Solvation

The interaction between solute and solvent molecules is known as solvation or dissolution; when
water is the solvent, it is also known as hydration and the resulting solution is known as an aqueous
solution. Solvation is possible when the attractive forces between solute and solvent are stronger
than those between the solute particles, i.e., when solute-solvent interactions overcome solute-
solute interactions. It then becomes more energetically favorable for the solute particles to be
surrounded each by the solvent rather than stay close together. For example, when NaCl dissolves in
water, its component ions (Na+ and Cl–) dissociate from one another and become surrounded by
water molecules. Because water is polar, ion-dipole interactions can occur between the Na+ and Cl–
ions and the water molecules. For nonionic solutes, solvation involves van der Waals forces between
the solute and solvent molecules. The general rule is that like dissolves like: Ionic and polar solutes
are soluble in polar solvents, and nonpolar solutes are soluble in nonpolar solvents.

This same generalization also applies to cases where the solute is not a solid. Water and oil do not
mix, for example, because they do not “dissolve each other.” The interactions between water
molecules and the nonpolar molecules of oil are nowhere near as strong as the hydrogen bonding
among water molecules themselves, which would need to be disrupted if mixing were to occur. The
two liquids therefore stay separate from each other.

Gases, in contrast, can dissolve in liquid. Gas solubility depends on pressure and temperature.
Henry’s law states that, at constant temperature, the solubility of a gas in a liquid is directly
proportional to the partial pressure of the gas on the surface of the liquid. For example, in a closed
container, the amount of carbon dioxide dissolved in water is proportional to the partial pressure of
the carbon dioxide above the liquid. If you open the container, the carbon dioxide escapes into the
atmosphere, and the solubility of the gas in the water decreases according to Henry’s law.