Assignment:
The question arises concerning the density of saltwater: is it because water molecules have moved out of the system, replaced by sodium and chlorine ions, which are heavier, or have the ions moved into the interstices, creating a greater number of particles in the solution?
By the gas laws, a cubic meter of gas always maintains a constant number of particles (at equal temp and pressure). Thus, damp air has the same number of particles as dry air, only some particles are lighter (the water). But does this apply to liquids?
The molarity and molality measures, without being rigorous, show that there is a difference in a solution of exactly 1 liter and a solution of 1 liter water plus some solute. So the inference is that solute takes us space; therefore, the added solute does not completely find hiding places among the water molecules. So the question is half answered: solute does not completely nestle among solvent, but must necessarily replace, or bump out, water from a solution. But does "any" of the salt find hiding places among the water, or is it a one-for-one replacement of water for, what? an ion of solute?