ABSTRACT
Solutions are important in environmental and biological systems as well as in industrial processes. Simple terms ‘pure substance’, ‘mixture’ and ‘solution’ often cause problems for new students. When a particular pure liquid is in equilibrium with its vapour at a particular temperature, the vapour pressure of that substance has a definite value. In the ideal mixture model, the vapour pressure of each component is proportional to the amount of that component present. The proportionality constant is the vapour pressure of the pure substance, so the vapour pressure of course reaches that of the pure liquid when it is present to the extent of 100%. Physical chemists and chemical engineers use phase diagrams not only for solids, liquids and gases, but also for other states of matter. The cohesion parameter approach to mixtures is that a material with a high cohesion parameter value requires more energy for dispersal than is gained by mixing it with a material of low cohesion parameter.
