Guide
Acetylene technical data sheet physical properties with their primary sources and the conditions they were measured at
Acetylene (C₂H₂, molecular weight 26.038) has a density of 1.17167 g/l at 0 °C and 1 atm, a specific gravity of 0.906 against air, a triple point at −80.55 °C and 1.2651 atm, and a critical point at 35.18 °C and 61.1 atm. Each figure here carries its primary source and its temperature.
- Molecular weight
- 26.038
- Density at NTP
- 1.17167 g/l at 0 °C and 1 atm; 1.08959 g/l at 20 °C; molar volume 22.223 l against 22.413 l for an ideal gas
- Triple point
- −80.55 ± 0.05 °C at 961.5 ± 0.5 mm Hg
- Critical point
- 35.18 ± 0.04 °C; critical pressure 61.1 atm
- Specific heats
- Cp 1.637 kJ/kg·K and Cv 1.309 kJ/kg·K at 0 °C and 1 atm; γ = 1.251 at 0 °C
Every acetylene datasheet on the internet prints roughly the same dozen numbers, and almost none of them says where a number came from or at what temperature it holds. This page does. It is KIGL's working reference for the physical properties of acetylene, built from the primary measurements (Ambrose, Bottomley, Din, Hölemann and Hasselmann and others) as compiled by S. A. Miller in 1965, cross-checked against IS 14814:2023, Acetylene — Code of Safety, and against the C.P.I. handbook Safety in the Production and Use of Acetylene (Paris, 1968). Where sources disagree, both figures are shown.
Conditions matter: acetylene's critical temperature sits inside the Indian ambient range and its solubility in acetone falls by a third between 15 °C and 30 °C. A number without a temperature is a guess.
Identity
| Property | Value | Source |
|---|---|---|
| Formula | C₂H₂, H–C≡C–H, linear and symmetrical | — |
| Molecular weight | 26.038 (C 12.01115, H 1.00797); IS 14814 prints 26.04 | International atomic weights 1961; IS 14814:2023 cl 4.3.2 |
| CAS number | 74-86-2 | IS 14814:2023 cl 4.2.2 |
| UN number and class | UN 1001, Class 2.1; Hazchem 2SE | IS 14814:2023 cl 4.2.3–4.2.5 |
| Bond lengths | C≡C 1.2034 Å; C–H 1.0605 Å (rₑ) | Lafferty, Plyler and Tidwell, J. Chem. Phys. 1962 |
| Dipole moment | zero (non-polar) | Watson, Rao and Ramaswamy, Proc. Roy. Soc. 1934 |
Rules that matter
The density figures below are the measurements of Bottomley, Reeves and Seitlow (Nature; J. Appl. Chem.), with a 15 °C row interpolated in the 1965 compilation. Pressures are international atmospheres (760 mm Hg).
| Temperature | Density at 1 atm, g/l | Density at 2 atm, g/l |
|---|---|---|
| 0 °C | 1.17167 | 2.36366 |
| 10 °C | 1.12909 | 2.27540 |
| 15 °C | 1.10898 | 2.23370 |
| 20 °C | 1.08959 | 2.19368 |
| 30 °C | 1.05284 | 2.11806 |
| 40 °C | 1.01858 | 2.04780 |
Three consequences follow from that table.
Gas-phase density and specific gravity
- The molar volume at NTP is 22.223 litres, against 22.413 litres for an ideal gas. Acetylene is 0.85 % denser than ideal at NTP; this is the compressibility correction a cylinder filler converting mass to volume needs.
- The specific gravity against air is 0.906 (1.17167 ÷ 1.2929 for air at 0 °C and 1 atm). The ideal-gas ratio would give 0.899. IS 14814:2023 cl 4.3.9 prints 0.906 at 15 °C. See Is acetylene lighter than air?.
- One kilogram of acetylene is about 0.918 m³ at 20 °C and 1 atm (1 ÷ 1.08959), which is the conversion behind cylinder contents stated by weight.
