Standard Temperature and Pressure
Definition and meaning of Standard Temperature and Pressure in chemistry.
Standard Temperature and Pressure is a specific set of normal conditions. Chemists use these fixed conditions to measure and compare different gases. The term is very frequently abbreviated as STP in science class.
In more detail
Gas volume changes wildly when it gets hot or cold. It also changes when you increase or decrease the outside pressure. This makes it very hard to compare two different gas experiments.
Scientists solved this problem by agreeing on one standard set of rules. The official temperature for STP is exactly 0 degrees Celsius. This value equals 273.15 Kelvin on the absolute temperature scale.
The official pressure is exactly 100 kilopascals, which equals one bar. Before the year 1982, the pressure standard was slightly different. The old rule used to be exactly one atmosphere of pressure.
One atmosphere of pressure is equal to exactly 101.325 kilopascals. You will still see this older rule in many modern textbooks. At the modern STP, one mole of any ideal gas takes up a specific amount of space.
It will occupy a physical volume of approximately 22.71 liters. The older standard gave a famous volume of 22.4 liters. Students often confuse these two different volume numbers on their exams.
You should always check which pressure rule your teacher wants you to use. Having one standard lets a chemist in Japan share data with a chemist in Brazil. They know they are talking about the exact same physical conditions.
Key facts
| Field | Physical Chemistry |
|---|---|
| Standard Temperature | 273.15 K (0 °C) |
| Current Standard Pressure | 100 kPa (1 bar) |
| Old Standard Pressure | 1 atm (101.325 kPa) |
| Current Molar Volume | 22.71 L/mol |
| Abbreviation | STP |
Imagine you have exactly one mole of helium gas inside a balloon. You bring this balloon into a room set to Standard Temperature and Pressure. The room is at 0 degrees Celsius and 100 kilopascals of pressure. The balloon will expand or shrink to reach a specific size. It will eventually occupy a volume of exactly 22.71 liters. This works for hydrogen or oxygen gas under the same rules.
Frequently asked questions
What is the molar volume of a gas at STP?
Under the current definition, one mole of gas occupies 22.71 liters. The older definition gives a slightly different volume of 22.4 liters.
Why do we need Standard Temperature and Pressure?
Gas volume changes quickly with temperature and pressure. A standard set of rules allows scientists to compare their results fairly.
Is STP the same thing as room temperature?
No, room temperature is usually around 20 to 25 degrees Celsius. Standard Temperature requires a freezing temperature of exactly 0 degrees Celsius.