Question

why the air holes of a gas burner have to be adjusted when the heart vessels get blackened by the flame

Answer 1

This is because when the gas burner are not adjusted properly , it burns with a sooty flame which wastes fuel and causes pollution . That is why the vessels get blackened due to soot. Therefore , to avoid this the air holes of gas burners are adjusted.

Answer 2

the under part of the vessels gets blackenes is because incomplete combustion. the presence of unsaturated compound gives that colour becoz when unsaturated compounds are burned in a lower supply of oxygen the unsaturated compounds gives out a sooty flame which in turn contributes to the blackening of the utensils