Phase-change constants
| Property | Value | Source |
|---|---|---|
| Triple point | −80.55 ± 0.05 °C (192.60 K) at 961.5 ± 0.5 mm Hg = 1.2651 atm = 1.282 bar | Ambrose, Trans. Faraday Soc. 1956 |
| Normal sublimation point | −83.80 °C (189.35 K) at 1.0000 atm | Ambrose (Antoine form) |
| Critical temperature | 35.18 ± 0.04 °C (308.33 K); IS 14814 prints 35.15 °C | Ambrose 1956; IS 14814:2023 cl 4.3.13 |
| Critical pressure | 61.1 atm = 61.91 bar = 6.191 MPa (selected value); IS 14814 prints 6,059 kPa | compiled 1965; IS 14814:2023 cl 4.3.14 |
| Critical volume and density | 112.8 cm³/mol → 0.2308 g/cm³; Zc = 0.272 | derived from the saturated-liquid table |
| Vapour pressure of liquid | 26.39 atm at 0 °C; ≈43.1 atm (43.7 bar) at 20 °C; IS 14814 4,406 kPa at 20 °C | Hölemann and Hasselmann, Forschungsber. NRW No. 792 (1959); IS 14814 cl 4.3.11 |
| Latent heat of vaporisation | 584 kJ/kg at the triple point; 383 kJ/kg at 0 °C; zero at the critical point; IS 14814 prints 614 kJ/kg at the triple point | Hölemann 1959; IS 14814 cl 4.3.15 |
| Liquid density | 0.617 g/cm³ at the triple point; 0.465 g/cm³ at 0 °C under its own vapour pressure; IS 14814 prints 0.613 at −80 °C | saturated-liquid molar volumes; IS 14814 cl 4.3.9 |
| Solid density | 0.729 g/cm³ at 189.1 K | compiled 1965 |
| Flash point | −17.7 °C (closed cup) | IS 14814:2023 cl 4.5.4 |
The critical pressure is a selected value, not Ambrose's measurement. Because the triple point lies above 1 atm, there is no liquid acetylene at atmospheric pressure; solid goes straight to vapour. See Vapour pressure and why liquid acetylene is banned. The critical temperature's awkward position inside the Indian summer is covered in The critical temperature and hot weather.
Thermodynamic properties of the gas
| Property | Value | Conditions and source |
|---|---|---|
| Cp | 10.18 cal/mol·K = 1.637 kJ/kg·K; 1.687 kJ/kg·K at 20 °C | 1 atm; Din, Thermodynamic Functions of Gases Vol. 2 (1956), from Heuse's measurements; Cp = 6.11 + 0.0149 T cal/mol·K |
| Cp° (ideal gas) | 10.531 cal/mol·K = 1.694 kJ/kg·K | 298.15 K; Wagman, Kilpatrick, Pitzer and Rossini, J. Res. NBS 1945; Gordon, J. Chem. Eng. Data 1963 |
| Cv | 8.14 cal/mol·K = 1.309 kJ/kg·K; 1.357 kJ/kg·K at 20 °C | 1 atm; Din 1956 |
| γ = Cp/Cv | 1.251 at 0 °C; 1.243 at 20 °C; 1.235 at 25 °C; 1.221 at 45 °C | 1 atm; Din; sound-velocity values 1.231 and 1.227 (Ingold, J. Chem. Soc. 1924) |
| Standard entropy S° | 48.002 cal/mol·K = 200.86 J/mol·K | 298.15 K, ideal gas; Wagman et al. 1945 — matches the modern NIST value |
| Heat of formation ΔHf | +54.2 kcal/mol = +226.9 kJ/mol = +8,714 kJ/kg | Valatin, J. Chem. Phys. 1948 gives 54.33; CPI 1968 uses 54.19 kcal |
| Heat of combustion | 312 kcal/mol; about 14,000 kcal/m³ at 0 °C | IS 14814:2023 cl 4.3.12; CPI 1968 |
| Bond dissociation energy, HC₂–H | 114 kcal/mol (best value, range 110–121) | Knox and Palmer, Chem. Rev. 1961 |
| Bond dissociation energy, HC≡CH | 228 kcal/mol | same |
IS 14814:2023 cl 4.3.16 prints a specific heat of 1.60 kJ/kg at 15.6 °C and 1 atm, about 2 % below the tabulated Cp at that temperature; KIGL uses the Din values with their temperatures stated. Cp rises very steeply with pressure near the critical region, which is one reason acetylene compressors are designed for the gas rather than adapted from air service.
Transport properties
| Property | Value | Conditions and source |
|---|---|---|
| Viscosity | 95.5 µP = 9.55 µPa·s = 0.00955 cP; 104.7 µP at 27 °C | 1 atm; Titani, Bull. Chem. Soc. Japan 1929; Adzumi 1937; Sutherland constant 320 over 20–100 °C |
| Viscosity under pressure | about 126 µP at 20 °C and 30 atm | Kiyama and Makita, Rev. Phys. Chem. Japan 1952 |
| Thermal conductivity | 44.0 µcal/s·cm·K = 0.01842 W/m·K at 0 °C; 48.3 µcal/s·cm·K = 0.0202 W/m·K at about 17 °C; 141 µcal/s·cm·°C at the critical point | 1 atm; Eucken, Phys. Z. 1913; Lindsay and Bromley, Ind. Eng. Chem. 1950 |
| Diffusion | self-diffusion 0.126 cm²/s; into air 0.191 cm²/s at 0 °C and 1 atm | Andrussov, Z. phys. Chem. 1952 |
| Velocity of sound | 341 m/s | 25 °C, 1 atm; Lambert and Salter, Proc. Roy. Soc. 1959 |
Rules that matter
These are treated in depth in Flammability limits and ignition temperature and Heat of formation and flame temperature; the summary values are:
Combustion and ignition — the headline figures
- Flammable range in air: 2.5–82 % v/v (IS 14814:2023 cl 4.5.3); the lower limit of 2.50 % for upward propagation is from Jones and Kennedy, US Bureau of Mines RI 3337 (1937). The upper limit depends on the apparatus and is effectively 100 % because acetylene decomposes without an oxidant.
