The Root-Beer Fountain
A memorable and fun demonstration of the effect of heat on the solubility of gases in water.
Solubility of gases
Heat capacity of Aluminum
Diffusion of gases
A 2 L bottle of Root Beer (diet type
An aluminum cylinder (about 1.8 cm diameter and 7cm or longer)
Dish pan or other shallow container
Open a cool (not cold) bottle of Root Beer very slowly to avoid formation of bubbles in the liquid.
Drill a 1 cm diameter hole in the plastic cap of an empty 2L bottle. Open the Root Beer bottle very slowly. Cover the top with several layers of plastic wrap or a plastic sandwich bag and screw on the drilled cap, so that the plastic film seals the hole in the cap.
Use a 1 cm diameter Aluminum retort rod about 10 cm long for heating.
Position the freshly opened 2-L bottle of Root Beer in the centre of a dishpan or other "mess-container". Using tongs, heat a small bar of aluminum metal as hot as you can get it using a Bunsen burner or a propane torch.
(I use a cylinder 1.8-cm diam. and 7-cm long . it has to fit through the neck of the bottle!)
Quickly move the Al out of the flame and drop it into the Root Beer.
[Or drop the retort rod so it melts the film and slides through the drilled hole in the cap.]
(Be careful not to touch the side of the plastic bottle. The bar may stick to it.)
A fountain of Root Beer will erupt, showing dramatically that gases are much less soluble at high temperatures. The odour of Root Beer will quickly reach every corner of the room, illustrating even more properties of gases. [Be prepared for a fountain 2 or 3 m high! ]
If the demonstration is a bit too messy for your facilities, try it with Soda Water or other clear, highly carbonated drink.
Try this demo with Tonic Water in a dark room illuminated with Ultra Violet light!
It will be talked about in the cafeteria at noon... Guaranteed!
[These suggestions are NOT intended to be a complete review of all the safety issues involved with this activity. Professional judgement and practices are essential. If you are unsure of the safety precautions that should be taken, seek experienced assistance.]
The aluminum cylinder will be very hot. Anticipate what would happen if you drop the hot bar on the bench and have a heat proof covering in that area.
Don't drink it, "sink it".
Remember to remove the Al cylinder from the 2 L bottle. It usually sticks to the bottom of the bottle, but it has never melted through the bottom.
This is an entertaining way to make a point about solubility of gases in cold and hot liquids.
Aluminum has twice the heat capacity of Iron or Copper, so it is the metal of choice for the demonstration, although any very hot metal will cause the CO2 to come out of solution.
An interesting student experiment could be performed with different metals and smaller bottles of Root Beer or just with soda water to which a small amount of detergent had been added. Compare remaining water levels in the bottles, or compare remaining weight of liquids in the bottles.
Root Beer has ingredients that help foam to form. Sugar in the solution seems to help in making foam, although the sticky residue is much more inconvenient to clean up than the residue from Diet types.
Materials available from Flinn Scientific:
Catalog No. Description
AP9234 Specific Gravity Metal Specimen Set
(use the Al or all of the metals in a "foam-off" competition)