Heating Safely with Bunsen Burners and Crucibles

A Bunsen burner is a gas burner that produces an adjustable flame for heating in the laboratory, and a crucible is a heat-resistant ceramic cup used to heat solids to very high temperatures. In brief: light the match first, then turn on the gas; use the yellow safety flame when not heating and the blue flame with the air hole open for hot, clean heating; and let a hot crucible cool before weighing it.

Heating is one of the most common operations in chemistry, and also one of the most hazardous. Burns and fires are avoided by correct technique, suitable equipment and attention to the flame. This article describes the parts of a Bunsen burner, how to light it and adjust it, how to heat test tubes, beakers and crucibles, compares flame types, works through a heating-to-constant-mass example and gives the history of the burner.

What are the parts of a Bunsen burner?

• Base: a heavy foot that keeps the burner stable.
• Gas inlet: the nozzle to which the rubber tubing from the gas tap is connected.
• Gas valve (needle valve): on some burners, controls the flow of gas.
• Barrel (chimney): the vertical tube in which the gas and air mix.
• Collar and air hole: a rotating collar that opens or closes the air holes at the bottom of the barrel to change the amount of air.

How do you light a Bunsen burner?

1. Check the tubing for cracks, and make sure it is firmly attached to the gas tap and the burner.
2. Tie back long hair, and clear flammable materials from the area.
3. Close the air hole completely.
4. Light a match or lighter and hold it beside the top of the barrel.
5. Turn on the gas at the tap, and the burner will light with a yellow flame.
6. Open the air hole slowly to get a blue flame. If the flame “strikes back” and burns at the base of the barrel, or makes a roaring sound, turn off the gas, let the burner cool and relight with less air.
7. When finished, turn off the gas at the tap and make sure the flame is out.

What are the different flames?

The yellow flame is cooler because the fuel is only partly burned. Its yellow glow comes from glowing particles of soot, which deposit on glassware as a black layer. The blue flame has enough oxygen for complete combustion, and burns hotter and without soot. A blue flame in bright light is hard to see, so look carefully before and after you turn it off.

How do you heat different containers?

• Test tube: fill to no more than one third, hold it with a test-tube holder at an angle of about 45°, and keep the opening pointed away from people. Heat the side of the tube near the liquid surface first, and move the tube gently through the flame to avoid local boiling and “bumping”.
• Beaker or flask: place a wire gauze on a tripod above the burner, and put the vessel on the gauze, which spreads the heat. Use a medium flame at first.
• Evaporating dish: heat gently on a gauze or a water bath, to avoid spitting.
• Crucible: support it in a clay triangle on a tripod and heat as described below.
• Never heat a sealed container, or leave a burner unattended.

How do you heat a substance in a crucible?

1. Clean and dry the crucible with its lid, and heat it with a strong flame for a few minutes to remove moisture. Let it cool on a heat-resistant mat or in a desiccator.
2. Weigh the empty crucible with the lid, and record the mass to the precision of the balance.
3. Add the sample, and weigh again.
4. Place the crucible on a clay triangle on a tripod. Heat gently at first, then more strongly, with the lid slightly open to let gases escape.
5. Use crucible tongs to move a hot crucible, and let it cool in a desiccator, which prevents it from absorbing moisture from the air.
6. Weigh it again. Repeat heating, cooling and weighing until two consecutive masses agree to within about 0.002 g. This is called heating to constant mass.

Worked example: water of crystallization

A student heats hydrated copper(II) sulfate, CuSO₄·xH₂O, in a crucible to drive off the water of crystallization. The blue crystals become a white powder.

1. Mass of hydrate. 26.318 − 24.318 = 2.000 g.
2. Mass of anhydrous salt. 25.601 − 24.318 = 1.283 g.
3. Mass of water lost. 2.000 − 1.283 = 0.717 g.
4. Moles. CuSO₄: 1.283 ÷ 159.61 = 0.008038 mol. H₂O: 0.717 ÷ 18.02 = 0.03979 mol.
5. Ratio. 0.03979 ÷ 0.008038 = 4.95, which rounds to 5. The formula is CuSO₄·5H₂O.
6. Percent water. 0.717 ÷ 2.000 × 100% = 35.9%. The accepted value is 90.08 ÷ 249.68 × 100% = 36.1%, so the percent error is about 0.6%.

The two last weighings, 25.603 and 25.601 g, differ by 0.002 g, so the sample is at constant mass, and the water has been completely removed.

When should you use a hot plate instead?

Open flames are not suitable near flammable solvents, and a hot plate, a heating mantle or a water bath is safer. Hot plates provide controlled, steady heating and are used for solutions and organic liquids. They look the same when they are hot and cold, so label a hot plate that is on, and let it cool before touching it.

Who invented the Bunsen burner?

The burner is named after the German chemist Robert Bunsen, who in 1855 worked with his laboratory technician Peter Desaga at the University of Heidelberg on a burner that produced a hot, nearly colorless flame. The design was based on earlier burners, including one by Michael Faraday. The colorless flame was important for the flame tests and emission spectra studied by Bunsen and Gustav Kirchhoff in 1859 and 1860, which led to the discovery of the elements cesium and rubidium.

Which mistakes are common?

Mistake 1: Turning on the gas before the match is lit

Gas accumulates and ignites with a sudden flash. Light the match first.

Mistake 2: Leaving the burner on a heating flame when it is not used

A nearly invisible blue flame is easy to touch. Return to the yellow safety flame.

Mistake 3: Weighing a hot crucible

Hot air currents make the weight look lower, and the pan may be damaged. Cool it first.

Mistake 4: Heating a test tube at the bottom only

The liquid may shoot out. Heat the side and keep moving the tube.