In this chapter, the relationships between pressure, specific volume, and temperature will be presented for a pure substance. A pure substance is homogeneous. It may exist in more than one phase, but each phase must have the same chemical composition. Water is a pure substance. The various combinations of its three phases have the same chemical composition. Air, in the gas phase, is a pure substance, but liquid air has a different chemical composition. Air is not a pure substance if it exists in more than one phase.