Physical properties
| Gas | Formula | Mol. wt | Density 0 °C (g/l) | Rel. to air | Boiling point | Critical point |
|---|---|---|---|---|---|---|
| Acetylene | C₂H₂ | 26.038 | 1.1716 | 0.906 | −83.6 °C (sublimes) | 35.18 °C / 60.4 atm |
| Oxygen | O₂ | 31.999 | 1.4290 | 1.105 | −183.0 °C | −118.6 °C / 49.8 atm |
| Nitrogen | N₂ | 28.013 | 1.2506 | 0.967 | −195.8 °C | −146.9 °C / 33.5 atm |
| Argon | Ar | 39.948 | 1.7840 | 1.380 | −185.8 °C | −122.4 °C / 48.1 atm |
| Carbon dioxide | CO₂ | 44.010 | 1.9770 | 1.529 | −78.5 °C (sublimes) | 31.0 °C / 72.8 atm |
| Hydrogen | H₂ | 2.016 | 0.0899 | 0.070 | −252.9 °C | −240.0 °C / 12.8 atm |
| Helium | He | 4.003 | 0.1786 | 0.138 | −268.9 °C | −267.9 °C / 2.24 atm |
| Nitrous oxide | N₂O | 44.013 | 1.9780 | 1.530 | −88.5 °C | 36.4 °C / 71.7 atm |
| Compressed air | Air | 28.960 | 1.2930 | 1.000 | −194.3 °C | −140.6 °C / 37.2 atm |
Heavier or lighter than air — and why it matters
Relative vapour density decides where a leak goes, which decides where it kills someone. Anything above 1.0 sinks and collects in pits, trenches, cellars and tank bottoms. Anything below 1.0 rises and gathers at roof level.
| Behaviour | Gases | Where it accumulates |
|---|---|---|
| Sinks — heavier than air | Oxygen, Argon, Carbon dioxide, Nitrous oxide | Pits, trenches, cellars, tank bottoms, basements |
| Roughly neutral | Nitrogen, Compressed air | Disperses with air movement |
| Rises — lighter than air | Acetylene, Hydrogen, Helium | Roof space, high points, under ceilings |
Flammability and hazard class
| Gas | Flammable in air | Hazard character | UN | CAS |
|---|---|---|---|---|
| Acetylene | 2.5 – 81 % | Flammable gas | 1001 | 74-86-2 |
| Oxygen | Not flammable — oxidiser | Oxidiser — makes other things burn | 1072 | 7782-44-7 |
| Nitrogen | Not flammable — inert | Inert / asphyxiant | 1066 | 7727-37-9 |
| Argon | Not flammable — inert | Inert / asphyxiant | 1006 | 7440-37-1 |
| Carbon dioxide | Not flammable | Inert / asphyxiant | 1013 | 124-38-9 |
| Hydrogen | 4 – 75 % | Flammable gas | 1049 | 1333-74-0 |
| Helium | Not flammable — inert | Inert / asphyxiant | 1046 | 7440-59-7 |
| Nitrous oxide | Not flammable — oxidiser | Oxidiser — makes other things burn | 1070 | 10024-97-2 |
| Compressed air | Not flammable | Inert / asphyxiant | 1002 | 132259-10-0 |
Acetylene's range in pure oxygen is 2.5 – 100% — there is no proportion of acetylene in oxygen that is not ignitable. That, plus its ability to decompose without any oxygen present, is why it is stored dissolved rather than compressed.
Indian Standard specifications
| Gas | Specification | Cylinder notes |
|---|---|---|
| Acetylene | IS 308 (1988, Amdt 1:2005) | Dissolved in acetone over a porous mass — sold by weight, not by cylinder pressure. |
| Oxygen | IS 309 (2005, 4th revision) | 47 L water capacity at 150 kg/cm² ≈ 7 Nm³. |
| Nitrogen | IS 1947 (1972, Amdt 1 & 2) | 47 L water capacity at 150 kg/cm² ≈ 6 Nm³. |
| Argon | IS 5760 * | 47 L water capacity at 150 kg/cm² ≈ 7 Nm³. |
| Carbon dioxide | IS 307 * | Liquid under its own vapour pressure (~57 kg/cm² at room temperature) — sold by weight. A CO₂ cylinder reads full on the gauge until nearly empty. |
| Hydrogen | IS 1090 * | 47 L water capacity at 150 kg/cm² ≈ 6–7 Nm³. |
| Nitrous oxide | IP * | Liquefied under pressure — sold by weight. |
* Confirm the exact standard and revision your inspecting authority requires before ordering against it.
Frequently asked questions
Which industrial gases are heavier than air?
Argon, carbon dioxide, nitrous oxide and oxygen are all denser than air and sink into pits, trenches, cellars and tank bottoms. Hydrogen and helium are lighter and rise. Nitrogen and acetylene are close to neutral. This determines where a leak accumulates — and where it becomes dangerous.
What is the density of argon gas?
1.784 g per litre at 0 °C and 1 atm, giving a relative vapour density of 1.38 against air. That is why argon pools in confined spaces and why welders working in tanks and trenches face the greatest risk from it.
What is the flammable range of acetylene?
2.5% to 81% in air, and 2.5% to 100% in pure oxygen — meaning there is no proportion of acetylene in oxygen that will not burn. Acetylene can also decompose without any oxygen present, which is why it is stored dissolved in acetone rather than compressed.
Which gases are oxidisers rather than fuels?
Oxygen and nitrous oxide. Neither burns itself; both make other materials burn far more readily. This is why oil and grease must be kept away from oxygen equipment — hydrocarbon contamination under high-pressure oxygen can ignite with no spark at all.
What is the boiling point of nitrogen?
−195.8 °C at 1 atm. Argon boils at −185.8 °C and oxygen at −183.0 °C, which is the difference cryogenic air separation exploits to split them apart.
What does relative vapour density mean?
The density of the gas compared with air at the same conditions. Above 1.0 the gas sinks; below 1.0 it rises. Hydrogen at 0.07 rises rapidly, while carbon dioxide at 1.53 sinks and stays low.