Guide
Critical temperature and pressure of industrial gases: why CO₂ and N₂O are liquid in the cylinder and oxygen nitrogen and argon never are
The critical temperature is the highest temperature at which a gas can be liquefied by pressure. Carbon dioxide's is 30.98 °C and nitrous oxide's 36.37 °C, so both are liquid in a cylinder on most Indian days. Oxygen (−118.57 °C), nitrogen (−146.96 °C) and argon (−122.46 °C) never liquefy at room temperature.
- Carbon dioxide
- Critical point 30.98 °C and 73.77 bar; vapour pressure about 57 bar at 20 °C
- Nitrous oxide
- Critical point 36.37 °C and 72.45 bar; vapour pressure about 51 bar at 20 °C
- Acetylene
- 35.18 ± 0.04 °C; critical pressure 61.1 atm
- Oxygen, nitrogen, argon
- −118.57 °C / 50.43 bar; −146.96 °C / 33.96 bar; −122.46 °C / 48.63 bar
- Indian legal classes
- Permanent gas below −10 °C; high-pressure liquefiable −10 °C to +70 °C; low-pressure liquefiable above +70 °C
The critical temperature of a gas is the highest temperature at which it can exist as a liquid. Below it, enough pressure will condense the gas; above it, no pressure will. That single number decides how a gas is stored, filled and sold: carbon dioxide (30.98 °C) and nitrous oxide (36.37 °C) sit in the cylinder as liquids on most Indian days, while oxygen, nitrogen and argon, with critical temperatures more than 100 °C below zero, are always compressed gases.
The table below gives the critical temperature and pressure of fourteen gases, mostly from the NIST Chemistry WebBook, with the class each falls into under the Gas Cylinders Rules 2016.
Critical temperature and pressure of industrial gases
| Gas | Critical temperature, °C | Critical temperature, K | Critical pressure, bar (abs) | Critical pressure, kg/cm² (abs) | GCR 2016 class | In a cylinder at 25 °C |
|---|---|---|---|---|---|---|
| Helium | −267.95 | 5.20 | 2.283 | 2.33 | Permanent | Compressed gas |
| Hydrogen | −240.01 | 33.15 | 12.96 | 13.2 | Permanent | Compressed gas |
| Nitrogen | −146.96 | 126.19 | 33.96 | 34.6 | Permanent | Compressed gas |
| Carbon monoxide | −140.29 | 132.86 | 34.94 | 35.6 | Permanent | Compressed gas |
| Air (pseudo-critical) | about −140.6 | about 132.5 | about 37.9 | about 38.6 | Permanent | Compressed gas |
| Argon | −122.46 | 150.69 | 48.63 | 49.6 | Permanent | Compressed gas |
| Oxygen | −118.57 | 154.58 | 50.43 | 51.4 | Permanent | Compressed gas |
| Methane | −82.59 | 190.56 | 45.99 | 46.9 | Permanent | Compressed gas |
| Ethylene | 9.20 | 282.35 | 50.42 | 51.4 | High-pressure liquefiable | Supercritical fluid |
| Carbon dioxide | 30.98 | 304.13 | 73.77 | 75.2 | High-pressure liquefiable | Liquid and vapour |
| Acetylene | 35.18 | 308.33 | 61.9 | 63.1 | Dissolved gas (DA cylinder) | Dissolved in acetone or DMF |
| Nitrous oxide | 36.37 | 309.52 | 72.45 | 73.9 | High-pressure liquefiable | Liquid and vapour |
| Propane | 96.74 | 369.89 | 42.51 | 43.3 | Low-pressure liquefiable | Liquid and vapour |
| Ammonia | about 132.3 | about 405.5 | about 113.5 | about 115.7 | Low-pressure liquefiable | Liquid and vapour |
Sources and conditions: NIST Chemistry WebBook equations of state for every gas except acetylene and air; ammonia's values differ slightly between equations of state and are rounded. Acetylene's critical temperature is Ambrose's measurement (Trans. Faraday Soc. 1956); its critical pressure of 61.1 atm is a selected literature value, and IS 14814:2023 prints 35.15 °C and 6,059 kPa. Air's figures are pseudo-critical values for the mixture from the NIST air model of Lemmon and co-workers (2000). Pressures are absolute; kg/cm² is converted at 1 bar = 1.0197 kg/cm².
Rules that matter
Along a liquid's vapour-pressure curve, the liquid gets less dense and the vapour gets denser as temperature rises. At the critical point the two densities meet, the meniscus vanishes, and the latent heat of vaporisation falls to zero. The critical pressure is simply the vapour pressure at that temperature.