- Flammable range in oxygen: 2.0–92.5 % in a 2 cm³ vessel (László and Németh, 1962), usually rounded to 2.0–93 %.
- Ignition temperature 325 °C and auto-ignition temperature 305 °C (IS 14814:2023 cl 4.5.1–4.5.2).
- Minimum ignition energy about 2 × 10⁻⁵ J (0.02 mJ) for the stoichiometric 7.73 % mixture in air at 1 atm and 25 °C (Fenn, Ind. Eng. Chem. 1951) — the lowest measured for any common combustible, shared only with hydrogen.
- Oxy-acetylene flame temperature: calculated adiabatic 3,114 °C at an equimolar (neutral) O₂:C₂H₂ ratio and 3,086 °C at the fully stoichiometric ratio of 2.5 (Edwards, Smith and Brinkley, US Bureau of Mines); the trade figure of 3,070–3,100 °C sits inside that band. In air the calculated maximum is 2,331 °C.
- Explosion group IIC under IS/IEC 60079; IS 14814:2023 cl 7.3.4 requires flameproof electrical fittings to IS/IEC 60079-1 in filling and storage premises.
Solubility — the figures that run the dissolved-acetylene trade
| Solvent | Value | Conditions and source |
|---|---|---|
| Water | 1.97 g/kg = 1.68 vol/vol at 0 °C; 1.229 g/kg = 1.05 vol/vol at 20 °C; IS 14814 prints 1.2 g/l at 20 °C | 1 atm; Winkler, Z. phys. Chem. 1900; IS 14814 cl 4.3.17 |
| Acetone | 36.0 g/kg = 24.3 vol/vol at 20 °C and 1 atm acetylene partial pressure; 472 g/kg (211 g per litre of solution) at 20 °C and 15 atm total; 546 g/kg at 15 °C and 15 atm | Hölemann and Hasselmann, Forschungsber. NRW Nos. 34 (1952), 78 (1954), 723 and 792 (1959) |
| Dimethylformamide | 42.7 g/kg = 37.4 vol/vol at 20 °C and 1 atm partial; 452 g/kg at 20 °C and 15 atm | as compiled by S. A. Miller (1965) from unpublished British Oxygen Co. data; Howard, Schoch and Majewski, Pet. Refiner 1954 |
| Heat of solution in acetone | 4.25–4.70 kcal/mol = 684–757 kJ per kg of acetylene dissolved | Schay, Székely, Rácz and Traply, Periodica Polytech. 1958 |
| Dilation of acetone | about 1.6 ml per gram of acetylene dissolved; loaded solvent expands 0.23 % per °C | Hölemann, Hasselmann and Strootman, Forschungsber. NRW No. 569 (1958) |
The often-quoted "300 volumes of acetylene per volume of acetone at 12 bar" is examined in The 300-volumes myth; the measured figure at 20 °C is about 247 volumes. The acetone–DMF comparison is in Acetone vs DMF, and the dissolved-acetylene product itself is on the acetylene gas page.
Rules that matter
Three numbers from IS 14814:2023 belong on any Indian datasheet because they are the ones an inspector will ask about:
Regulatory values that sit beside the physics
- Free-state acetylene outside the cylinder is normally permissible at 0.612 atm, and at a maximum of 1.021 atm only with clearance from the Chief Controller of Explosives (cl 7.2.1). See Where the 1.5 bar rule comes from.
- The maximum working pressure of a cylinder charged with acetylene at 15 °C is 15.495 atm when acetone is the solvent (cl 7.2.2) — the same 15 kg/cm² settled-pressure rule the C.P.I. recorded in 1968.
- Notification GSR 625(E) of 7 August 1983 declares acetylene, when liquid or under any pressure or mixed with air or oxygen in any proportion, to be an explosive (cl 4.5.5).
Product purity limits under IS 308:2024 are on the dissolved acetylene standards page; hazard and first-aid data are on the acetylene safety data sheet.
