Acetylene (\(C_2H_2\)) is a gas at standard temperature and pressure (STP). This two-carbon unsaturated hydrocarbon is widely used in industry as a high-energy fuel gas, particularly in applications requiring intense heat. The gas is colorless, highly flammable, and lighter than air.
Physical State and Chemical Identity
Acetylene’s chemical formula is \(C_2H_2\), representing the simplest member of the alkyne family of hydrocarbons. The structure is defined by a carbon-carbon triple bond, which gives the molecule high energy content and reactivity. This triple bond makes the unsaturated molecule an important building block in chemical synthesis.
Pure acetylene is naturally odorless, but the commercial grades used in industry possess a distinctive, often garlic-like smell. This odor comes from impurities such as phosphine, which are created during its industrial production from calcium carbide and water. This noticeable scent acts as a practical safety warning for leaks in an industrial setting. Acetylene is slightly soluble in water, but it is highly soluble in organic solvents like acetone.
The Necessity of Stabilized Storage
Acetylene cannot be safely compressed and stored in a hollow cylinder like gases such as oxygen or nitrogen. When compressed above approximately 15 pounds per square inch gauge (psig), the gas becomes highly unstable. It can spontaneously decompose, triggered by heat or shock, releasing a large amount of energy and leading to an explosion even without oxygen.
Acetylene cylinders are not hollow; instead, they are packed with a porous filler material, such as diatomaceous earth or calcium silicate. This filler ensures there are no large empty spaces where free gaseous acetylene could collect and build up pressure.
The porous material is then saturated with a solvent, most commonly acetone or Dimethylformamide (DMF). Acetone is highly effective at dissolving acetylene, allowing it to absorb a large volume of the gas under moderate pressure—up to 250 times its own volume at 10 atmospheres of pressure. The acetylene molecules are safely dissolved in the liquid solvent, which prevents them from reacting with each other and undergoing explosive decomposition.
This method allows for the safe storage of acetylene at pressures up to about 250 psig, which is necessary for commercial use and transportation. Users must ensure that the cylinder remains upright during storage and use, as tilting can cause the solvent to escape and create unstable pockets of free acetylene gas.
Key Industrial Uses
The primary industrial application for acetylene is in oxy-acetylene welding and cutting because it produces the hottest flame of any common fuel gas. When combined with pure oxygen, the combustion of acetylene can reach flame temperatures as high as 3,090°C (5,594°F). This intense, concentrated heat is ideal for efficiently cutting thick metals and for fusion welding applications.
The gas is also widely used in metal fabrication for processes like brazing, soldering, and flame hardening. Its high energy content and clean-burning nature make it a preferred choice in these specialized metalworking fields.
Beyond its role as a fuel, acetylene is a valuable starting material, or chemical feedstock, for the synthesis of various organic compounds. Its reactive triple bond makes it a versatile precursor in the plastics industry, where it is used in the manufacturing of polymers. These include compounds like vinyl chloride, which is a building block for the common plastic polyvinyl chloride (PVC). Acetylene is also used in the production of other specialty chemicals, including some solvents and acetic acid.