The Three Hole Bottle Demo

Author(s): Nabila Jahchan and Peggy Klipfel LeDuff

Discrepant Event - Teacher's Guide
SED 695B; Fall 2005

Detailed Explanation of Discrepant Event

Principles illustrated

· Another discrepant event problem is explored, to help students get familiar with the process of science. A plastic bottle with three holes covered with tape is filled with water and capped. Students predict what may happen as the tape is removed from each hole in turn, and hypothesize about the cause of the observed results.

· Liquid flow under air pressure.

· Balanced and unbalanced forces

· Surface tension of water

MA Materials

A plastic soda bottle, with 3 small holes vertically placed on one side, about 6 cm apart, and covered with a vertical strip of duct tape or electrician's tape (about 18 cm long), so as to temporarily seal all three holes. The bottle is then filled with water, and tightly capped.

The holes can be made by heating a dissecting/teasing needle, small stirring rod (or cork-hole borer of about the same diameter), and "melting" each hole through the plastic.

Have a basin and towel close at hand during the demo.

Standards

8-Physical Science

2. Unbalanced forces cause changes in velocity. As a basis for understanding this concept:

a.	Students know a force has both direction and magnitude.
b.	Students know when an object is subject to two or more forces at once, the result is the cumulative effect of all the forces.
c.	Students know when the forces on an object are balanced, the motion of the object does not change.
d.	Students know how to identify separately the two or more forces that are acting on a single static object, including gravity, elastic forces due to tension or compression in matter, and friction.
e.	Students know that when the forces on an object are unbalanced, the object will change its velocity (that is, it will speed up, slow down, or change direction).

9. Scientific progress is made by asking meaningful questions and conducting careful investigations. As a basis for understanding this concept and addressing the content in the other three strands, students should develop their own questions and perform investigations. Students will:

Plan and conduct a scientific investigation to test a hypothesis.

ASSESSABLE OBJECTIVES

Students will....

1. Demonstrate their ability to form testable hypotheses, given a sample problem.

2. Suggest valid and discriminating tests to challenge a hypothesis.

3. Predict the reasonable alternative outcomes of those tests, depending on whether the hypothesis is correct or not correct.

Questioning Script The Three Hole Bottle Demo Report

1. Compare the bottle used with the diagram at right. Point out any differences noted, or sketch the demo bottle as you see it.

2. PREDICT what you think will happen when your teacher pulls the tape off the top hole. You can describe the expected result, or show it on the diagram at right.

3. Show (and/or describe) what actually DID happen.

4. How would you explain what happened? (In other words, what do you think caused to happen what did happen?). You might be able to list 2 or 3 reasonable, possible explanations.

5. Select one of your explanations (your "most likely" one, mark that one with an X), and based on that possible explanation, what do you predict will happen when the tape is pulled down exposing the second hole? (Describe, and/or show on the diagram).

6. Show (and/or describe) what actually DID happen.

7. Did the results match your prediction?

----- If so, what does that suggest about your possible explanation (hypothesis)?

----- If not, what does THAT suggest about your hypothesis?

8. If necessary, propose a second hypothesis:

9. Now, based on your best hypothesis, what do you predict will happen when the tape is fully removed from all three holes?

10. Show (and/or describe) what actually DID happen.

11. Did the results match your prediction?

----- If so, what does that suggest about your possible explanation (hypothesis)?

----- If not, what does THAT suggest about your hypothesis?

12. Summarize briefly what this entire experience tells you about how science solves problems.

13. Explain why some hypotheses are better than others; in other words, what are the characteristics of good hypotheses which may be lacking from poor hypotheses?

14. Now we'll go back to the bottle, with the tape covering the three holes and the bottle full of water, but the bottle will be held horizontally, with all the holes facing down.

