Often we find that different phases of pure substances can exist in equilibrium with one another. Let us consider an important gedankenexperiment (Latin-German for "thought experiment") in which we boil water. Ordinary water boiling is shown in Fig. 3.1. However,this ordinary experiment has constraints which are too loose. Most importantly, the mass of water leaks into the atmosphere; thus, the water vapor and the air become a mixture and no longer a pure substance