It forms carbon dioxide and water and gives out heat. The sooty flame is due to higher amount of carbon in ethyne than in methane. All the carbon atoms cannot get oxidized while burning this makes the flame sooty. But if ethyne is burnt with a proper control, for example, if the gas is made to pass through a small nozzle, then it gets ample air mixture to burn completely. This type of complete combustion is used for acetylene lamps in industries.
Acetylene lamps produce very luminous non-sooty flame.
This oxy-acetylene flame is used for welding metals, where very high temperatures are required. Alkynes are more reactive than the alkanes or alkenes due to the presence of unsaturated bonds. The pi bond is not localized and hence can be broken easily in a reaction. Such a reaction is called addition reaction. In an addition reaction, the pi bond converts into a sigma bond and the alkyne will become an alkane.