A Bunsen burner is a type of gas burner commonly used as a heat source in laboratory experiments. The burner consists of a flat base with a straight tube extending vertically, known as the barrel or chimney. Natural gas (predominantly methane) or a liquified petroleum gas such as propane or butane is supplied at the bottom of the chimney.
Bunsen burners are normally fitted with a hose barb at the base of the chimney to allow rubber tubing to supply the gas from a gas nozzle on the laboratory bench. There may also be a gas value on the Bunsen burner. The other critical component of a Bunsen burner is the air hole. This is located near the bottom of the chimney, just above the gas inlet. The air hole allows pre-mixing of air and gas before combustion occurs at the top of the chimney. A collar around the base of the chimney, with a hole that aligns with the air hole, acts as an air regulator, allowing the air in the pre-mixture to be adjusted.
Air is drawn into the air hole due to the Venturi effect. A fluid flow transfers energy in three ways, potential energy, pressure and kinetic energy. Bernoulli’s principle states that, due to conservation of energy, a change in velocity must result in either a change in the potential energy or a change in the fluid’s pressure. When a fluid flow increases in velocity, normally it is the pressure which decreases. Because the gas in a Bunsen burner is flowing through the chimney, it has a lower pressure than the static air surrounding it. This difference in pressure causes air to be drawn into the air hole as the gas flows past it, a phenomenon known as the Venturi effect.
As the air hole is opened the flame progresses from an unsteady orange flame to a more steady orange, a steady purple and finally a roaring blue flame. This progression results in increasing flame temperature. The unsteady orange flame produced when the air hole is completely closed is highly visible and of lower temperature. This safety flame is, therefore, used for lighting and as the default position when the Bunsen burner is not in use.