Guide
Nitrous oxide technical data: vapour pressure critical point, densities, decomposition and the medical and industrial grades
Nitrous oxide (N₂O, 44.013 g/mol) is a liquefied gas: at 20 °C its cylinder pressure is 50.5 bar and its liquid density 785 kg/m³. Its critical temperature is 36.4 °C, so it is sold by weight, not pressure. It is an oxidiser that can decompose exothermically, and medical cylinders are French blue.
- Critical point
- 36.4 °C
- Vapour pressure
- 31.2 bar at 0 °C
- Densities
- Saturated liquid 785 kg/m³ and vapour 158 kg/m³ at 20 °C; gas 1.978 kg/m³ at 0 °C and 1 atm
- Boiling and triple points
- Boils at −88.5 °C at 1 atm; triple point −90.8 °C at 0.879 bar
- Decomposition
- 2 N₂O → 2 N₂ + O₂ releases about 82 kJ per mole of N₂O; the products are one-third oxygen
Nitrous oxide is a gas that lives as a liquid in its cylinder. Its critical temperature, 36.4 °C, lies just above a normal Indian working day, so below that temperature the cylinder holds liquid under its own vapour pressure, about 50.5 bar at 20 °C, and above it the contents turn into a single dense fluid. That one fact explains why N₂O is sold by weight, why its gauge says nothing about contents and why its filling is limited by a ratio. This page gives the physical data from the NIST Chemistry WebBook, the decomposition chemistry that makes N₂O more than an inert gas, and the difference between medical and industrial grades.
Nitrous oxide properties table
| Property | Value | Conditions | Source |
|---|---|---|---|
| Formula | N₂O | — | NIST Chemistry WebBook |
| CAS number | 10024-97-2 | — | NIST Chemistry WebBook |
| Molar mass | 44.013 g/mol | — | NIST Chemistry WebBook |
| Normal boiling point | −88.5 °C | 1 atm | NIST Chemistry WebBook |
| Triple point | −90.8 °C at 0.879 bar | — | NIST Chemistry WebBook (Fonseca and Lobo, 1989) |
| Critical temperature | 36.4 °C (309.6 K) | — | NIST Chemistry WebBook (Ohgaki et al., 1990) |
| Critical pressure | 72.4 bar | — | NIST Chemistry WebBook (Ohgaki et al., 1990) |
| Gas density | 1.978 kg/m³ | 0 °C, 1.01325 bar | NIST Chemistry WebBook |
| Gas density | 1.873 kg/m³ | 15 °C, 1.01325 bar | NIST Chemistry WebBook |
| Relative density | about 1.53 (air = 1) | 0 °C | NIST Chemistry WebBook |
| Specific heat, Cp, of the gas | 0.878 kJ/kg·K | 20 °C, 1 atm | NIST Chemistry WebBook |
| Enthalpy of vaporisation | 16.5 kJ/mol | 184.7 K | NIST Chemistry WebBook (Atake and Chihara, 1974) |
| Enthalpy of formation | about +82 kJ/mol | 25 °C | NIST Chemistry WebBook |
| Flammability | Not flammable; oxidiser, supports combustion | — | UN Class 2.2, subsidiary 5.1 |
| Transport identity | UN 1070 liquefied; UN 2201 refrigerated liquid | — | IS 4379:2021 Table 1 |
| Occupational exposure limit | 50 ppm 8-hour TWA (ACGIH); 25 ppm REL during anaesthetic use (NIOSH) | — | ACGIH; NIOSH |
The relative density of 1.53 is the same as that of carbon dioxide, which has the same molar mass to within 0.01 g/mol. Leaked nitrous oxide sinks and collects in pits, trenches and basements, so oxygen and N₂O monitoring belongs near the floor.
