There are two important sets of conditions which are referred to as 'standard':

• Standard Temperature and Pressure (STP) is taken to mean a temperature of 0 °C (273.15 K) and 1 atm (1.01325 × 10⁵ Pa) pressure.
• Standard Ambient Temperature and Pressure (SATP) refers to 25 °C (298.15 K) and 1 bar (10⁵ Pa) pressure.
• The standard pressure of 1 bar is frequently used in thermodynamics, and is given its own symbol: p° = 1 bar.
• The standard state of a substance is its pure form at p° = 1 bar and a specified temperature.

Avogadro's principle states that equal volumes of gases at the same temperature and pressure contain the same number of molecules, regardless of the identity of the gases. It applies to ideal (or perfect) gases, and is a good approximation for real gases in many instances. Stated another way, Avogadro's principle implies that a given amount of any gas at a specified temperature and pressure occupies a particular volume. Specifically one mole of any ideal gas has a molar volume, V_m, equal to:

• V_m = 22.414 L mol^-1 at STP (273.15 K, 1 atm)
• V_m = 24.790 L mol^-1 at SATP (298.15 K, 1 bar)

In the case of liquid solutions, the standard state of a solute may be defined as unit activity in a specified solvent at a given temperature, a = 1. For very dilute aqueous solutions, activities are almost equal to molalities, m, in the sense:

a ≈ m / m°

where m° = 1 mol kg^-1, and to molarities (amount concentrations):

a ≈ c / c°

where c° = 1 M = 1 mol dm^-3.