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I think these two terms are sometimes used as synonyms and other times as different concepts. Can some explain what is the difference? An example would be good.

Also, can state variable be the state function at the same time? Eg. in ideal gas let's take pressure P... is it a state variable or state function? What about volume V and temperature T?

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Pressure $P$ is state variable when we specify equilibrium state of a system by giving numerical value of $P$.

Pressure $P$ is a state function when we discuss how the above value depends on other state variables like $V,T$.

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  • $\begingroup$ That makes sense.. So can we say in other words that: "state variable" is something that we take as independent variable, while "state function" is something that depends on previously selected "state variables" where this dependence is given in the equation of state for the particular thermodynamic system. Would this statement be correct? $\endgroup$ – matori82 Mar 18 '16 at 22:59
  • $\begingroup$ @matori82, yes, I think that's the way people use these terms. Of course, value of every state function can be used to define a state variable. Usually the symbols for the function and the corresponding variable are adopted to be the same. $\endgroup$ – Ján Lalinský Mar 19 '16 at 3:06

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