Vapour pressure and liquid density by temperature
| Temperature | Vapour pressure | Saturated liquid density | Saturated vapour density | State in the cylinder |
|---|---|---|---|---|
| 0 °C | 31.2 bar | 907 kg/m³ | 85 kg/m³ | Liquid with vapour above |
| 20 °C | 50.5 bar | 785 kg/m³ | 158 kg/m³ | Liquid with vapour above |
| 30 °C | 63.1 bar | 688 kg/m³ | 233 kg/m³ | Liquid with vapour above, close to critical |
| 36.4 °C | 72.4 bar | — | — | Critical point; liquid and vapour become one phase |
| Above 36.4 °C | rises steeply | — | — | Single supercritical phase |
Values are from the NIST Chemistry WebBook saturation and isothermal tables. Two things stand out. First, the pressure rises by about 1.3 bar for every degree between 20 and 30 °C, so a cylinder brought from an air-conditioned store into a hot yard changes pressure with no change in contents. Second, the liquid expands by about 14 % between 20 and 30 °C, from 785 to 688 kg/m³. If a cylinder were filled until liquid occupied all the space at 20 °C, warming it by ten degrees would leave the liquid nowhere to go and the pressure would rise hydraulically, far faster than vapour pressure. That is the physical basis of the filling ratio, the maximum mass of gas per litre of water capacity, which in India is set under the Gas Cylinders Rules, 2016 and the cylinder standards they call up. The comparison with CO₂, the other high-pressure liquefiable gas KIGL customers meet, is on critical temperature and pressure of gases.
Why nitrous oxide is sold by weight
Because the pressure of a liquefied gas tracks temperature, not contents, the gauge on an N₂O cylinder stays near the vapour pressure until the last of the liquid has boiled away. Only then does it fall, and by then the cylinder is nearly empty. The contents are therefore measured by weighing: gross weight minus the tare stamped on the cylinder. For converting to volume, the NIST gas densities give:
| Basis | Mass per m³ | Volume per kg |
|---|---|---|
| 0 °C and 1.01325 bar | 1.978 kg | 0.506 m³ |
| 15 °C and 1.01325 bar | 1.873 kg | 0.534 m³ |
The same logic applies to carbon dioxide, and the general principle, with the Gas Cylinders Rules definitions of compressed, liquefiable and dissolved gases, is set out in permanent vs liquefied gases.
Rules that matter
Nitrous oxide is not inert. Its enthalpy of formation is about +82 kJ/mol (NIST), which means the molecule holds energy that it can release by breaking down:
2 N₂O → 2 N₂ + O₂
The reaction gives off about 82 kJ for every mole of N₂O and produces a mixture that is one-third oxygen. It needs a high temperature to start, but once started it can propagate through pure nitrous oxide with no fuel present, and the oxygen it produces makes any nearby combustible burn harder. The practical triggers are:
The decomposition hazard
- oil, grease or other organic contamination in a valve, regulator or hose, which can ignite on fast pressurisation;
- rapid opening of a cylinder valve into a dead-ended regulator, which heats the gas by adiabatic compression;
- fire exposure of a cylinder or manifold.
The controls are the same as for oxygen: oxygen-clean fittings, no lubricants except those approved for oxidiser service, valves opened slowly, and regulators and hoses rated for N₂O. The full hazard picture, including exposure limits and first aid, is on nitrous oxide safety data explained.
Medical and industrial grades
| Grade | Governing document | Typical use | Notes |
|---|---|---|---|
| Medical (IP) | Indian Pharmacopoeia monograph, under the Drugs and Cosmetics Act, 1940 | Anaesthesia and analgesia, often as a 50 % mixture with oxygen | Manufactured and released as a drug, with batch records and a certificate of analysis |
| Food | Food-additive specification (E942) | Propellant and whipping agent for cream | Must meet food-grade purity; supply terms differ |
| Industrial and analytical | Supplier specification | Nitrous oxide–acetylene flame in atomic absorption spectrometry; semiconductor and process uses | Purity and moisture to the application |
Medical nitrous oxide is a drug, not industrial gas in a blue cylinder. In India it is made and tested to the Indian Pharmacopoeia monograph under a manufacturing licence, and the medical cylinder is identified under IS 3933:2021 Table 1 by French blue (166) on body and shoulder. The monograph's impurity limits are not reproduced here; ask for the certificate of analysis against the current IP edition. The medical gas framework is summarised on the medical gas standards page.
