Question

What is a solute that would give a water solution whose density is greater than `1.6*g*cm^-3`?

Answer 1

You want a water solution denser than `1.6*g*cm^(-3)`?

Explanation:

You've tried with salt, but sodium chloride is not particularly soluble in water. A saturated solution of salt in water has a density (`rho`) of approx. `1.2*g*mL^-1`. This density does not change much with temperature. For reasons I am not going to go into, halite dissolution in water does not vary with temperature.

There might be other (and cheaper!) alternatives, but a polytungstate solution might fit your requirements. These solutions have very high densities (approx. `3*g*mL^-1)`. Zinc chloride solutions can also have very high densities, but you will have to dig for the details.