Above the critical temperature the substance is a supercritical fluid. It can be compressed to a density close to that of a liquid, but it does not boil, does not separate into two phases, and fills the container evenly. Three consequences follow for anyone handling cylinders:
What the critical point actually is
- A gas with a critical temperature below ambient can only be compressed. Its contents follow the pressure, roughly in proportion, and are measured with a gauge. These are the gases the Rules call permanent.
- A gas with a critical temperature above ambient condenses in the cylinder. Its pressure is the vapour pressure of the liquid, which depends on temperature alone while liquid remains, so the gauge does not show the contents. These gases are filled and sold by weight.
- A gas whose critical temperature lies inside the ambient range changes behaviour on a warm day. In India that is carbon dioxide, nitrous oxide and, in principle, acetylene.
Why carbon dioxide and nitrous oxide liquefy and oxygen does not
Carbon dioxide at 20 °C is 11 °C below its critical temperature. Compress it and it condenses at about 57 bar; keep compressing and you add liquid, not pressure. Oxygen at 20 °C is nearly 140 °C above its critical temperature. Compress it to 150 or 200 bar and it remains a gas, just a dense one: 215.6 kg/m³ at 150 bar and 15 °C by NIST, about a fifth of the density of liquid oxygen.
Vapour pressure of the two liquefied gases KIGL's customers meet most often:
| Temperature | CO₂ vapour pressure, bar abs | N₂O vapour pressure, bar abs | State |
|---|---|---|---|
| 0 °C | about 34.9 | about 31 | Liquid and vapour |
| 15 °C | about 50.9 | — | Liquid and vapour |
| 20 °C | about 57.3 | about 51 | Liquid and vapour |
| 25 °C | about 64.3 | — | Liquid and vapour |
| 30 °C | about 72 | about 63 | Liquid and vapour, near critical |
| 30.98 °C | 73.77 (critical) | — | CO₂ supercritical above this |
| 36.37 °C | — | 72.45 (critical) | N₂O supercritical above this |
Source: NIST Chemistry WebBook saturation data, as tabulated on the carbon dioxide technical data and nitrous oxide safety data pages. For comparison, propane's vapour pressure at 15 °C is only 7.3 bar absolute (NIST), because its critical temperature is far above ambient.
What happens when a CO₂ cylinder passes 31 °C
Below the critical temperature, a correctly filled CO₂ cylinder has a vapour space. As it warms, the liquid expands into that space and the pressure follows the vapour-pressure curve. If the cylinder is overfilled, the liquid can fill the cylinder completely before the critical temperature is reached, and from that point the pressure rises very steeply, because a liquid is nearly incompressible. Above 30.98 °C the contents become a single dense phase whose pressure depends on the mass charged per litre of cylinder.
That is why liquefied gases are filled to a filling ratio, defined in Rule 2(xxv) of the Gas Cylinders Rules 2016 as the weight of gas divided by the weight of water the cylinder holds at 15 °C, and why Rule 4(2) requires a CO₂ cylinder valve to carry a softened copper bursting disc set to burst between 200 and 220 kg/cm². The Indian CO₂ filling ratio now sits in IS 15975:2020; the practical treatment is on permanent vs liquefied gases.
Acetylene's 35.18 °C
Acetylene's critical temperature, measured by Ambrose in 1956 as 35.18 ± 0.04 °C, is passed in the shade for weeks every year across most of India. In a dissolved acetylene cylinder this matters less than it seems, because there is no liquid acetylene in the cylinder: the gas is dissolved in acetone or DMF held in a porous mass. What the heat does change is the solvent: acetone holds about a third less acetylene at 30 °C than at 15 °C. Liquid acetylene is never handled commercially because it is unstable, and Indian law, through notification GSR 625(E) of 1983, declares liquid acetylene an explosive. The detail is in acetylene critical temperature and hot weather and why liquid acetylene is banned.
Rules that matter
The Gas Cylinders Rules 2016 sort gases by critical temperature in Rule 2:
How Indian law uses the critical temperature
- Permanent gas: critical temperature below −10 °C (Rule 2(xliii)). Oxygen, nitrogen, argon, hydrogen, helium, air, methane and carbon monoxide. Working pressure is the internal pressure at 15 °C (Rule 2(liii)).
- High-pressure liquefiable gas: critical temperature between −10 °C and +70 °C (Rule 2(xxx)). Carbon dioxide, nitrous oxide, ethylene.
- Low-pressure liquefiable gas: critical temperature above +70 °C (Rule 2(xl)). Propane, butane, ammonia. Working pressure is the vapour pressure at 65 °C (Rule 2(lii)).
- Cryogenic liquid: the liquid form of a permanent gas with a normal boiling point below −150 °C (Rule 2(xvii)). Liquid oxygen, nitrogen and argon qualify; liquid CO₂ does not.
