Standard State
The reference condition of a pure substance at 1 bar pressure and a stated temperature, conventionally 298.15 K.
Formula / equation
denoted by °
Unit
Dimensionless
In depth
For a solute the standard state is a 1 mol/L concentration; for an element it is the most stable allotrope under those conditions. Defining a standard state makes thermodynamic data from different sources comparable and lets tabulated values be combined reliably.
Examples in the real world
Graphite rather than diamond is standard-state carbon; O₂ gas rather than ozone is standard-state oxygen.