Light a Bunsen burner. Open and close the air hole at the bottom of the burner by means of the movable ring around it. When do you get yellow sooty flame? When do you get blue flame?

Explanation

When the air hole at the bottom of Bunsen burner is open, sufficient amount of oxygen is available for complete combustion of the gas (fuel). Hence, the gas burns with clean blue flame. When the air hole is partially blocked by movable ring around it, the air supply is limited resulting in incomplete combustion of gas and hence, yellow sooty flame is produced.