The Bunsen burner is the workhorse of the science laboratory. It is a quick and cheap way to produce high temperatures for softening glass, boiling water or encouraging chemicals to react with one another. It probably dates back to about 1850 when Robert Bunsen was working with an engineer, kitting out his new laboratory. At the time, coal-gas was used for lighting and so it was a natural choice to develop a way of using it for heating as well. The clever thing about a Bunsen burner is that the air is mixed with the fuel gas at the base of the chimney so that the luminosity and temperature of the flame can be controlled. Our Bunsen burners use methane (natural gas) because this is the gas commonly supplied to houses for heating and cooking. It is possible to get Bunsen burners designed to burn propane or butane.
The Bunsen flame can be controlled in two ways. Firstly, the flow of gas can be regulated at the tap to increase or decrease the size of the flame. There are occasions when gentle heating is required and having a smaller flame offers more control. The other adjustment to make is opening or closing the air-hole. The picture above shows the sequence of flames from ‘air-hole fully closed’ to ‘air-hole fully open’. The first flame, known as the safety flame, is about 300 °C and very sooty. If you heat anything with this flame it will become blackened as soot is deposited. Its main use is to show that the Bunsen is lit with its bright, luminous flame. As you let more air in to mix with the gas the fuel burns more efficiently, resulting in less soot and less orangey glow. With the air-hole fully open you have the classic roaring blue flame with the distinct inner and outer cones.
The picture above shows the various regions of the blue flame. The unburned area is quite cold – it is possible to suspend an unburned match in this region without it igniting. The BBC has a good page about Bunsen burners.
Questions…
- Suggest a safety precaution you might take when lighting a Bunsen burner. (There are several, pick one.)
- What are the two main products released when methane burns in plenty of oxygen?
- If a hydrocarbon burns in a shortage of oxygen, what poisonous gas might be released?
- Roughly what temperature can be achieved by a Bunsen with the air-hole fully open?
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