A non-volatile solute has a low tendency to escape as vapor from a solution. Its vapor pressure is less than the vapor pressure of the solvent.
When a non-volatile solute is added to a solution, the solvent becomes diluted. This alters the chemical potential of the solvent while it is in the liquid phase, but not when it is in the gas phase. This results in the equilibrium temperature between the liquid and gas phases being higher for the solution. The addition of a non-volatile solute also causes freezing-point depression. Solvents with non-volatile solutes remain in the liquid phase for a larger temperature range.