In general, in what type of solvent (non-polar, moderately polar, or highly polar) are polar solutes most soluble? Explain why.

In general, polar solutes are most soluble in highly polar solvents.

Explanation:

The general rule is "like dissolves like" which means that polar solvents dissolve polar (or ionic) solutes and non-polar solvents dissolve non-polar solutes.

In order for a solvent dissolve a solute, the strength of the interacttion (force) between the solute and the solvent units (atoms, molecules, or ions) must be stronger than the strength of the forces that keep together he particles of the pure substances (known as intermolecular forces).

Since the nature of the interactions between the units are electrostatic, the more polar is the solvent the better it will be able to attract and surround the solute particles, keeping them separated and in solution. That mechanism explains why polar solutes will be most soluble in highly polar solvents.