Are you reffering to heat of solution?

When a solute is dissolved in a solvent to form a solution, there is frequently an evolution or absorption of heat. The heat change per mole of solute dissolved is not constant, but usually varies with the concentration of the solution. If the total change of enthalpy at constant temperature observed when m moles of solute are added to a definite quantity,e.g., 1000 grams, of solvent is plotted against m, for various values of the entalpy change, a curve is obtained. The curve reaches a limit when the solution is saturated at the experimental temperature; the solution then contains ms moles of solute to 1000grams of solvent. The height of the ordinate(enthalpy change) at any point,e.g.X, divided by the corresponding number of moles mx of solute dissolved,i.e.,enthalpy change delta Hx/mx , represents the increase of enthalpy per mole of solute when it dissolves to form a solution of a particular concentration; thisn quantity is called the integral heat of solution at the given concentration. It is observed that the integral heat of solution is, in general, approximately constant in dilute solution, but becomes smaller with increasing concentration.

The fact that the heat of solution of a solute varies with its concentration implies that there must be a change of enthalpy when a solution is diluted by the addition of solvent. The internal heat of dilution is the change in enthalpy when a solution containing 1 mole of solute is diluted from one concentration to another.