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Empty the Bottle
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Developer:
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Charles Scaife
Department of Chemistry
Union College
Schenectady, NY 12308
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Grade Levels:
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2 through 8
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Discipline:
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Physical Science/Chemistry
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Goals:
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Examine the effects of atmospheric air pressure on the filling of a large bottle with water and on emptying water from the bottle.
Empty a large bottle of water as fast as you can.
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Background:
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I first heard about aspects of this experiment from Bill Brent (1) and Bob Becker (2) at the ChemEd 93 conference at Butler University in Indianapolis, IN. Bill teaches at Rolla Senior High School in Rolla, MO, and Bob teaches at Kirkwood High School in Kirkwood, MO. Bill sets up a whirlpool between two mouth-to-mouth plastic bottles. Bob invites students to participate in a race to empty a bottle as in the following activity.
Several concepts are critical to this experiment. Emptying a bottle rapidly does not just involve simply inverting the bottle and letting gravity do the rest. Emptying the bottle rapidly also requires getting air into the bottle to displace the water and fill the partial vacuum that is produced. Otherwise there is a higher air pressure outside the bottle than inside, and that higher air pressure inhibits the water from flowing out the bottom. Several ways of getting air into the bottle are plausible. One using centrifugal force to provide an air flow up the center of the bottle works best if no other items of equipment are allowed.
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Teacher's Notes:
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A pair of students does this experiment. One functions as the timer while the other empties the bottle. Challenge both of them about ideas as to how to perform the experiment.
This experiment generates a great deal of excitement and interest when a race is set up to see which student can empty the bottle in the shortest time. Many will gleefully say that they can better the time, but it usually takes a few tries before a student solves the gurgle dilemma.
Obviously no methods are allowed which involve breaking or puncturing the bottle. I have frequently used a 5-pint glass reagent bottle with neck handle for middle school students. They can lift a little more than younger children, and the slightly larger bottle spreads the times out somewhat.
Be open to other suggestions which students may make, and try to test their ideas experimentally if possible. For example, a teacher at a recent workshop suggested an even faster way to e teacher at a recent workshop suggested an even faster way to empty the bottle, but additional equipment is required. Insert a piece of 1/4" ID x 3/8" OD x 1/16" wall Tygon tubing or rubber tugh a drinking straw connected to the tubing as the water emptiebing through the mouth and into the top of the bottle. At the GO signal, blow through a drinking straw connected to the tubing as the water empties. If you blow sufficiently in one long breath, no swirling of the bottle is required, and the bottle empties in less than half the time. Clearly there is a sanitation factor here, and drinking straws must be provided for each participant. CAUTION: Do not let a number of students blow directly through the same piece of tubing or drinking straw.
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Materials:
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Plastic dishpan or tub half-filled with water
2-liter plastic soda bottle (or 5-pint glass bottle)
Smaller plastic container such as a plastic tennis ball container (for easy filling of the larger bottle)
Cloth towel or paper towels (to clean spills)
Clock or watch with a second hand
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Explanation:
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When the bottle is first turned upside down, gravity acts on the water in the bottle and pulls it downward out of the bottle. Since the water leaving the bottle is blocking the path, no air can get in to take the place of the water that left the bottle, and a partial vacuum is set up above the water. This slows the flow of water out of the bottle because the atmospheric air pressure pushing up on the water is greater than the much smaller water vapor pressure pushing down on the bottle. A gurgling or chug/chug/ch on the water is greater than the much smaller water v pushing down on the bottle. A gurgling or chug/chug/chug action is set up at the mouth of the bottle because the more dense water trying to get out gets in the way of less dense air trying to enter the bottle and move to the top to take the place of emptied water. This gurgling causes inefficient exchange of water and air, and emptying the bottle in this way takes a long time.
Students usually suggest that an obvious solution to the gurgling might involve tipping the bottle gradually so that there is always water pouring out the bottom side of the mouth and air entering through the top side of the mouth. This procedure requires about the same amount of time as the gurgling method above because it does not utilize gravity to its fullest extent. The sides of the bottle hold back some of the watme of the water even though gravity is pulling it downward.
The best solution has to overcome both of these problems. We want to invert the bottle vertically and still let air in somehow. That can be accomplished by swirling the bottle in a circular motion. The more dense water is carried by centrifugal force to the sides of the bottle whereas the less dense air forms a vortex or tornado up the mf the bottle. The water can then pour out around the outer edge of the mouth of the bottle whereas the air enters the center of the mouth. Obviously there are degrees of success with this method. If the bottle is not swirled fast enough initially, the vortex of air is insufficient to take the place of the water, and some gurgling still occurs. If the bottle is swirled too rapidly, much of the water simply swirls around the edge of the bottle rather than being pulled by gravity out the narrow mouth. Shortening the time by the last couple of seconds requires considerable practice.
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Questions:
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When water empties from your bathtub, does it gurgle the way at least some of the water did as it was leaving your bottle? Why?
The water in the bathtub generally does not gurgle because a swirling circular motion is set up quickly in the bath water leaving the drain. That swirling motion is just like the the one set up in the bottle in this experiment. The swirling motion provides a central pathway for air to escape from the drain and prevents a build up of pressure in the drain which would inhibit water from going down the drain.
What sets up the swirling motion at the bathtub drain?
The earth rotates on its axis and sets up a centrifugal force that imparts a circular motion to the water. This is the same kind of motion that a tether ball on the end of a rope would have when the rope is being swung around by a person. The momentum of the water carries it to the outer edge of its circular path leading to the swirling motion.
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References:
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Bill Brent, Very Quick and Easy Demonstrations, Rolla Senior High School, Rolla, MO 65401, 1993, p. 15.
Robert Becker, An Incredible Evening of Chemistry: 20 Demonstrations Guaranteed to Knock Your Socks Off, Flinn Scientific, Inc., (P.O. Box 219, Batavia, IL 60510), 1993, pp. 45-46.
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Procedure:
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Be careful not to spill any water outside of the dishpan or tub while performing this activity. Partially fill the 2-liter soda bottle with water by holding the mouth of the bottle under the water level in the plastic dishpan or tub. What problems do you observe as you fill the bottle? Why do they happen? How could the problems that you observed be avoided? Keep these problems in mind as you later empty the bottle! Fill the bottle the rest of the way by dipping water from the tub into the smaller plastic container and pouring the water into the bottle. Keep the bottle inside the tub during the entire filling process.
Your task is now to empty the completely filled bottle into the tub as fast as you can. Your partner will time you as you tip the bottle over and pour the water back into the tub. A prize may be given to the person with the shortest time.
On the signal of GO, pour the bottle as quickly as possible into the tub. When you have emptied the bottle completely, dry your hands on the towel provided. If you get an idea as to how to do it faster during the first trial, you may test your idea by trying the experiment a second time.
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Recording Results:
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On a sheet of paper, write your name and the shortest time you required for emptying the bottle.
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