A colourless unstable gas, C2H2, with a characteristic sweet odour; r.d. 0.618; m.p. –80.8°C; b.p. –84.0°C. It is the simplest member of the alkyne series of unsaturated hydrocarbons, and is prepared by the action of water on calcium dicarbide or by adding alcoholic potassium hydroxide to 1,2-dibromoethane. It can be manufactured by heating methane to 1500°C in the presence of a catalyst. It is used in oxyacetylene welding and in the manufacture of ethanal and ethanoic acid. Ethyne can be polymerized easily at high temperatures to give a range of products. The inorganic saltlike dicarbides contain the ion C22−, although ethyne itself is a neutral compound (i.e. not a protonic acid).