[kuttaman]

Why does CH2Cl2 solvent (lower layer) have higher density than water while non-chlorinated solvents like hexanes and ether (top layer) have lower density than water, i.e. what exactly is it about Cl that causes this difference?

[Schrödinger]

For starters, one atom of Cl weighs the same as two molecules of water. Cl is certainly heavier.

But that alone wont do. Methane is a gas and is considerably less dense than water. Consider CH₃Cl. It is heavier (molar mass) than water, has one Cl atom yet is less dense than water. On the contrary, H₂O < CH₂Cl₂ < CHCl₃ < CCl₄ (increasing order of density)

So, it probably makes sense when you have some 'heavy atoms' in the molecule. And in this case, for our liking, the entire molecule happens to be compact too.