Fluids consist of many fluid elements, and each fluid element consists of many particles. The state of matter in a given fluid element is determined thermodynamically , meaning that only a few parameters are tracked as the fluid element evolves. Generally, not all the thermodynamic variables are independent, being connected through the so-called equation of state. The number of independent variables can also be reduced if the system has an overall additivity property. As this is a very instructive example, we will now illustrate such additivity in detail.
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