How Solutions Form

A solution is a homogeneous mixture of two or more substances. The substance that dissolves a material to form a solution is called the solvent. The dissolved material in a solution is a solute. Thus the solvent dissolves the solute, and together they form a solution. A solution is perhaps most commonly thought of as the liquid that results when a solid solute dissolves into a liquid solvent, as in the case of sugar dissolved in water. In fact solids, liquids, and gases can participate as a solution. A mixture of two or more gases, such as air, is a solution. Gas can also be dissolved in a liquid or in a solid. Carbonated beverages contain dissolved CO₂ gas, and H₂ gas can be dissolved in solid palladium (Pd). Any mixture of two liquids, such as ethanol and water, is also a solution. Two or more solids can also be mixed to form a new solid, as in the case of many alloys. An alloy is a mixture of two or more metallic elements.

In a solution, the solute particles are evenly distributed throughout the mixture with the solvent, and a solution is therefore homogeneous by definition. When a solution forms, three things must happen. First, the solute-solute molecules move apart from one another as the solute dissolves. The solvent-solvent molecules must also move apart from one another to make room for the dissolved solute molecules. Because the like molecules are attracted to one another, both of these processes are endothermic. Lastly, as the molecules mix, the solute and solvent molecules are drawn to one another by attractive intermolecular forces, an exothermic process. Energy is absorbed by an endothermic reaction and released by an exothermic reaction. Enthalpy (H) is the internal energy of a system plus the work needed to displace the environment to produce the components of the system. The enthalpy change (${\Delta H}$) of endothermic processes is always positive, and the enthalpy change of exothermic processes is always negative. The sum of the ${\Delta H}$ associated with the three steps in the formation of a solution, called the enthalpy of solution (${\Delta H}_{\rm{soln}}$), is the net enthalpy change of the process.

This three-step process can be summarized as:

1. Solute molecules spread apart: $\Delta H_1>0$.

2. Solvent molecules spread apart: $\Delta H_2>0$.

3. Solvent and solute molecules move together: $\Delta H_3<0$.

And $\Delta H_{\rm{soln}}=\Delta H_1+\Delta H_2+\Delta H_3$. When a solute molecule is completely surrounded by solvent molecules, it is said to be solvated. A solute is hydrated if it is solvated by water molecules. Hydration enthalpy is the energy released when one mole of solute becomes completely hydrated. This energy explains why water is able to dissolve ionic compounds, which have strong attractive forces between the positive and negative ions. The hydration enthalpy for ionic compounds is approximately equal to the energy needed to separate the ionic compound into its ion components.