Acetylene gas formula structure and properties — the numbers behind the hottest flame in the workshop

The acetylene gas formula is C₂H₂. Written to show its structure it is HC≡CH: two carbon atoms sharing a triple bond, each with one hydrogen attached, in a straight line. It is the simplest alkyne, and its IUPAC name is ethyne.

Everything useful and everything dangerous about acetylene follows from that triple bond. Forming it took energy in, when calcium carbide was made in an electric furnace, and that energy is still stored in the molecule. Acetylene is endothermic: it holds more energy than the carbon and hydrogen it is made of. Burn it and you get that energy back on top of ordinary combustion heat, which is why the flame is so hot. Compress it, and the molecule can release that energy without burning at all.

Oxy-acetylene is the hottest flame in common industrial use. Three things make it so. The triple bond releases its stored energy when it breaks. Acetylene needs only 2.5 volumes of oxygen per volume of gas for complete combustion, against 5 for propane, so less heat is spent warming surplus combustion products. And the flame burns in two stages, with the hottest zone, the inner cone, sitting a few millimetres from the torch tip, where the welder puts the work.

That is why acetylene still cuts, welds and brazes where LPG cannot. The acetylene versus LPG guide sets out where each wins.

Most fuel gases burn in air over a band of a few percent. Propane ignites between about 2 % and 10 %. Acetylene ignites from 2.5 % to 82 %, the widest range of any common industrial gas, and needs very little energy to light. Almost any leak that mixes with air produces an atmosphere that can ignite.

Two handling rules come straight from that number. Acetylene stores and filling halls ventilate at high level, because the gas is slightly lighter than air and collects under the roof. And leaks are found with soap solution, never with a flame.

Above roughly 1.5 bar g, free acetylene can decompose into carbon and hydrogen explosively with no oxygen present at all. It does not need to meet air to be dangerous. A compressed-gas cylinder of acetylene would be a bomb.

So acetylene is never compressed into an empty cylinder. It is dissolved in acetone held in a porous mass that fills the cylinder, and acetone takes up about 300 volumes of acetylene per volume at working pressure. The porous mass stops any decomposition that starts from propagating. That is why an acetylene cylinder is heavy, why it stands upright, why the gauge reads acetone vapour pressure rather than contents, and why the cylinder is weighed rather than gauged. The acetone guide and the decomposition guide go further.

Industrially, acetylene is made by reacting calcium carbide with water: CaC₂ + 2H₂O → C₂H₂ + Ca(OH)₂. A kilogram of good carbide gives roughly 300 litres of gas and a lime slurry by-product. The gas leaves the generator carrying phosphine and hydrogen sulphide from the carbide, which the plant's purification train removes before the gas is dried, compressed and dissolved into cylinders. KIGL builds those plants and fills acetylene gas at its own works, and has done since 1947.