Compressed Gas
Definition and meaning of Compressed Gas in chemistry.
Compressed gas is a gas held in a sealed container at a pressure significantly greater than atmospheric pressure, existing as a pure gas, a liquefied gas, or a gas dissolved in a solvent under pressure.
In more detail
Forcing a gas into a smaller volume packs its molecules closer together, following Boyle's law (P1V1 = P2V2 at constant temperature), which lets large quantities of gas be stored in a compact, portable cylinder. Because the pressurized contents store substantial potential energy, cylinders must be kept upright, secured, and shielded from heat, since rising temperature raises internal pressure and can lead to rupture or violent release. Some compressed gases, such as propane and carbon dioxide, partly liquefy under storage pressure, while others, like nitrogen and oxygen, remain fully gaseous. Agencies such as OSHA and the U.S. Department of Transportation define "compressed gas" using specific pressure thresholds measured at set reference temperatures.
Key facts
| Field | General Chemistry |
|---|---|
| Governing Law | Boyle's Law (P1V1 = P2V2) |
| Typical Cylinder Pressure | ~2,000-2,500 psi (140-170 atm) |
| Common Examples | O2, N2, CO2, propane (C3H8) |
A steel cylinder of compressed medical oxygen stores gas at roughly 2,000-2,200 psi (about 140 atm) and releases it through a pressure-reducing regulator for safe delivery to a patient.
Frequently asked questions
What is the difference between a compressed gas and a liquefied gas?
A compressed gas stays entirely gaseous at normal storage temperatures (e.g., nitrogen or oxygen), whereas a liquefied compressed gas partly condenses into liquid inside the cylinder under its own storage pressure (e.g., propane or carbon dioxide).
Why does a compressed gas cylinder feel cold when gas is released quickly?
Rapid expansion from high to low pressure causes the escaping gas to cool through adiabatic expansion (related to the Joule-Thomson effect), which can chill the valve and cylinder surface.