Rules that matter
What this means for you
- Weigh nitrous oxide cylinders to know the contents; the gauge only tells you the temperature.
- Keep cylinders below 36.4 °C, out of direct sun, so the contents stay as liquid and vapour.
- Treat N₂O fittings as oxygen fittings: no oil, no grease, slow valve opening.
- Use the medical grade only where it is supplied and labelled as a drug in a French blue cylinder; industrial gas is not interchangeable.
- Read the nitrous oxide safety data sheet before first use.
Nitrous oxide technical data — frequently asked questions
What is the critical temperature of nitrous oxide?
36.4 °C, with a critical pressure of 72.4 bar (NIST, from Ohgaki and co-workers, 1990). Below 36.4 °C a nitrous oxide cylinder holds liquid with vapour above it; above that temperature the contents become a single supercritical fluid and the pressure rises steeply with temperature. In an Indian summer a cylinder left in the sun can pass the critical point.
What is the pressure in a nitrous oxide cylinder?
Whatever the vapour pressure is at the cylinder's temperature, as long as some liquid remains: 31.2 bar at 0 °C, 50.5 bar at 20 °C and 63.1 bar at 30 °C (NIST). The gauge stays near that figure until the liquid is almost gone, so it shows temperature, not how much is left. Weigh the cylinder to know the contents.
How much gas is in 1 kg of nitrous oxide?
About 0.51 m³ of gas at 0 °C and 1 atm, or 0.53 m³ at 15 °C, from the NIST gas densities of 1.978 and 1.873 kg/m³. That conversion matters because N₂O is sold by weight: most of it is stored as liquid, and the pressure of a liquefied gas depends on temperature, not contents. Weigh the cylinder and subtract the stamped tare.
What is the density of liquid nitrous oxide?
About 907 kg/m³ at 0 °C, 785 kg/m³ at 20 °C and 688 kg/m³ at 30 °C for the saturated liquid (NIST). The liquid expands sharply as it warms towards 36.4 °C, which is why a liquefied-gas cylinder must never be filled so full that the liquid can expand to fill it: the filling ratio limit exists for that reason.
How much energy does nitrous oxide release when it decomposes?
About 82 kJ per mole, roughly 1.9 MJ per kilogram, and it can do so without any fuel. Nitrous oxide is thermodynamically unstable, with an enthalpy of formation of about +82 kJ/mol (NIST), and once decomposition is started by enough heat it can propagate through the gas, releasing nitrogen, oxygen and heat. Contamination with oil, rapid valve opening and fire exposure are the usual triggers.
What colour is a medical nitrous oxide cylinder in India?
French blue (shade 166) on the body and shoulder, under IS 3933:2021 Table 1, the Indian standard for medical gas cylinders. The cylinder carries UN 1070 and the symbol N₂O. The industrial colour table in IS 4379:2021 has no separate nitrous oxide row, so confirm the marking of any industrial cylinder from its label.
References
- Bureau of Indian Standards. (2021). Colour identification of medical gas cylinders (IS 3933:2021).
- Bureau of Indian Standards. (2021). Identification of the contents of industrial gas cylinders (IS 4379:2021, Second Revision).
- CGA nitrous oxide publications (G-8 series), Compressed Gas Association
- EIGA nitrous oxide safety guidance, European Industrial Gases Association
- 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. [Nitrous oxide, phase-change data and thermophysical properties] https://webbook.nist.gov/cgi/cbook.cgi?ID=C10024972
- NIOSH Pocket Guide to Chemical Hazards and criteria document on nitrous oxide https://www.cdc.gov/niosh/npg/
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