The enthalpy of dilution, also called the heat of dilution, is the enthalpy change associated with the process of diluting a component in a solution at constant pressure. It represents the heat absorbed or released when a certain amount of solvent is added to a solution, thereby changing its concentration. When the initial state is a pure liquid component dissolved in solution, the enthalpy of dilution is equivalent to the enthalpy of dissolution (or heat of solution). Enthalpy of dilution can be characterized in two ways:

• Differential enthalpy of dilution: This is the enthalpy change caused by adding a very small amount (one mole) of solvent to a large amount of solution, such that the concentration changes negligibly. It captures the enthalpy change on a microscopic scale.
• Integral enthalpy of dilution: This reflects the enthalpy change over a macroscopic scale when a known amount of solution is diluted from an initial to a final concentration, normalized per mole of solute. If the dilution is to infinite dilution, this gives the limiting enthalpy change when the solute is infinitely diluted.

The enthalpy of dilution is usually expressed in units of energy per mole of solute (e.g., kJ/mol) or per unit mass. In summary, the enthalpy of dilution quantifies the heat effect when a solution undergoes dilution due to solvent addition at constant pressure and temperature.