Principles of solution chemistry

A solution is a homogeneous liquid, formed by the addition of solutes to a solvent. The behaviour of solutions is determined by the type of solutes involved and by their proportions, relative to the solvent. Many laboratory exercises involve calculation of concentrations, e.g. when preparing an experimental solution at a particular concentration, or when expressing data in terms of solute concentration. Make sure that you understand the basic principles set out in this topic before you tackle such exercises.

Solutes can affect the properties of solutions in several ways, as follows.

Electrolytic dissociation
This occurs where a substance dissociates to give charged particles (ions). For a strong electrolyte, e.g. Na⁺CI⁻, dissociation is essentially complete. In contrast, a weak electrolyte, e.g. ethanoic acid, will be only partly dissociated, depending upon the pH and temperature of the solution.

Osmotic effects
These are the result of solute particles lowering the effective concentration of the solvent (water). These effects are particularly relevant to biological systems since membranes are far more permeable to water than to most solutes. Water moves across biological membranes from the solution with the higher effective water concentration to that with the lower effective water concentration (osmosis).

Ideal/non-ideal behaviour
This occurs because solutions of real substances do not necessarily conform to the theoretical relationships predicted for dilute solutions of so-called ideal solutes. It is often necessary to take account of the non-ideal behaviour of real solutions, especially at high solute concentrations.