The goals of this chapter are to show how the free energies of mixtures depend on the concentrations of the various species, and how these concentration dependences determine the thermodynamics of mixing and of chemical reactions. As with single-component phase transitions described in Chapter 6, the chemical potential plays a central role in analyzing equilibrium in mixing and in reaction. However, unlike single-component phase transitions, where each phase is pure, here we will need to calculate chemical potentials of species that are combined in various proportions. As we will see, the chemical potential varies logarithmically with concentration. In the process of deriving this logarithmic dependence, we will introduce the concept of “standard state” concentrations. Together, the standard state and logarithmic concentration terms will be used to develop expressions for mixing and reaction thermodynamics. A key concept that emerges from the reaction free energy equation is the equilibrium constant. We will discuss the relationship between the equilibrium constant and the standard state, and how the equilibrium constant depends on temperature and pressure.