The Entropy of Mixing is Not Due to Mixing Itself

The entropy of mixing is frequently misunderstood as a special force to drive the spontaneous phenomena in nature, for example, diffusion and chemical reactions. The misconception results from the confusion of mixing with expansion. This article deals with only mixing of the components of gases or solutes without expansion and gives formulations of the entropies under the mixed and unmixed conditions. It has been found that the entropy of mixing does not arise from mixing but just reflects the number densities of the components themselves in the unmixed system. The conclusion of this article will be valuable for teaching and learning not only the entropy of mixing but also the essence of the spontaneous phenomena in thermodynamics.