15. Assuming your best hypothesis for the actions with the vertical bottle might apply as well to the horizontal bottle, predict what is likely to happen when the tape is removed from the first hole (show and/or describe):

16. Predict what is likely to happen now when the tape is peeled down further to reveal the second hole (show and/or describe):

17. Finally, predict what is likely to happen when the tape is removed from all three holes (show and/or describe):

18. If possible, test your hypothesis by using the teacher's bottle, or building your own bottle and performing the above actions. Record all observations. Write up your work using our lab report format. Your peers will critique your work in our "Three-Hole Bottle Symposium".

19. Explain how the modern flush toilet (that miracle of human technology) is similar to the bottle systems used in this activity. Do this neatly, using diagrams as appropriate.

Prior knowledge & experience: Water leaks through holes in any types of bottles

Root question: Question -   Why does water not leak out of a closed plastic bottle

(Cap tightly secured on top) with a hole poked in it?

Target response:

   That is because the surface tension of water makes a rather good seal if the

hole is small enough.  Water molecules attract one another, which is the

cause of surface tension.  Thus molecules in the middle of the hole feel a

force pulling them back into the bottle from molecules inside the bottle.

There are obviously no molecules  outside the bottle to exert a balancing

force by attempting to pull the surface molecules out.  This is why surface

tension is apparent mostly at surfaces.  If the hole is large enough, the

pressure pushing the molecules out overcomes the surface tension forces

holding them in.

Surface Tension of Water

The surface tension of water is 72 dynes/cm at 25°C. It would take a force of 72 dynes to break a surface film of water 1 cm long. The surface tension of water decreases significantly with temperature as shown in the graph. The surface tension arises from the polar nature of the water molecule.

Cohesion and Surface Tension

The cohesive forces between molecules down into a liquid are shared with all neighboring atoms. Those on the surface have no neighboring atoms above and exhibit stronger attractive forces upon their nearest neighbors on the surface. This enhancement of the intermolecular attractive forces at the surface is called surface tension.

WHY? WHICH FORCES ARE ACTING ON THE BOTTLE

As long as no air can get into the bottle to push the water out, the water stays inside. Once the cap is loosened or removed, the air pushes on the water and forces it out the hole. Depending on the placement of the holes and the water level, the water will remain in the bottle as long as no air can get in, either through the holes or the top.

The underlying reason is in the way pressure and volume is related in a gas.

Let us start with the top hole uncovered, with air flowing into the bottle and

water flowing out the bottom hole.  Now cover the top hole and let us look at

the forces on the cylinder of water that extends from the bottom hole up

to the top surface of the water inside the bottle.

1) Atmospheric pressure below the bottom hole, acting over the area of the

bottom hole; this is your only upward force and it is equal to P_o * A, where

P_o is the pressure outside the bottle, and A is the area of the hole.

2) The gravitational force W of the cylinder of water; this is a downward

force, obviously.

3) Air trapped in the bottle, with a pressure that will vary as water

Leaves the bottle (with the top plugged).  This pressure also acts over the area of the

hole, and it produces a downward force equal to P_i * A, where P_i is the

pressure in the bottle.  (Do not get sucked in by the notion of some sort of

Vacuum inside the bottle sucking the water in.  The air inside the bottle

pushes water down.

All sideways forces on the cylinder cancel out, and are therefore ignorable.

Initially, the air in the bottle is at atmospheric pressure, and its force

cancels the upward force from the air under the bottle.  The remaining force

is the weight of the water, and this causes water to move down through the

Hole.

As water leaves the bottle, the air inside increases in volume, and this

Causes its pressure to decrease.  If the amount of air stays constant, and its

temperature does not change, the equation that relates pressure and volume is

     Pressure * Volume = constant,

So, as the volume increases the pressure decreases.  As the pressure decreases,

the total downward force on the cylinder decreases.  Here is an expression for

that force:

Force = P_o * A + W - P_i * A.

If P_i decreases to the point that the total downward force is zero, then

Water will stop flowing out the bottom hole.

Common Misconceptions:

Water doesn’t flow from any of the 3 holes

When held horizontally

Water flows under hand pressure

References & Links:

http://www.indiana.edu/~ensiweb/lessons/threehol.html

http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html

http://www.newton.dep.anl.gov/askasci/phy00/phy00875.htm