Rules that matter
Rules to apply when you use these numbers
- Always carry the temperature with a density, solubility or vapour-pressure figure. At 15 atm, acetone holds 546 g of acetylene per kg at 15 °C and 364 g at 30 °C; a figure without a temperature is wrong by up to a third.
- Use the measured density (1.17167 g/l at NTP), not the ideal-gas value, when converting cylinder contents between mass and volume.
- Quote IS 14814:2023 for any figure a regulator will act on, and the primary measurement for any figure a scientist will check. Where the two differ by a per cent or two, say so rather than choosing silently.
Acetylene technical data sheet — frequently asked questions
What is the density of acetylene gas in kg/m³?
1.17167 kg/m³ at 0 °C and 1 atm, 1.10898 kg/m³ at 15 °C and 1.08959 kg/m³ at 20 °C, from the measurements of Bottomley, Reeves and Seitlow. Acetylene is 0.85 % denser than an ideal gas at NTP, so a cubic metre at NTP weighs 1.172 kg, not the 1.162 kg the ideal gas law gives. Filling calculations should use the measured figure.
What is the relative density of acetylene against air?
Slightly below 1. The real-gas specific gravity is 0.906 (air = 1) at 0 °C and 1 atm, and IS 14814:2023 prints the same 0.906 at 15 °C. That is close enough to neutral that leaking acetylene neither reliably rises nor reliably sinks, which is why acetylene rooms are ventilated at both floor and ceiling level. See the separate guide on buoyancy.
What are the triple point and critical point of acetylene?
The triple point is −80.55 °C at 1.2651 atm (Ambrose, 1956), which is above atmospheric pressure, so acetylene has no liquid phase at 1 atm and sublimes at −83.80 °C. The critical temperature is 35.18 °C with a critical pressure of 61.1 atm. IS 14814:2023 prints 35.15 °C and 6,059 kPa, about 2 % lower on pressure.
What is the vapour pressure of liquid acetylene?
About 43.1 atm absolute (43.7 bar, roughly 43.5 kg/cm² gauge) from the vapour-pressure equation of Hölemann and Hasselmann fitted over 213–293 K; IS 14814:2023 gives 4,406 kPa and the CPI (Paris, 1968) liquefaction table 44.5 kg/cm² absolute. At 0 °C it is 26.39 atm. These are the pressures of liquid acetylene, never the pressure of a dissolved acetylene cylinder.
What is the specific heat ratio (Cp/Cv) of acetylene?
1.251 at 0 °C and 1.235 at 25 °C, from the tabulated real-gas specific heats of Din (Cp 10.18 and Cv 8.14 cal/mol·K at 0 °C, 1 atm). Sound-velocity measurements by Ingold gave 1.231 and 1.227. The older literature value of 1.26 reported by Giacomini and Leduc at 15 °C is not the preferred figure.
What is the viscosity and thermal conductivity of acetylene gas?
Viscosity 95.5 micropoise (9.55 µPa·s, 0.00955 cP) at 0 °C and 1 atm, rising to 104.7 µP at 27 °C, from Titani and Adzumi. Thermal conductivity 44.0 µcal/s·cm·K (0.0184 W/m·K) at 0 °C and 0.0202 W/m·K at about 17 °C. IS 14814:2023 prints 0.00943 cP at 25 °C, which does not match the measured temperature trend, so the measured values are given here.
How much energy does acetylene release on decomposition?
Its heat of formation, +226.9 kJ/mol or 8,714 kJ/kg, is returned when it decomposes to carbon and hydrogen with no air present. The CPI (1968) expresses the same quantity as 2,313 kcal per cubic metre at 15 °C. This single positive number is the physical reason acetylene is sold dissolved in acetone inside a porous mass rather than compressed.
What is the speed of sound in acetylene?
341 m/s at 25 °C and 1 atm (Lambert and Salter, Proc. Roy. Soc. 1959), with a vibrational relaxation time of 0.12 ms from sound absorption. The figure matters to anyone calibrating ultrasonic flow meters or acoustic gas detectors on acetylene service, because such instruments assume a sound speed for the gas they are set up on.
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).
- C.P.I. (1968). Safety in the production and use of acetylene. Paris.
- Linstrom, P. J., & Mallard, W. G. (Eds.). (n.d.). NIST Chemistry WebBook (NIST Standard Reference Database 69). National Institute of Standards and Technology. [acetylene thermochemical data] https://webbook.nist.gov/cgi/cbook.cgi?ID=C74862
- Wagman, Kilpatrick, Pitzer and Rossini, J. Res. Nat. Bur. Stand. 1945, 35, 467 — ideal-gas thermodynamic functions
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