A alkyne with the formula C2H2. It is made by reacting calcium carbide with water. It is a gas at room temperature, but can be liquefied by pressure alone. It has an extremely positive enthalpy of formation of +250 kJ/mol, which means that it produces much heat when it is burned. It is used in welding torches because of this. It decomposes at pressures above 2 atmospheres, releasing carbon, hydrogen, and a large amount of heat.