In Chapters 1 and 2 we noted that a system was a part of the universe that we were interested in examining and that the state of a system is a snapshot of the configuration of the system at some moment in time. The state of any system is determined by the properties of the system. The properties of the system are the complete set of descriptors that allow us to unambiguously define the state of the system. Knowing the properties of the system is similar to knowing someone’s DNA. Once we know the properties, then we know everything there is to know, thermodynamically, about the system. In this chapter will find the basic rules for identifying a system based upon its properties. We will also determine approaches for finding the properties of a system from equations that allow calculation of properties, called equations of state, and tables.

Because systems can be composed of many different types of materials, we often simplify analysis of thermodynamic systems by initially considering a system composed of a single substance. We call this a pure substance. In Chapter 12, consideration is given to descriptions of mixtures of substances.