Ethylene is the instructive borderline case. With a critical temperature of 9.20 °C it is legally a liquefiable gas, yet on any Indian day above 9 °C a cylinder of ethylene contains no liquid at all.
Liquefying a permanent gas needs cold, not pressure
Because oxygen, nitrogen and argon cannot be condensed at room temperature, liquid oxygen and liquid nitrogen are made by cooling air below their critical temperatures in a cryogenic air separation unit and then distilling it. They are stored near their boiling points (−182.96 °C and −195.80 °C) in vacuum-insulated tanks at modest pressure. KIGL operates an air separation plant and supplies liquid oxygen and liquid nitrogen. The process is described in how oxygen is made, and the containers in cryogenic cylinders, dewars and tanks.
Rules that matter
Rules to apply
- Look up the critical temperature before trusting a cylinder gauge. If it is above the room temperature, weigh the cylinder.
- Never top up a liquefied gas cylinder by pressure. Fill by weight to the permitted filling ratio, and never overfill.
- Keep CO₂ and N₂O cylinders shaded. Both cross their critical temperatures in Indian summer sun, and their pressure then climbs faster with every degree.
- Do not assume a gas is liquid because it is classed as liquefiable: check ethylene's 9.20 °C against the day's temperature.
Critical temperature — frequently asked questions
What is the critical temperature of carbon dioxide?
30.98 °C, with a critical pressure of 73.77 bar (75.2 kg/cm² absolute) and a critical density of about 468 kg/m³, from the NIST Chemistry WebBook. Below 30.98 °C a filled CO₂ cylinder holds liquid with vapour above it at about 57 bar at 20 °C; above it the contents become a single supercritical fluid whose pressure depends on how full the cylinder was filled.
Why can oxygen not be liquefied in a cylinder at room temperature?
Because oxygen's critical temperature is −118.57 °C. Above that temperature no pressure, however high, will turn it into a liquid; compressing it only makes a denser gas. Liquid oxygen exists only when it is cooled below its critical temperature, and in practice it is stored near its boiling point of −182.96 °C in insulated tanks, not in high-pressure cylinders.
What happens to a gas above its critical temperature?
It becomes a supercritical fluid: there is no longer a boundary between liquid and vapour, no meniscus and no boiling. Its density can still be high, close to a liquid's, but it fills the container uniformly and its pressure rises steadily with temperature instead of following a vapour-pressure curve. For a liquefied gas cylinder this is where overfilling becomes dangerous.
What is the difference between critical temperature and boiling point?
The boiling point is where the vapour pressure equals one atmosphere: −78.46 °C (sublimation) for carbon dioxide, −88.47 °C for nitrous oxide. The critical temperature is the top end of the same vapour-pressure curve, where liquid and vapour become identical: 30.98 °C for CO₂ and 36.37 °C for N₂O. Between the two, the gas can be held as a liquid under pressure.
Is nitrous oxide liquid or gas inside the cylinder?
Liquid, with vapour above it, at any temperature below 36.37 °C. The pressure is the vapour pressure, about 51 bar at 20 °C, and it stays nearly constant as gas is drawn off until the liquid is gone. So the gauge does not show contents; the cylinder is filled and checked by weight. On a hot afternoon above 36.4 °C the contents become supercritical.
What is a high-pressure liquefiable gas under the Gas Cylinders Rules?
A gas whose critical temperature lies between −10 °C and +70 °C (Rule 2(xxx)). Carbon dioxide, nitrous oxide and ethylene are the common examples. They are filled by weight to a filling ratio rather than to a pressure. Gases with a critical temperature above 70 °C, such as propane and ammonia, are low-pressure liquefiable; those below −10 °C are permanent gases.
References
- Ambrose, D., Trans. Faraday Soc. 1956, 52, 772 — triple point and critical temperature of acetylene
- Bureau of Indian Standards. (2023). Acetylene — Code of Safety (IS 14814:2023, First Revision).
- Government of India. (2016). Gas Cylinders Rules, 2016 (G.S.R. 1081(E)). Petroleum and Explosives Safety Organisation.
- Linstrom, P. J., & Mallard, W. G. (Eds.). (n.d.). NIST Chemistry WebBook (NIST Standard Reference Database 69). National Institute of Standards and Technology. [, SRD 69 — Thermophysical Properties of Fluid Systems (critical constants)] https://webbook.nist.gov/chemistry/fluid/
Enquiries
Request a quotation
State the gas or equipment, the quantity or capacity, and your location. KIGL will confirm the grade, the cylinder or plant size, and whether the location can be served from Noonmati, or through the agent network.