Dynamite Demo? Dale D. Clyde Department of Chemistly and Earth Sciences, San Antonio College, 1300 San Pedro Avenue, San Antonio, TX 78212 Intriguing Demonstration of a Phase Change
The teacher gently rubs ice along the outside surface of a n inverted, stoppered, glass flask half-filled with water. Presto! To the amazement of the class, the water boils! This "oldie hut goodie" demonstration of a phase change (1) is a n old favorite of mine and many of my colleagues in secondary education. I t is effective in initiating class discussion because students invariably ask the question, "Why?" The materials are accessible and inexpensive: water, a flask with a rubber stopper, a burner or hot plate, and ice or a stream of cold water. The preparation is simple. Water in a half-filled, roundbottomed flask is brought to a boil. The flask is removed from the heat source, and a stopper is inserted. The flask is allowed to cool some and is ready to be placed under a stream of cold water, or have ice placed on it. Cooling the walls of the flask causes the water vapor to condense, thereby lowering the vapor pressure above the liquid water and, thus, the water boils. The Unexpected
I have performed this demonstration many times over the years, and I have carefully inspected each flask for flaws. Therefore, I was surprised last year, when I experienced a mishap. Upon reflection I recall that I did not disassemble the apparatus in my usual manner. I had just
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Journal of Chemical Education
finished the demonstration and was still holding the inverted, stoppered flask under a stream of cold water above a sink when I noticed the formation of sediment due to our hard water. The residue was settling onto the rubber stopper. While I was in conversation with the class, my attention was diverted by a n interest in flushing out the sedier ment. Unconsciouslv. I removed the rubber s t o ~ ~ from the inverted flask, h:ping that the sediment wouii fall out. But to mv bewilderment when I removed the stoo~er. .. . air quickly entkred, pushing the layer of water against the inside surface of the flask with such force that the flask exploded! I have not repeated this procedure to test whether the inverted position was in any way responsible for the explosion. I have a question, "Would the incoming air behind the mass of the water create a greater momentum or force?" I welcome opinions from physicists. At any rate I can say that in my experiences, only this one time, when I removed the stopper from a n inverted position, have I experienced a n explosion.
The Moral of this Explosion No matter haw routine a demonstration becomes, the unerpected can happen. Teachers always must he on guard.
Literature Cited 1.Sears. Jerry A; Grieve, C. J. Sci. Tpoeh. February L
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