The term ‘mass transfer’ or ‘mass transport’ encompasses a vast range of processes involving the movement of matter within a system. It is not possible to provide a simple definition of its scope, except to state that we are concerned with the movement of particular molecular species within a system and with the factors which affect the movement. We can introduce the subject by means of two simple examples, although these in no way describe its full breadth.

A puddle of water on the road surface slowly evaporates, the liquid water progressively being transferred as vapour to the air above it. The rate of evaporation depends upon such factors as humidity, the ambient air temperature relative to the ground and the speed of the wind over the surface of the puddle.

If a crystal of copper sulphate is dropped into a beaker of water it slowly dissolves in the water and produces a concentrated solution around the crystal surface. In time this dissolved material diffuses further and further into the surrounding water. The speed with which the crystal dissolves and the rate of transfer of dissolved copper sulphate to the bulk of the water phase can be modified by a number of factors. For example, the process would occur more quickly in hot water than cold, or if the beaker were stirred.