One of the primary challenges that we face as engineers is how to communicate large quantities of complex information to our peers and superiors. For instance, we may need to report what thermodynamic changes our working fluid is undergoing. Here, the familiar saying: A picture is worth a thousand words applies. Although most non-engineers believe that the only language an engineer speaks is mathematics, in actuality, the most effective means that engineers use to communicate information involves the use of pictures or diagrams. Phase behavior is not an exception. In phase behavior thermodynamics, phase diagrams are used precisely for this purpose.
A phase diagram is a concise graphical method of representing phase behavior of fluids. It provides an effective tool for communicating a large amount of information about how fluids behave at different conditions.
First of all, let us classify fluids into two broad groups on the basis of the number of components that are present in the system: pure component systems and mixtures.
Two Classes of Fluids
Although this classification may seem trivial at a first glance, it recognizes the paramount influence of composition on the phase behavior of a fluid system. Whereas for a single-component system, composition is not a variable and therefore cannot influence behavior, the behavior of a mixture is strongly controlled by composition. In fact, as the number of components in the system increases, the complexity of the phase diagram increases as well. Therefore, the simplest form of a phase diagram is that of a system made of only one component (a pure-component system).