In the Classroom edited by
Tested Demonstrations
Ed Vitz
Variations on the “Whoosh” Bottle Alcohol Explosion Demonstration Including Safety Notes
Kutztown University Kutztown, PA 19530
submitted by:
John J. Fortman,* Andrea C. Rush, and Jennifer E. Stamper Department of Chemistry, Wright State University, Dayton, OH 45435; *
[email protected] checked by:
Jay A. Young Chemical Consultant, 12916 Allerton Lane, Silver Spring, MD 20904 Mark J. Waner Division of Sciences and Mathematics, Spring Hill College, Mobile, AL 36608-1791
The “whoosh” bottle alcohol explosion demonstration has become a very popular and common demonstration, although its origin is unknown (1) and the basic directions have not even been published in the open literature (perhaps because the procedure is so simple!). Basically, it involves the combustion of methanol or ethanol vapors in large smallneck bottles when ignited with a match. This causes a blue flame to shoot out the mouth of the bottle, after which dancing flames pulsate in the jug as more air is sucked in. There have been a number of variations suggested (2–4), which involve things like using 70% 2-propanol for other effects such as a “ring of fire” (4). Plastic bottles have generally replaced glass bottles for safety reasons; however, there have been recent published (5, 6 ) and unpublished reports of jugs violently shattering. We have found that various effects may be produced depending on the alcohol used, its dilution with water, and the temperature. Except for the double effect observed in one variation (4), we have found no advantage to using a glass jug. CAUTIONS: (i) These alcohol explosions with undiluted alcohols should never be done in glass bottles. (ii) Oxygen or any gas mixture with a concentration of oxygen higher than air should not be used. (iii) The alcohols and the bottles should never be heated above room temperature when using methanol or ethanol. (iv) After the bottle is shaken, the pressure inside may cause alcohol to be sprayed from its mouth when the stopper or lid is removed. Therefore presenters should be careful to avoid spraying themselves or the audience. (v) If excess liquid alcohol is left in the bottle, it will increase the amount of gaseous afterburning; but the liquid may also ignite, which may cause the plastic bottle to melt. (It is wise to keep a lid handy, which can be inverted over the mouth of the bottle to extinguish the flame if it continues so long as to begin melting the bottle.) (vi) Excess alcohol on the outside of the bottle should be wiped off in order to avoid its igniting and also softening the plastic jug. (vii) Plastic jugs should be replaced when they begin to show grazing, frosting, or cracking. (One reviewer suggested using clear packing tape to wrap the outside of the jug to keep plastic shards from scattering should it shatter, but this does reduce the visibility of the burning.) (vii) Safety shields must always be in place, for even the mildest of these explosions have some chance of causing shattering; and of course safety goggles must be worn. 1092
Like all demonstrations involving fires or explosions, this one should not be attempted if the demonstrater is too fearful of an accident occurring. Moreover, teachers should stop doing the demonstration if they are tempted not to take all safety precautions in anticipation of the worst. The Observable Effects Two basic effects may be observed in these demonstrations. The first is the usual “whoosh”, which involves a moderately violent thrust of flames and gas out the mouth of the bottle, with or without afterburning or dancing flames of burning vapor in the body of the bottle. The second basic effect is a slower burn of gas down the inside surface of the bottle, producing a ring, plate, or cone of fire, which may or may not be accompanied by an upward thrust of flames in the center. The sound accompanying these slow burns is actually more of a “whomp”. Deese has described a procedure for obtaining both effects sequentially by using a glass bottle and 70% 2propanol (4 ), but if a plastic bottle is used for this variation the walls will collapse inward during the closed cooling between ignitions. Since both effects can be produced separately in plastic jugs by changing other variables, it is advised that glass containers no longer be used. A good contrast of effects is obtained if one uses pure methanol in one bottle and 70% 2-propanol or 70% propanol in a second. Basic Procedure About 25 to 40 mL of alcohol is shaken and swirled in a 5-gallon plastic water jug for 10 to 15 seconds to hasten the volatilization of the liquid and to make the gas concentration uniform throughout the bottle. The stopper or lid is removed and the excess liquid alcohol is poured out. The jug is placed behind a shield, the lights are dimmed, and a long fireplace match or kitchen match taped to a meter stick is brought over or slightly into the mouth of the bottle. In almost 200 trials testing the effects of various variables, 1-gallon plastic jugs, designed for pet watering bowls, were used for initial trials before triplicate tests with the 5-gallon jugs. The jugs were totally filled with water after each ignition to exhaust the oxygen-depleted air. If the jug was to be used for a 100% alcohol test, it was either allowed to fully dry or was given a triple rinse with the pure alcohol before a second test was done.
Journal of Chemical Education • Vol. 76 No. 8 August 1999 • JChemEd.chem.wisc.edu
In the Classroom
Variations with Different Pure Alcohols
Variations of Oxygen Concentrations
The demonstration was tested at room temperature using pure methanol, ethanol, 2-propanol, and propanol. The methanol and ethanol both give a thrust of blue flames out of the mouth of the bottle followed by some dancing flames of burning vapor in the center of the jug, the violence of the methanol explosion being somewhat greater than that of the ethanol. The 2-propanol starts with a less violent “whoosh” of escaping flames that is sometimes followed by a fast ring of flame down the inside of the container. The probability of the demonstration ending with a ring, plate, or cone of flame after the initial thrust increases somewhat if some liquid 2-propanol is left in the bottom of the jug. The propanol gives the least initial thrust of blue flame out of the mouth of the bottle and almost always is followed by a ring of flames down the side walls, with an orange cone trailing to the center until all the flames come together in the center without dancing. The decrease in violence from an explosion to more of a wall-surface burn when going from methanol to ethanol to 2-propanol to propanol is consistent with the decreasing room-temperature vapor pressures, which also is reflected in the increasing boiling points of the alcohols. Butyl alcohols did not have sufficient vapor concentrations to ignite.
