A collection of experiments that demonstrate chemical concepts and processes.
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Controlled explosion of a methane-air mixture

Demonstration

A large tin fitted with a press-on lid and a glass chimney is filled with methane. The gas is lit at the top of the chimney. After a while the flame burns down the chimney and, as the methane-air mixture in the tin changes in composition, an explosive mixture is reached and the lid of the tin is blown off with loud bang.
 

Lesson organisation


This simple demonstration can be used for fun, such as at Open Days, or to provide an entertaining illustration of the effect of the composition of a mixture of an inflammable gas and air on its explosive properties. A link can be made to domestic gas explosions.

A darkened room will heighten visibility of the explosion flame.

The time for carrying out the demonstration should be about 5 min.

Chemicals

Methane (natural gas) supply (EXTREMELY FLAMMABLE)

Refer to Health & Safety and Technical notes section below for additional information.

Apparatus

Eye protection for the demonstrator

Safety screens

A large tin with a press-on lid (Note 1)

Glass tubing, 2 - 3 cm in diameter, 30 - 50 cm long

Epoxy resin adhesive (e.g. Araldite)

Length of rubber tubing

Boss, clamp and stand

Wooden splints  

Health & Safety and Technical notes


Read our standard health & safety guidance

Methane (natural gas), CH4(g), (EXTREMELY FLAMMABLE) - see CLEAPSS Hazcard.

1 A catering size (500 – 750 g) instant coffee tin works well. Make a small hole about 1 cm in diameter in the base of the can. Make a larger hole halfway up the side of the tin to take the glass tubing. Use epoxy adhesive to glue the glass tube in place – see the figure below.

Apparatus set-up

 

Procedure


a Put the lid on the tin, place it on its side and use the glass chimney to clamp it in position so that the lid is facing away from the class – see the figure above. Place a safety screen between the tin and the class.

b Insert the rubber tube attached to the gas source into the hole in the base of the tin and fill the tin with methane. Allow at least a minute to ensure that the methane has displaced all the air in the tin.

c Turn off the gas supply and remove the tubing. Without delay use a lighted splint at arms length to ignite the gas emerging from the chimney. It will initially burn with a yellow luminous flame. This will change to a blue flame as more air is drawn into the tin.

d After a short while the flame will start to descend the chimney. As it reaches the bottom, the gas mixture in the can will explode, blowing the lid off the can. The explosion is fairly gentle and the lid must not be too tightly in place or it will not be blown off.

 

Teaching notes


The reaction is the combustion of methane to form carbon dioxide and water:

CH4(g) + 2O2(g) → CO2(g) + 2H2O(l), ΔHPlimsol symbol width 12 = - 890 kJ mol–1

The similarity of the flame to the flames of a Bunsen burner with the air hole open and shut should be pointed out. The flame descends the chimney because the combustion reaction is using up gas faster than the gas is rising up the chimney.

The energy released appears as heat, light, sound and kinetic energy (of the flying lid), similar to the situation in an internal combustion engine. Methane-air mixtures have quite narrow explosive limits (4 -17 mol %), whereas hydrogen-air mixtures are explosive over a much broader range (4 -77 mol %).

The source of the energy produced by the reaction could be discussed in terms of the breaking and making of bonds.

This demonstration will not work with other domestic gases, such as propane and butane, because they are denser than air. This difference also affects how these gases behave in the event of leaks. Methane (natural gas) would concentrate near the ceiling or move upstairs and could escape through open high windows. Propane and butane would sink, building up at floor level, and could migrate down stairs or into cellars.

Many mining disasters, especially coal mining, have been caused by explosion of methane-air mixtures. Methane levels have to be constantly monitored. Canaries and the Davy safety lamp have been used in the past.

Health & Safety checked March 2009

 

Related experiment


Controlled explosion of hydrogen-air mixture

 

Weblinks


YouTube - News report of methane explosion in Ukranian coal mine in 1999.

USGS Bolcano Hazards program - photo and details of a volcanic methane explosion.

 

Page last updated on 14 December 2011