12.1: Solution Formation
There is no one solvent that can dissolve every type of solute. Some substances that readily dissolve in a certain solvent might be insoluble in a different solvent. A simple way to predict which substances dissolve in which solvent is the phrase "like dissolves like". This means that polar substances, such as salt and sugar, dissolve in a polar substance like water. In contrast, non-polar substances are more soluble in non-polar solvents such as carbon tetrachloride.
This selective solubility can be explained by the intermolecular forces within the molecules of solute and solvent and those between solute and solvent molecules in solution. Stronger intermolecular forces between the solute molecules and solvent molecules ensure greater solubility of the solute in the solvent. Ionic interactions and hydrogen bonding that usually hold polar solutes together can only be overcome by other strong forces, like the dipole-dipole attractions between polar solute molecules and polar solvent molecules.
Dispersion forces between non-polar solutes are overcome predominantly by dispersion forces between the non-polar solute molecules and the non-polar solvent molecules and are not strong enough to break polar interactions. While non-polar substances like iodine and carbon dioxide can dissolve in water, their solubility is limited.
A solution must be homogeneous; that is, it must have a uniform appearance and the same concentration of the solute throughout the solvent. Consider a sugar syrup and pure water are in the same tank but separated by a barrier. When the barrier is removed, the liquids mix together spontaneously to form a homogeneous solution. This phenomenon is called concentration equilibration.