An Experiment on Thermal Pollution,
or Why We Should Leave Trees Growing Along Rivers
Concepts:
An experiment is a way to test a hypothesis to see if it can be disproved.
Objectives:
To use the scientific method to test whether shade will prevent water from becoming
heated by the sun.
Subject:
General Science, Physics
Materials:
2 shallow pans, about 9"x14"x2" (glass baking dishes work well, or whatever you have)
2 thermometers
4 small props to suspend the thermometers ¼ to ½ inch above the tray bottom
6 sheets corrugated cardboard, large enough to cover and overlap the edges of the trays all around
Procedure:
At the beginning of the period place the two pans on a bench or countertop where it will be exposed to direct sunlight. Under each pan place 2 of the corrugated cardboard sheets to act as insulation from the underlying bench/counter top.
Lay a thermometer in each of the pans. Use two props on under each thermometer to keep it off the bottom of the pan.
Add cold tap water to each of the pans. Use enough to cover the thermometers. Fill the pans to the same depth.
Record the temperature in each of the pans
Cover one of the pans with the last two sheets of corrugated cardboard. Leave the other one uncovered. (Artistic students could be encouraged to draw leaves on the top piece of cardboard before it's put in place.)
At the end of the period record the temperature in each of the pans.
Discussion:
As you set up the experiment simply explain that you're going to see if the sun will warm up the water, the way it might warm up a stream where the trees along the bank had been cut down.
While the experiment is in progress explain that the sunlight is a form of energy, and according to what we understand about the nature of energy, how it might cause the water to become warmer.
Explain that a scientific experiment is a way of asking a question.
In a scientific experiment we make a hypothesis. In this case our hypothesis is that the sun shining on the water will warm it up. If this is so, then the water should be warmer at the end of the period than it was at the beginning. If it isn't so, then the water should remain the same temperature or get cooler.
But there is a problem. The room is probably warmer than the water. Won't that warm up the water anyway? Good question ! That's why we have the second pan, the one that's covered. It's just like the first pan, it started out at the same temperature. The only difference is that one is covered, the other is not. This is our control.
If our hypothesis is correct, then the pan we left uncovered will be warmer at the end of the period than the pan we covered. If our hypothesis is incorrect, then the two pans will be the same temperature at the end of the period or the uncovered pan will be cooler. Logical isn't it?
We've performed a scientific experiment. We've demonstrated that the sun can warm the water.
We've looked at the logic behind a scientific experiment. Ours was a simple experiment, but any experiment, even the most elaborate and sophisticated, involves the same thought processes and the same logical structure.
Stress that to be valid a scientific experiment always has to have a potential outcome that can disprove the hypothesis. You have to be able to prove the theory is wrong. If you don't have a theoretically possible outcome to the experiment that would disprove the hypothesis, you don't have a scientific experiment.
After discussing the experiment as a piece of science, discuss what it means. In some cases we might want the sun to warm water. For example, if we fill our swimming pool with water, we probably are going to be happy if the sun warms it for us. On the other hand, there may be times when we don't want the sun to warm the water. Brook Trout and Salmon are very sensitive to heat and if we want these fish in our streams we have to keep the water cool (below 68 degrees F or 20 degrees C) . One way we can do that is by preventing the sun from shining on the water's surface by leaving trees along the edge of the stream to shade it.
We've seen that sunlight can warm the water in a laboratory situation. Does the same thing happen in the real world of the river? You can only answer this by going out and seeing if it does. You can measure the water temperature in a stretch of the river that is shaded by overhanging vegetation and compare it to the temperature of the water in a stretch that is exposed to the sun. (It works best if you measure the temperatures in midday, after there has been time for the sun to affect the temperature.) When we try to perform experiments in a real world situation like this we have to ask whether there are any other variables except shade which might affect temperature. And we then have to ask ourselves how we could control for these variables.