This is a simple experiment to show the movement of molecules.
Heat is a measure of the kinetic energy in a material. The hotter something is, the faster its molecules move. In this experiment, we will not be able to see molecules (since we do not have an electron microscope), but we will be able to see a very obvious effect caused by this movement.
All you need is:
Pour equal amounts of water into each of 2 glasses. Heat the water in one of the glasses. Then put 2 drops of food coloring into each glass. (Do not mix)
Why does the food coloring spread out faster in one of the glasses?
Because the molecules are moving faster in the hot water, so they mix the food coloring in faster. The food coloring in the other glass will eventually spread out too, but it will take much longer.
I also used this experiment to discuss with the kids how to have a controlled experiment. I asked them questions like:
What was the same with the 2 glasses?
The size and shape of the glass.
The amount of water that was in each.
The high from which the food coloring was dropped.
What would happen if I dropped the food coloring from different heights?
Some drops would hit the water faster and that would cause the coloring to spread faster.
What if we used 2 different colors?
This was perhaps the most interesting question. Elijah said that it would do the same thing. I pointed out that the company that made the food coloring may have changed the formula in order to produce the different colors, so one color might spread more easily than another. I pointed this out just to say that we really want to control every factor in our experiment (I did not think there would actually be a significant difference between the colors). Then we repeated the experiment with blue. The blue coloring did not spread nearly as quickly as the green. There really was a difference. It was a good thing we controlled parts of the experiment even if they did not seem important.
What would happen if we changed 2 things with the glasses: say, the temperature and the shape of the cup?
This question was to drive home the point of controlling our experiment. If there was a difference we would not know if it was caused by the temperature, the shape, or a combination of the two.
How does this experiment relate to the third law of thermodynamics?
Our kids had just memorized a simple version of this law, which says, “It is impossible to reach the state of absolute zero temperature.” The kids had been thinking that if matter is hotter the molecules move faster. I pointed out that you could also state the conclusion of our experiment as – if matter is colder, the molecules move more slowly. Then they got the answer, “If you could reach absolute zero, there would be absolutely no movement of the molecules.”
I hope you enjoy this easy experiment and can make a good discussion out of it too.
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