heat of solution

Have you noticed that some powders make the water feel cold as they dissolve, while others warm it up? That temperature change is the heat of solution: the heat absorbed or released when a solute dissolves in a solvent. It is the net result of energy spent pulling the solute apart and energy gained when solvent molecules wrap around the freed particles.

If wrapping the dissolved particles releases more energy than was needed to break the solid apart, the process gives off heat and warms the solution (exothermic, a negative heat of solution by convention). If breaking the solid apart costs more, the process pulls heat from the surroundings and cools the solution (endothermic, positive). Many salts dissolve endothermically, which is why instant cold packs use a salt dissolving in water.

For formulators the heat of solution is more than a curiosity. It is linked to how solubility changes with temperature: an endothermic dissolution usually means solubility rises as you heat the solvent, while an exothermic one can mean the reverse. It also matters in large-scale manufacture, where dissolving a lot of material quickly can heat or chill a batch enough to affect stability or processing.