $O-H$ bonds.

However in case of Heavy water the $O-D$ bond is slightly stronger and hence cannot be broken as easily as $O-H$ bond, hence the $Na$ and $Cl$ are less solvated making them less soluble in heavy water.