Guide

Industrial gas properties table: molar mass boiling point, density, relative density, critical point and flammable range for 12 gases

By Mithillessh Garg, Director (Gas Supply)

This industrial gas properties table compares 12 gases on one basis: oxygen weighs 1.429 kg/m³ at 0 °C and 1 atm, nitrogen 1.250, argon 1.784, carbon dioxide 1.977 and hydrogen 0.0899. Argon, CO₂, N₂O, propane and oxygen are heavier than air; hydrogen, helium, methane and acetylene are lighter.

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Gas cylinders held upright in pallet racks at a KIGL works
Densest gas in the table
Propane
Lightest gas
Hydrogen
Lowest boiling point
Helium
Critical temperatures above Indian ambient
Propane 96.74 °C
Air reference density
1.2929 kg/m³ at 0 °C and 1 atm; 1.225 kg/m³ at 15 °C

Every industrial gas data sheet quotes a density, a boiling point and a critical point, but rarely on the same basis, so two suppliers' figures for the same gas can differ by 5 % or more. This page puts twelve gases on one footing: densities at 0 °C and at 15 °C, both at 1 atm (101.325 kPa); boiling points at 1 atm; critical constants and flammable ranges from named sources. Nearly every figure comes from the NIST Chemistry WebBook equations of state; acetylene comes from the primary measurements used on the acetylene technical data sheet.

The table

Gas Molar mass, g/mol Boiling point at 1 atm, °C Density at 0 °C, 1 atm, kg/m³ Density at 15 °C, 1 atm, kg/m³ Relative density (air = 1, 0 °C) Critical temperature, °C Critical pressure, bar Flammable range in air, % v/v
Oxygen, O₂ 31.999 −182.96 1.4290 1.354 1.105 −118.57 50.43 Oxidiser
Nitrogen, N₂ 28.013 −195.80 1.2504 1.185 0.967 −146.96 33.96 Non-flammable
Argon, Ar 39.948 −185.85 1.7840 1.690 1.380 −122.46 48.63 Non-flammable
Carbon dioxide, CO₂ 44.010 −78.46 (sublimes) 1.977 1.872 1.529 30.98 73.77 Non-flammable
Helium, He 4.0026 −268.93 0.17848 0.1692 0.138 −267.95 2.283 Non-flammable
Hydrogen, H₂ 2.016 −252.78 0.08988 0.0852 0.070 −240.01 12.96 4–75
Nitrous oxide, N₂O 44.013 −88.47 1.9779 1.873 1.530 36.37 72.45 Oxidiser
Acetylene, C₂H₂ 26.038 −83.80 (sublimes) 1.17167 1.109 0.906 35.18 61.9 2.5–82
Air (dry) 28.96 about −194 to −191 1.2929 1.225 1.000 about −140.6 about 37.9 Non-flammable
Propane, C₃H₈ 44.096 −42.11 2.0105 1.899 1.555 96.74 42.51 2.1–9.5
Methane, CH₄ 16.043 −161.48 0.71746 0.6798 0.555 −82.59 45.99 5–15
Carbon monoxide, CO 28.010 −191.51 1.2505 1.185 0.967 −140.29 34.94 12.5–74

Sources, column by column

  • Molar mass, boiling point, both densities and the critical constants: NIST Chemistry WebBook, Thermophysical Properties of Fluid Systems, for every gas except acetylene and air. Oxygen, nitrogen, argon and carbon dioxide match the figures on KIGL's existing technical data pages.
  • Acetylene: density 1.17167 kg/m³ at 0 °C and 1.10898 kg/m³ at 15 °C from Bottomley, Reeves and Seitlow; sublimation point −83.80 °C and critical temperature 35.18 °C from Ambrose (Trans. Faraday Soc. 1956); critical pressure 61.1 atm (61.9 bar) as a selected literature value. IS 14814:2023 prints 35.15 °C and 6,059 kPa.
  • Air: 1.2929 kg/m³ for standard dry air at 0 °C and 1 atm, and 1.225 kg/m³ at 15 °C from the ICAO standard atmosphere. Air's boiling range and pseudo-critical point are those of the NIST air model of Lemmon and co-workers (2000); as a mixture it has no single boiling point.
  • Relative density: each gas's measured density at 0 °C divided by 1.2929.
  • Flammable ranges: hydrogen and methane from the ILO/WHO International Chemical Safety Cards 0001 and 0291; propane and carbon monoxide from the NIOSH Pocket Guide; acetylene from IS 14814:2023 cl 4.5.3.

