Dissolving a solute in a solution is either an exothermic or an endothermic process.

When sodium hydroxide dissolves in water, heat is transferred from the solution to the surrounding water causing the temperature of the water to increase. This is an exothermic process.

In endothermic processes, such as dissolving ammonium chloride in water, heat is absorbed by the solution causing the temperature of the water to decrease.

At constant pressure, the heat released or absorbed is called the enthalpy change.

Solution formation has three steps, each associated with a corresponding enthalpy change.

Step one is the separation of the solute particles. This requires an input of energy to overcome the attractive forces between the solute particles.

Step two is the separation of the solvent particles. This is also an endothermic step since energy is required to disrupt the attractive forces between the solvent particles.

Step three occurs when the solute and solvent particles mix. This step is exothermic because the attractive interactions between solute particles and solvent particles release energy.

For a stepwise process, Hess’s Law states that the net enthalpy change is the sum of the enthalpy changes in each step. The sign of the net enthalpy depends on the magnitudes of the enthalpies of the components.

If the sum of the component enthalpies is less than the enthalpy of mixing, the net enthalpy change is negative, and the dissolution process is exothermic.

If the sum of the component enthalpies is greater than the enthalpy of mixing, the enthalpy change is positive, and the dissolution process is endothermic.

If the two are equal, heat is neither released nor absorbed.

Solution formation is different from a chemical reaction. When a solute is dissolved in a solvent, the change is physical. Upon evaporating the solution, the solute can be recovered.

On the contrary, a chemical reaction alters the properties of the reactants. When copper hydroxide is dissolved in hydrochloric acid, evaporating the solution will not return copper hydroxide.

Instead, we will obtain the product, copper chloride.