Standard states of pure substances
The standard state of a pure substance is defined as follows. For a pure solid or a pure liquid, the standard state is defined as the state with pressure of exactly 1 bar (100,000 Pa) that we donete by P° ≡ 1 bar, and temperature T, where T is some temperature of interest. Thus for each value of T there is a single standard state for a pure substance. The symbol for a standard state is a degree superscript "°" (read as “naught,” “zero,” or “standard”), with the temperature written as a subscript. For example, the molar volume of a pure solid or liquid at 1 bar and 300 K is symbolized by V°m,300, where the degree superscript indicates the standard pressure of 1 bar and 300 stands for 300 K.
For a pure gas, the standard state at temperature T is chosen as the state where P = 1 bar and the gas behaves as an ideal gas. Since real gases do not behave ideally at 1 bar, the standard state of a pure gas is a fictitious state. Summarizing, the standard states for pure substances are:
Solid or liquid: P = 1 bar, T
Gas: P = 1 bar, T, gas ideal
We disccus in the next parahraphs the standard states for components of solutions and we'll se how to Calculate the properties of the gas in the fictitious standard state from properties of the real gas.
A standard-state reaction is one in which all substances are in their standard states before and after the reaction.