Standard temperature and pressure

Standard temperature and pressure (STP) is a reference set of temperature and pressure conditions used to compare gas properties. Its numerical values depend on the convention being used.

Temperature: A physical quantity that describes thermal state and determines the direction of heat transfer. STP fixes a temperature so gas measurements can be compared on the same basis.
IUPAC standard conditions for gases: IUPAC's recommended reference conditions for gases: 273.15 K and 100 kPa. Its 100 kPa pressure differs from the traditional one-atmosphere STP convention.
Molar volume: The volume occupied by one mole of a substance under specified conditions. At STP, molar volume gives a convenient comparison for gases.
Boyle's law: The gas-law relationship stating that pressure and volume are inversely proportional at constant temperature and amount. It describes the volume correction needed when measured pressure differs from STP.
Pressure: Force exerted per unit area by particles or fields acting on a surface. STP fixes pressure because gas volume and density depend strongly on it.
Standard ambient temperature and pressure: A reference condition commonly set at 25 °C and 100 kPa. Its warmer reference temperature distinguishes it from common STP definitions.
Gas density: The mass of a gas per unit volume under specified conditions. A stated STP convention is necessary for meaningful density comparisons.
Charles's law: The gas-law relationship stating that volume is proportional to absolute temperature at constant pressure and amount. It describes temperature-based volume corrections to a reference condition.
Ideal gas law: The equation PV = nRT relating pressure, volume, amount, and absolute temperature for an ideal gas. It predicts how a gas property changes when STP conditions change.
Normal conditions: A gas-reference convention commonly using 0 °C and 1 atmosphere, though usage varies. The term often overlaps with STP but does not guarantee one universal convention.

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