We did not conduct these explosions with concentrations of oxygen gas above the 21% in air because it would appear from other alcohol explosions (7) and calculations based on this demonstration (4) that oxygen is the limiting reagent, and increasing the amount of combustion as well as the rate of reaction would not be safe. This would be particularly true for methanol, which has a much higher upper explosive limit in air than the other three alcohols. Methanol has an upper limit of explosiveness of 36% by volume in air at one atmosphere pressure and room temperature, compared to 19% for ethanol (8). This high concentration limit for methanol combined with its high volatility means that one must be particularly cautious when using methanol as it has the potential for the most violent combustion.
Variations with Temperature As one might expect, cooling the alcohols and jugs in ice baths reduced the violence of each explosion. The propanol was in fact difficult to ignite but produced a slow ring of blue flames down the inside of the container. This decrease in violence in each case would be consistent with both (i) a decrease in reaction rate due to the reduced temperature and (ii) the decreased vapor pressure of each alcohol, which would result in a lower concentration of alcohol vapor—which again reduces the both the rate and the amount available for reaction. We did not test methanol and ethanol at higher temperatures because their roomtemperature reactions were already violent enough. Slightly warming the propanol did cause it to react with a more violent explosive exit of flames out the mouth of the jug and a reduction in the ring effect. Variations of Alcohol/Water Solution Concentrations For each alcohol, dilution with water decreased the violence of the initial thrust and sufficient dilution produced a ring, plate, or cone of burning emanating from the surface of the container’s inside walls. This would be in keeping with the decreased vapor pressure and thus reduced fuel vapor concentration predicted by Raoult’s law. All four alcohols give a ring effect when diluted with 20 to 30% water. Interestingly, ethanol seems to be most affected by the addition of water, as little as 5% water sometimes producing the burning ring instead of the explosive thrust. In most instances the water left inside the jug after filling it with water to exhaust the burnt gases was enough to make a noticeable effect on the observations when using pure alcohols. Therefore we had to wait for the inside to dry or do a triple alcohol rinse before repeating those experiments.
Plastic vs Glass Containers We have not been able to detect any reproducible differences between identical procedures done in plastic or glass bottles. If there is any change due to such differences as the adherence of the alcohols, water, or solutions to the walls of the containers, they are very slight. Therefore there is no reason for doing any of these explosions in glass except perhaps for the double effect described by Deese (4) with 70% 2propanol. The two extremes may be obtained separately in plastic with methanol or ethanol for the “whoosh” effect and propanol for the ring of flames. Adding Salts for Color Although the results are not particularly dark colored and are masked by the blue tint of the plastic bottles, the addition of metal salts to alcohol solutions will add color to the explosions or flames. The best results are obtained using a 75% solution of methanol or a 70% solution of 2-propanol saturated with the salt. Sodium chloride gives a yellow-orange color, copper(II) chloride gives a green flame, and strontium nitrate adds a reddish pink tint. Boric acid adds a bright pale green color to pure and diluted methanol and ethanol, but adds no noticeable green with 2-propanol or propanol. Summary It would seem that two different effects can be obtained in these experiments. The alcohol vapor–air mixture may detonate or the alcohol in the bottle may simply burn in the presence of air. Therefore both forms of combustion (explosion and burning) can be demonstrated. If the precautions given earlier in this paper are followed, this is a relatively safe demonstration; but with so many variable factors it should always be done in ways anticipating the possibility of an accident. A safety shield should always be used. We may never know what caused past accidents, but a thorough understanding of the variables involved should help those doing these demonstrations to avoid the greatest dangers by taking proper precautions in the future. Literature Cited 1. The first recorded reference that I have for this demonstration is: Bailey, M. H. Abstracts of the Tenth Biennial Conference on Chemical
JChemEd.chem.wisc.edu • Vol. 76 No. 8 August 1999 • Journal of Chemical Education
1093
In the Classroom Education, Purdue University, West Lafeyette, IN, 1988; Abstract No. 295 and handout. I have talked with numerous teachers who knew of this demo much earlier, but were not aware of its source. 2. Robinson, D. A. 209th ACS National Meeting, Anaheim, CA, April 1995; Abstract No. CHED-038. 3. Robinson, D. A. 211th ACS National Meeting, New Orleans, LA, March 1996; Abstract No. CHED-061.
1094
4. 5. 6. 7.
Deese, W. C. CHEM 13 News 1996, Nov, 8–9. Lee, C. J. Chem. Educ. 1998, 75, 543. Deese, W. C. CHEM 13 News 1998, May, 3. Shakhashiri, B. Z. Chemical Demonstrations; University of Wisconsin Press: Madison, WI, 1985; Vol. 2, pp 216–219. 8. Lange’s Handbook of Chemistry, 9th ed.; Handbook Publishers: Sandusky, OH, 1956; pp 32–49, 824.
Journal of Chemical Education • Vol. 76 No. 8 August 1999 • JChemEd.chem.wisc.edu
