Thus far in our study of thermodynamics, we have considered only single-component systems. In this chapter, we will study the two-component system of air and water vapor, generally called psychometrics. The more general presentation in which several components make up the system is not considered in this text. The two-component mixture of air and water vapor must be considered when designing air-conditioning systems, whether they are for heating, for cooling, or simply for dehumidification.