Electron-dot structure for ethyne: H:C:::C:H
When ethyne is burnt in air, incomplete burning takes place and produces a sooty flame due to the limited supply of oxygen present in the air. However, if ethyne is burnt in oxygen, it gives a clean flame with a high temperature (3000°C) due to the complete combustion of ethyne. Hence, this oxy-acetylene flame is used for welding, and it is not possible to attain such a high temperature with air. Due to this reason, a mixture of ethyne and air is not used for welding.