What each column tells you

Molar mass

The mass of one mole of gas. Divided by 22.414 litres it gives the ideal-gas density at 0 °C and 1 atm, which is close for oxygen, nitrogen, argon, hydrogen and helium and less close for gases near condensing. Propane's ideal density is 1.967 kg/m³ against the real 2.011, a 2 % difference; acetylene's is 1.162 against the measured 1.172.

Boiling point at 1 atm

The temperature at which the liquid boils at atmospheric pressure. For the cryogenic gases (helium, hydrogen, nitrogen, carbon monoxide, argon, oxygen, methane) it is the temperature of the liquid in an open dewar or the bottom of a storage tank. Carbon dioxide and acetylene have no liquid at 1 atm: their triple points (5.18 bar for CO₂, 1.282 bar for acetylene) are above atmospheric pressure, so the solid turns straight to gas. This is why dry ice sublimes.

Gas density at 0 °C and at 15 °C

The mass of one cubic metre of gas at the stated temperature and 1 atm. The 0 °C figure is the basis of the normal cubic metre (Nm³); the 15 °C figure is the basis of the standard cubic metre (Sm³), and 15 °C is also the temperature at which the Gas Cylinders Rules 2016 define water capacity and filling pressure. Between the two, the density of an ideal gas falls by 5.2 %, because 288.15 ÷ 273.15 = 1.055; real gases follow within a fraction of a per cent at 1 atm. Conversions are worked through in Nm³ to kg.

Relative density

The ratio of the gas's density to that of air at the same conditions. Above 1, a leak sinks; below 1, it rises. Real leaks are also governed by temperature: cold gas from a cryogenic liquid is far denser than the warm figure suggests, so even nitrogen boil-off flows along the floor until it warms. Oxygen at 1.105 is only slightly denser than air, so an enriched pocket does not clear quickly from a low, unventilated space or from clothing.

Critical temperature and pressure

The critical temperature is the highest temperature at which a gas can be liquefied by pressure alone. The critical pressure is the vapour pressure at that temperature. If the critical temperature is below room temperature, the gas in a cylinder is a compressed gas sold by pressure; if it is above, the gas is liquid in the cylinder and is sold by weight. The full treatment is in critical temperature and pressure of gases and permanent vs liquefied gases.

Flammable range

The lower and upper limits, in per cent by volume in air, between which a mixture will carry a flame. Five of the twelve are flammable; oxygen and nitrous oxide are oxidisers. The classification of every gas is in which industrial gases are flammable.

Rules that matter

For full data, grades and safety detail on each gas KIGL supplies:

Gas pages for each gas

Propane, methane and carbon monoxide are included for comparison; KIGL does not sell them as pure gases, although carbon monoxide and methane appear as components of gas mixtures.

Rules that matter

Patterns worth knowing

  • The three air gases boil within 13 °C of each other: nitrogen at −195.80 °C, argon at −185.85 °C and oxygen at −182.96 °C. Argon sits between the other two, which is why it is drawn off from the middle of the column in an air separation plant.
  • Nitrogen and carbon monoxide have almost identical molar masses (28.013 and 28.010) and densities (1.2504 and 1.2505 kg/m³ at 0 °C). Carbon monoxide therefore mixes into air rather than layering, and it cannot be located by where it collects.
  • Carbon dioxide and nitrous oxide are near-twins: molar mass 44.01, density about 1.98 kg/m³, critical temperatures of 31.0 °C and 36.4 °C. Both are sold by weight, and both reach their critical temperature on an Indian summer afternoon.
  • Hydrogen and helium have the lowest critical temperatures in the table, −240.01 °C and −267.95 °C, so at any temperature a cylinder meets in service they are compressed gases and cannot be liquefied however high the pressure. Their critical pressures are also very low: 12.96 bar and 2.28 bar.
  • Acetylene's critical temperature of 35.18 °C falls inside the Indian ambient range. The practical consequences are covered in acetylene critical temperature and hot weather.

