Heats of solution and crystallization

When a solute dissolves in a solvent without reaction, heat is usually absorbed from the surrounding medium (still commonly referred to as the heat of solution), i.e. if the dissolution occurs adiabatically the solution temperature falls. When a solute crystallizes out of its solution, heat is usually liberated (still commonly referred to as the heat of crystallization) and the solution temperature rises. The reverse cases, viz. heat evolution on dissolution and heat absorption on crystallization, may be encountered with solutes that exhibit an inverted solubility characteristic, e.g. anhydrous sodium sulphate in water. 

The dissolution of an anhydrous salt in water at a temperature at which the hydrated salt is the stable crystalline form frequently leads to the release of heat energy, owing to the exothermic nature of the hydration process  

The enthalpy changes associated with dissolution (A/7soi) and crystallization (AT/crys) are generally recorded as the number of heat units liberated by the system when the process takes place isothermally. According to this system of nomenclature, if an adiabatic operation is considered, the expression AT/soi = (heat units per unit mass of solute) means that the solution temperature will increase A/7soi = —q means that it will fall. 

The magnitude of the heat effect accompanying the dissolution of solute in a given solvent or undersaturated solution depends on the quantities of solute and solvent involved, the initial and final concentrations and the temperature at 

In crystallization practice, however, it is usual to take the heat of crystallization as being equal in magnitude, but opposite in sign, to the heat of solution at infinite dilution, since this is the quantity most commonly available 

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