Editing Entropy (section)

=== State variables and functions of state ===
Many [[thermodynamic properties]] are defined by physical variables that define a state of [[thermodynamic equilibrium]], which essentially are [[state variable]]s. State variables depend only on the equilibrium condition, not on the path evolution to that state. State variables can be functions of state, also called [[state function]]s, in a sense that one state variable is a [[Function (mathematics)|mathematical function]] of other state variables. Often, if some properties of a system are determined, they are sufficient to determine the state of the system and thus other properties' values. For example, temperature and pressure of a given quantity of gas determine its state, and thus also its volume via the [[ideal gas law]]. A system composed of a pure substance of a single [[Phase (matter)|phase]] at a particular uniform temperature and pressure is determined, and is thus a particular state, and has a particular volume.  The fact that entropy is a function of state makes it useful. In the [[Carnot cycle]], the working fluid returns to the same state that it had at the start of the cycle, hence the change or [[line integral]] of any state function, such as entropy, over this reversible cycle is zero.