This is an excellent time for review. In fact, a review is practically forced upon you in this chapter.

A body composed of a single species is a limiting case of multicomponent materials. In what follows, we reformulate the fundamental postulates made in Chapters 1, 2, and 5, to enable their application to multicomponent bodies. With a few relatively minor modifications, all we have said about single-component systems can be applied to multicomponent ones.

Viewpoint

Up to this point we have been primarily concerned with single-component materials or materials of uniform composition. Hereafter, we shall be treating a material consisting of N species or constituents that is undergoing an arbitrary number of homogeneous and heterogeneous chemical reactions. Not only are we interested in the velocity and temperature distributions in such a material, but we wish to follow its composition as a function of time and position. We may ask, for example, how rapidly a particular species formed by a catalytic reaction at an adjacent surface will distribute itself throughout a material; or we may wish to determine the rate at which a liquid droplet will evaporate into a surrounding gas stream.

Our first task is to choose a continuum model for an N -component material. We will wish to follow each species individually as the N-component mixture goes through some operation, possibly involving deformation, flow, and chemical reactions. We will view each species as a continuous medium with a variable mass-density field.