Answer
In aqueous solutions, ΔHsolvent and ΔH mix combine to make the heat of hydration (ΔH hydration) which is the enthalpy change that happens when one mole of gaseous solute ions is dissolved in water.
Work Step by Step
This process is detailed in Chapter 12 on pages 554-555.
The enthalpy of a solution depends on the relative magnitudes of ΔH solute and ΔH hydration because the ion-dipole interactions that occur are stronger than hydrogen bonds in water, ΔH hydration is mostly negative (therefore exothermic) for ionic compounds. For ionic compounds, the energy required to separate the solute into its separate parts is the negative of the solute's lattice energy (ΔH solute = -ΔH lattice). For aqueous ionic solutions, the overall enthalpy depends on the (relative) magnitude of ΔH solute and ΔH hydration with three possible scenarios (the magnitude is the absolute value of ΔH).
