For any meaningful discussion of solvents we need to know how they work. The process of dissolving involves the interaction of solvent and solute to form a solution. For example water, a solvent, will dissolve salt, a solute, to form salt solution whereas mixing paraffin wax into water will never produce a solution no matter how much effort is invested.

Without some basic knowledge, anyone involved in making solvent-based preparations, or even adding solvents to aqueous systems, is left with nothing more to go on than a set of trial and error experiments a costly, lengthy and possibly risky exercise. In considering the process of dissolving we need to start at the molecular level, work through some basic theory and examine how one substance interacts with another. And to do that requires a detailed look at the particles that make up the solvent and solute: molecules, ions, atoms.

Consider a substance made up of particles A that is to be mixed with a substance comprising particles B. In substance A there are attractive forces between the particles that may be represented as A A; in a similar manner substance B may be represented as B B.

For dissolving to occur these attractive forces must be overcome by new attractive forces between A and B represented as A B. If the A B forces are stronger than the A A and B B forces then dissolving occurs. Conversely if the A B forces are less than the A A and B B forces then dissolving...