In this chapter, we will consider different phases in which a pure substance exists and learn how to find important properties of engineering substances of interest in thermodynamics. Substances used in power generation and refrigeration systems will be of primary importance. The thermodynamic properties of internal energy and enthalpy will be defined and used throughout the remainder of the text. They will be calculated for several pure substances at various states so that you can become familiar with the terms and learn how to determine values of these properties. All substances of interest are pure substances. That is, they have the same chemical composition throughout; they are chemically homogeneous. Water is a good example of a homogeneous substance, even if it is an ice–water mixture, but water mixed with oil is not homogeneous since oil is not soluble in water. They must also be simple substances; that is, the relationships between internal energy enthalpy, temperature, pressure and density are not affected by gravitational, electrical, or magnetic fields.