An equation of state is a quantitative relationship among the variables that determine the state of a given system, that is, whether it is solid, liquid, or vapor, its enthalpy or heat content, etc. The state of a system is its condition or situation, at a given instant, and is determined by its properties. Such an equation can be used to calculate the value of one state variable, for example, pressure, if the values of the other variable are known.

Scientific studies generally involve the examination of changes in the states of systems. A system is the portion of the physical world which we set apart for study, e.g., a chemical reaction or a container of gas. The values of a specific number of the properties or variables of the system must be specified in order to completely define its state.

Variables of...