Rules that matter

Rules to apply when you use these numbers

  • State the temperature and pressure with every density. A figure at 0 °C is about 5.5 % higher than the same gas at 15 °C.
  • Use measured densities, not molar mass ÷ 22.414, when converting cylinder weight to volume for propane, carbon dioxide, nitrous oxide and acetylene.
  • Use relative density to place detectors: high for hydrogen, helium and methane, low for propane, carbon dioxide, argon and nitrous oxide.
  • Use the critical temperature to decide whether a cylinder's gauge tells you its contents. Above the room temperature, it does not.

Industrial gas properties table — frequently asked questions

Which industrial gases are heavier than air?

At 0 °C and 1 atm, propane (relative density 1.55), nitrous oxide (1.53), carbon dioxide (1.53), argon (1.38) and oxygen (1.105) are all denser than air. They sink, flow into pits, trenches and basements and stay there in still air. Nitrogen and carbon monoxide (0.967) are very slightly lighter and mix readily. Hydrogen, helium, methane and acetylene are lighter and rise, acetylene only slightly at 0.906.

What molar mass of air should be used in gas calculations?

28.96 g/mol for dry air, which gives an ideal-gas density of 1.292 kg/m³ at 0 °C and 1 atm, within 0.1 % of the measured 1.2929 kg/m³. For relative density, divide the gas's own measured density by 1.2929 at the same temperature and pressure, rather than dividing molar masses, so that both are real-gas figures on the same basis.

Why do gas density figures differ between data sheets?

Mostly because of the reference temperature. A cubic metre of oxygen weighs 1.429 kg at 0 °C and 1 atm, 1.354 kg at 15 °C and about 1.33 kg at 20 °C. Indian cylinder standards IS 309 and IS 1747 state contents at 27 °C and 760 mm Hg. Always quote the temperature and pressure alongside a density, and compare like with like.

Which gas has the lowest boiling point of all?

Helium, at −268.93 °C (4.22 K) at 1 atm by the NIST equation of state. It has no solid phase at atmospheric pressure at all. Next comes hydrogen at −252.78 °C, then nitrogen at −195.80 °C, carbon monoxide at −191.51 °C, argon at −185.85 °C and oxygen at −182.96 °C. These boiling points are why air separation can split nitrogen, argon and oxygen by distillation.

Why is relative density not simply molar mass divided by 29?

Because real gases are not ideal. Dividing molar masses gives 0.899 for acetylene, but the measured densities give 0.906, and propane comes out 1.52 by molar mass against 1.555 from NIST densities. The difference grows for gases close to condensing. For ventilation design it rarely matters; for converting cylinder weight to gas volume it does.

Which of these gases can be liquid inside a cylinder at room temperature?

Only those with a critical temperature above the room: propane (96.74 °C), nitrous oxide (36.37 °C), carbon dioxide (30.98 °C) and, in principle, acetylene (35.18 °C), although liquid acetylene is never handled because it is unstable. Oxygen, nitrogen, argon, hydrogen, helium, methane, carbon monoxide and air have critical temperatures far below zero and stay gaseous at any pressure.

References

  1. Ambrose, D., Trans. Faraday Soc. 1956, 52, 772 — acetylene critical temperature
  2. Bureau of Indian Standards. (2023). Acetylene — Code of Safety (IS 14814:2023, First Revision).
  3. ILO/WHO International Chemical Safety Cards — hydrogen (0001), methane (0291) https://chemicalsafety.ilo.org/dyn/icsc/showcard.display?p_card_id=0001&p_lang=en&p_version=2
  4. 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] https://webbook.nist.gov/chemistry/fluid/
  5. NIOSH Pocket Guide to Chemical Hazards — propane and carbon monoxide https://www.cdc.gov/niosh/npg/

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