Question

Why is the mixture of ethyne and air not used in welding?

Answer 1

We have to know that ethyne is a hydrocarbon that is composed of carbon and hydrogen atoms. Ethyne molecule has a triple bond between carbon atoms thus it lies under the category of alkynes. It is commonly known as acetylene having chemical formula ${C_2}{H_2}$.

Complete answer:
Before starting this question we must know what welding is? And what is the principle behind it?
So the answer is. Welding is a fabrication process whereby two or more parts are fused together by means of heat, pressure or both forming a join as the parts cool. Welding is usually used on metals and thermoplastics but can also be used on wood. A weld can be defined as a coalescence of metals produced by heating to a suitable temperature with or without the application of pressure, and with or without the use of a filler material. When ethyne is burnt in the air incomplete burning takes place and produces sooty flame due to the limited supply of oxygen present in the air. If ethyne is burnt in oxygen, it gives a clean flame with high temperature ($3000^\circ C$) due to the complete combustion of ethyne. Hence, this oxyacetylene flame is used for welding, and it is not possible to attain such a high temperature with air. That is why a mixture of ethyne and air is not used for welding.

Note:
We need to remember that when acetylene burns in air it gives a sooty flame or you can say it will lead to combustion. If a mixture of acetylene and pure oxygen is burnt the acetylene burns completely producing a blue flame. Among all other gases, acetylene is capable of producing the hottest flame.