Solubility
The solubility of a substance in a liquid is determined by the two following factors:
- The amount of energy required to break the intermolecular forces in the substance and the liquid
- The amount of energy released by the intermolecular bonds forming between the substance and the liquid
In general, a substance is soluble if the amount of energy released is greater than the amount of energy required, and the solubility increases with the difference between these two.
One rule that may generally be applied to determining the solubility of a substance is that 'like dissolves like'. (This is not a scientific term but rather a useful guideline that may be used to determine the solubility of a substance in a liquid.
The following is a discussion of the reason that the 'like dissolves like' rule works, considering cases for the solute (the substance being dissolved) and the solute (the liquid the substance is dissolved in).
- A polar / ionic solute is dissolved in a polar solvent. In this case, similar amounts of energy are required to break the intermolecular forces in each and are released by the formation of dipole-dipole / ion-dipole forces between the solute and the solvent, and therefore the solute is soluble.
- A non-polar solute is dissolved in a polar solvent. In this case, a large amount of energy is required to break the intermolecular forces between the molecules of the polar solvent (the dipole-dipole forces). However, the non-polar solute can only form intermolecular bonds with the polar solvent through relatively weak dispersion forces. Therefore, more energy is required to break the intermolecular forces within the polar solvent and the non-polar solute than is released by the formation of dispersion forces between solute and solvent, and the solute is unlikely to dissolve i.e. is insoluble.
- A polar solute is dissolved in a non-polar solvent. This has a similar result to the above.
- A non-polar solute is dissolved in a non-polar solvent. Only dispersion forces must be broken and only dispersion forces are formed, so the energy required and energy released are approximately balanced, so the solute is likely to dissolve.