Expt 010 -- Water Tornado

Students are invited to participate in a race to see who can empty their jug of water the fastest. They soon find out that gravity is not the only force that can help them! This activity is a problem solving exercise which works especially well when coupled with the Flame Tornado demonstration (009).

Chemical Concepts

(and physical)

1. Atmospheric pressure works in all directions.
2. Compared to their surroundings, less dense objects tend to rise upward; more dense objects tend to sink.
3. Inertia: An object in a straight line motion tends to continue in that straight line motion unless acted upon by an outside force. When forced to travel in a circular path, an object still tends to continue in a straight line motion, thus its momentum always acts to carry an object to the outer edge of its circular path. This gives rise to a fictitious phenomenon known as centrifugal force.
4. With all else equal, denser substances tend to have more inertia, and are thus more subject to this centrifugal "force." Hence, a centrifuge can act to separate out substances by their different densities.

Safety

Use normal laboratory safety procedures. Clean up any water that you spill.

Procedure

Fill two glass jugs up completely with water, and invite two students to come up and race one another to see who can empty out their bottle into the basin in the least amount of time. Let the winner stay in, and invite other challengers to come up and try their ideas.

!!!Click here to See Movie.

!!!Click here to See Movie.

Questions

1. Suggest a reason for the faster emptying of the bottle that is swirled making a "tornado" compared with the bottle that is just turned upside-down.
2. Identify natural situations in which water tornadoes appear around the house.

Handout Makeup

Name ___________________________ Class _______

Teacher __________________________

BeckerDemos 010 Water Race

Watch the movies.

Describe the method used to empty the jug most quickly.

Curriculum-

This activity may be used as a way of introducing the concepts of centrifuges and how they function. The race is a general problem solving activity when it follows the Flame Tornado (BBExperiment 009).

Activity-

Demonstration -- Student Contest

The race is a general problem solving activity when it follows the Flame Tornado (BBExperiment 009). Show the tornado at the beginning of class, discuss it and then move on to something else. Then at the end of class that same day (or at the beginning of class the next day), without mentioning any tie-in with the tornado, conduct the race. See how long it takes for a student to take what they learned from the tornado demonstration, generalize it and apply it for solving this water jug challenge.

Safety-

Use normal laboratory safety procedures. Provide paper tools to clean up any water spills.

Time-

Teacher Preparation: 2 minutes

Class Time: 10 minutes

Materials-

• 2 equivalent large clear jugs, made of rigid material and with a relatively narrow opening
• 2 wash basin or buckets with volumes greater that those of the jugs
• 2 large funnels
• water

Disposal-

Dispose at the sink.

Lab Hints-

1. The large (2.5 L) glass jugs used for acids work well for this activity, though they must obviously be rinsed out thoroughly and have their labels removed. Apple cider jugs work too. Plastic jugs may be used, but they must be relatively rigid: 1-gal milk jugs and 2-L soda bottles tend to get demolished during the contest!
2. Refill the bottles from the water in the basins (the funnels help). You may wish to get two additional basins to place below the bottles as you refill them, to catch any splashing or spills!
3. A stop-watch can be incorporated to time the races and to see how effective and how consistent the various methods prove to be.

Observations-

• When the bottle is tipped over, the water cannot all pour out at once. Since atmospheric pressure is pushing upward on the water surface as the water is pushing downward on the air, the water tends to come out in a chug-chug-chug manner. Essentially the denser water and the less dense air are trying to exchange places, and they get in each other's way. This chugging can be likened to the flickering and dancing of a flame as the rising hot gases and the sinking cooler gases push back and forth against one another.
• This applies to a typical jug of water being emptied. If the jug is tipped over gradually, so that the water pours out through the bottom of the opening as the air pours in through the top, then this chugging can be eliminated. This proves, however, to be a very inefficient way to empty the bottle, essentially because it does not take full advantage of gravity! If only there were a way to hold the bottle in an upside-down position, and yet overcome the chugging...
• Eventually a student will come up with a solution: the notion of swirling the bottle in a circular manner and creating a vortex inside the bottle. The above mentioned "centrifugal force" would act more on the water than on the air, pulling it to the sides of the bottle -- or pushing it (...it's a fictitious force anyway, so what difference does it make!?). With a directionality established, the water can pour out unimpeded along the perimeter of the opening while the air pours in unimpeded through the center.
• The optimum swirl of the bottle is not by any means the fastest. If too large a vortex is established, the water spends more time sloshing around inside the bottle than it does pouring out. In fact, if you keep swirling the bottle, you can prevent the water from ever emptying completely. The optimum swirl is one that quickly establishes a long, thin vortex, just enough to give the water and air easy passage-way by one another. Once this vortex is established the bottle can be held still; any additional swirling just increases the time it takes for the bottle to empty.

Answers-

Q1. Suggest a reason for the faster emptying of the bottle that is swirled making a "tornado" compared with the bottle that is just turned upside-down.

A1. Swirling pushes (pulls) the water to the outside leaving a center path for air to enter. In this way, the exiting water is not competing with the entering air for the same space at the same time -- causing an ebb and flow as first one and then the other "wins out."

Q2. Identify natural situations in which water tornadoes appear around the house.

A2. When water drains from a sink or tub, a swirling vortex like a tornado appears. (Even if the water is completely still in a stationary sink, these vortexes occur due to the rotation of the Earth, and something called the "Coriolis" effect.)

Reference-

Acknowledgements: I was first shown this demonstration in 1985 by Ron Perkins at Greenwich High School, Greenwich, CT. He says he learned the technique from a fellow lab assistant (AKA: glassware washer) in college who kept getting finished with his glassware before Ron did! The original variation incorporated here is simply that of coupling the activity with the flame tornado demonstration mentioned above (Tik Liem, Chem Ed '91).

Key Words 1-

centrifuge, vortex