溶液中の分子間力

When two substances mix, the forces between the constituent molecules, or intermolecular forces, are disrupted. Intermolecular forces can be of different types, such as dispersion forces between nitrogen molecules, dipole-dipole attractions between hydrochloric acid molecules, hydrogen bonding between ammonia molecules, and ion-dipole interaction between potassium ions and water. For a solute to dissolve in a solvent, solute–solute interactions between solute particles must be disrupted so that solute particles are distributed evenly through the solvent. Solvent–solvent interactions between solvent particles must be disrupted to accommodate the solute particles between the solvent molecules. Solvent–solute interactions between solvent and solute particles must be established so that the substances can mix. The extent to which a solute can dissolve in a solvent depends on how strong these three types of interactions are compared to each other. If the solvent-solute interactions are strong enough to overcome the solute–solute and solvent–solvent interactions, then the solute will readily dissolve in the solvent. Consider a salt solution. Before mixing, sodium and chloride ions in the salt are held together in a crystal by ionic bonding. For water, the molecules interact with each other through hydrogen bonds. When sodium chloride is added to water, the water molecules arrange themselves in such a way that the positive end of the dipole faces the negative chloride ions, and the negative end of the dipole faces the positively charged sodium ions. These ion-dipole attractions weaken the ionic bonds between the sodium and chloride ions so that the ions separate and the crystal structure breaks down. The separated ions are encircled by water molecules. Such interactions are collectively called hydration. The hydrated ions also overcome some of the hydrogen bonding between the water molecules. Now, the salt is said to be dissolved in water.