13.5. Magic, trick or physics?

Nature of Science with a siphon

Author:	Peter Dekkers
Time:	30 minutes
Age group:	14 - 18
Concepts:	Nature of Science, siphon

Note

The video is a little different from the performance described below. In the actual demonstration pouring the fluid collected on the right in the funnel on the left should start the water flow. Moreover, the larger the bottle inside, the longer the water keeps coming and the more surprising the phenomenon!

Introduction

Demonstrate a surprising phenomenon and use it to develop understanding of the Nature of Science. It’s about the journey, not the destination.

This demonstration is an idea from Lederman and Abd-El-Khalick [1998].

Equipment

• 2 large beakers

• 1 small beaker

• Food coloring

• Water

• Box

• About 1.5 m of water tubes;

• 2 funnels

• Double-sided tape or tape

• Water-resistant caulk or glue

• Empty bottle

Preparation

Place the box on a short side, lid at the back. Drill two holes on top, place the funnels in them. Drill a hole on each side of the box. Attach a tube directly from one funnel to the outside through a hole on the side. Place a large beaker underneath.

Fig. 13.10 The Magic box (using longer tubes and elevating the box would produce a more impressive result, see Figure 13.11).

Do the same for the other funnel, except that there is a siphon between the inlet and outlet. For a use the plastic bottle, cut off the bottom. Drill a hole in the cap of the bottle, attach a long piece of tube, glue the tube securely, and put the cap back on the bottle. Now invert the bottle and suspend it inside the box, below the funnel on the left. The outlet of the second funnel should let the water enter the inverted bottle: make sure there is no leakage. Guide the tube coming out of the cap from under in the box back up, and let it turn back down just below the top edge of the inverted bottle. Then lead it out of the second hole on the side. The outlet of this tube shoul ideally is situated lower than the lowest point of the inverted bottle (so that it empties all the way). A large beaker should fit under the outlets. Secure everything well. Make the tubes on the outside of equal length. Ideally, the box looks symmetrical from the outside.

Fill the siphon with water and some food coloring up to a point just below the point where the tube turns back down. The amount of water that you start off with in the small beaker must be enough to make this level rise and the water to go ‘over the top’ in the tube, and start the siphoning process.

Procedure

Ask students to pay close attention so they can later describe exactly what they saw. Then pour the water carefully through the first funnel. Collect it in the large beaker. Pour it back into the small one. Have students write down their observations.

Expected statements: you pour water into the funnel on one side and it flows out through the hose on that side.

Next, pour the same liquid thaty came out into the second funnel. If properly tuned, nothing will happen at first, but then liquid will start flowing out of the tube. Soon students will notice that the color is different. Then they’ll see that the outflow goes on for a long time, filling the large beaker almost completely. Does it stop? Yes, eventually it does. What’s in the box? Ask the students to come up with an explanation and make a sketch of what they think is in the box and explains what they saw. Let them discuss with each other: do their sketches explain the observations?

Select questions for the follow-up discussion:

1. You’ve come up with an explanation, but where did it come from? What did you use to find it? (Creativity, imagination, experience, knowledge.) Scientists depend on the same traits.

2. Is it necessary to test if your explanation is correct? Why or why not? How can you test your explanation? Is that how scientists work too?

3. Different explanations have been suggested in class. Which explanations are better and why? What are characteristics of good or better explanations?

On the side

The following aspects of Nature of Science can be clarified based on discussing these questions. See also the introduction on NoS.

1. Curiosity is a key driving force for scientists. If they don’t understand something, they don’t become disappointed but interested and excited.

2. What you observe depends on what exists, but also on ‘how’ you observe. Scientists, too, often only see something when they know what to look for.

3. To explain what you don’t understand, you use knowledge you already have. Scientists do the same. For example, because you know water as a clear liquid whose color and quantity do not change easily, you probably did not find the first event surprising. There was nothing to explain there, but later on suddenly there was. The explanation you came up with is based on knowledge and experience, but also on your creativity and imagination. Scientists also use creativity and imagination when inventing explanations!

4. Explanations are not simply accepted by other scientists, they will thoroughly test them first. There are various ways to do this. For example, can you predict what will happen according to your explanation if another, second cup of water is poured into the box?

Physics background

Looking at Figure 13.11, initially, the siphon is almost filled to the top (left). With the little extra liquid from the small beaker, the liquid at A turns the corner. The upward and downward parts of the tube remains completely filled throughout the process. The force that drives the water out of the siphon and into the tube is due to the pressure difference between A (1 bar) and B (liquid pressure at depth AB). This becomes smaller as it empties, but is zero only when the siphon is completely empty (if the set-up accords with the diagram).

Fig. 13.11 Diagram of the siphon, connected to one of the funnels. At the start, the siphon is almost completely filled with water and some dye.

Tip

Completely emptying a siphon remains peculiar, of course. Feel free to turn the box around, show the inside after the discussion and show how it works. Perhaps searching for good explanations is just as important as having them.

References

LAEK98

Norman Lederman and Fouad Abd-El-Khalick. Avoiding de-natured science: activities that promote understandings of the nature of science. In The nature of science in science education: Rationales and strategies, pages 83–126. Springer, 1998.