Chemical Education Today
Letters Another Variation on the “Whoosh Bottle” Theme The article “An Alcohol Rocket Car—A Variation on the ‘Whoosh Bottle’ Theme” by Dean J. Campbell (1) is an interesting variation of the popular demonstration. It includes many safety precautions, including the use of a safer plastic milk jug rather than a glass container in case of an explosion. I have used a similar demonstration in my chemistry classes over the past 10 years, which includes some additional safety measures worth mentioning. The demonstration is performed with a dry one-gallon milk jug. A volume of 7.0 mL of ethanol is added to the jug. The jug is capped and shaken for 30 seconds. After removing the top, excess ethanol is shaken from the jug, and placed in a stoppered container away from the demonstration table. Ethanol is less explosive in nature than methanol (2), and the larger volume of ethanol allows for sufficient vaporization in 30 seconds such that the demonstration may be continued rather than sitting for 15–20 minutes. Since these “Whoosh Bottle” demonstrations have caused a variety of accidents (2), it is imperative to protect against shattering when the ignition of the vapor in the jug occurs. After removing the top and drying the opening of the jug, place the jug on its side under the protection of a small three-sided safety shield1 that is lying on its side on the bench top. This allows for protection of both the demonstrator and audience when ignition occurs. The sideways safety shield also directs the propulsion of the jug in the desired direction (away from the audience and into a cleared area). This would alleviate the possible problem described in the Campbell article (1) of the alcohol rocket car rolling on its side or being propelled in the wrong direction. I have done this demonstration over 100 times, and the only time the jug shattered was when I (inappropriately) increased the oxygen gas concentration in the jug. The shattered/exploded pieces of the jug did remain under the safety shield and no accidents occurred, thus demonstrating the important use of the safety shield. The demonstration is used to discuss the combustion process. I normally first ignite a small volume of ethanol on a watch glass (behind a safety shield) to describe the slow burning process of the vapors. This provides a sharp contrast to the ignition of a small volume of ethanol vaporized in the jug. A variety of combustion/detonation processes may then be discussed (1).
Literature Cited 1. Campbell, Dean J. J. Chem. Educ. 2001, 78, 910. 2. Fortman, J. J.; Rush, A. C.; Stamper, J. E. J. Chem. Educ. 1999, 76, 1092. Edward G. Senkbeil Chemistry Department Salisbury University Salisbury, MD 21801
[email protected] The author replies: Dr. Senkbeil has added excellent safety considerations to the milk jug rocket car demonstration. A couple of additional safety comments should be made: 1. The tunnel-like blast shield will help keep the rocket car from initially tumbling, but may not keep the car from tumbling after leaving the shield. 2. It is imperative to make sure that all liquid ethanol is removed from the jug to help prevent it from catching on fire.
I had initially used ethanol in my rocket car but I encountered difficulties with the lower volatility of ethanol compared to methanol (1). It seemed in my hands that the ethanol would not completely volatilize and the film of excess liquid ethanol coating the inside of the jug would continue to burn if not manually blown out. On one occasion this continued burning of an alcohol layer actually melted and burned part of the milk jug in a rather spectacular fire. The damaged jug may be viewed at http://bradley.bradley.edu/~campbell/ demopix2.html (accessed Oct 2003). Since then I have opted to use the much smaller quantity of the more volatile methanol. Another type of rocket car that circumvents the whole issue of fire as the basis for propulsion uses the pressure from chemical production of carbon dioxide as a source of thrust. The carbon dioxide is produced by reaction between acids and carbonate salts. We have built a rocket car using a 35mm film canister attached to a chassis made from LEGO parts and propelled it using water and AlkaSeltzer tablets. This car is shown and described in more detail at http:// bradley.bradley.edu/~campbell/demopix5.html (accessed Nov 2003). Literature Cited
Note 1. The standard three-sided safety shield, which is recommended for use in performing any hazardous demonstrations, may be purchased directly from safety suppliers such as VWR Scientific Products or Flinn Scientific. Alternatively, a less expensive ($20– $30) shatterproof plexiglass safety shield may be fabricated by local auto glass/plastic businesses. Suggested dimensions are 75-cm length, 30-cm width, and 20-cm height.
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1. Ohe, S. Vapor Pressure Data. http://www.s-ohe.com/ vp_data.html (accessed Nov 2003). Dean J. Campbell Department of Chemistry Bradley University Peoria, IL 61625-0208
[email protected] Vol. 81 No. 1 January 2004
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Journal of Chemical Education
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