Theory of Molecular Motion

Time requirement: 30 minutes as an activity, 20 minutes as a demonstration.

Astronomical tidbit: Nearly all the heat energy in our atmosphere and on the surface of the Earth comes from the Sun. The Sun appears different in the photo on the left than it does when we see it with "our own eyes" (caution: don't look directly at the Sun without proper glasses or filters) because the picture was taken with a camera which is only sensitive to X-rays. The photograph was taken on January 24, 1992 by the Soft X-ray Telescope aboard the Yokkoh spacecraft. The bright features in the photo are extremely hot gases (over 1 million degrees!) trapped in the Sun's magnetic field. The dark regions are holes in the solar corona through which the solar wind emerges.

Materials

• 3 graduated cylinders (200 ml)
• 3 medicine droppers
• 3 thermometers
• Sources of hot water, cold water and tap water
• Food coloring
• Wall clock with second hand

Procedures

1. To the first graduated cylinder, add about 200 ml of hot water (the hotter, the better).
2. To the second graduated cylinder, add about 200 ml of tap water at about room temperature.
3. To the third graduated cylinder, add about 200 ml of cold water (the colder, the better). Remove any ice cubes first.
4. Record the temperature of the water in each cylinder.
5. Allow the water to settle for about two minutes.
6. Add two drops of food coloring to each cylinder at the same time.

Observation questions

1. What was the temperature of the water in each cylinder?
2. In which cylinder did the dye spread out the fastest?
3. In which cylinder did the dye spread out the slowest?
4. What was the difference between these two cylinders?
5. How long did it take the dye to spread completely in the cylinder with the hot water?
6. How long did it take the dye to spread completely in the cylinder with the cold water?
7. Why was it necessary for the water to stop moving before the dye was added?

To think about

Gases which are enclosed in a container exert pressure on the walls of the container. The pressure is caused by collisions between the moving gas molecules and the enclosing walls. If the temperature of an enclosed gas in increased (so that the velocity of the gas molecules also increases), do you think that the pressure exerted by the gas will increase, decrease or remain the same? Why???

A gas which is not enclosed will expand when it is heated. Why??? Is hot air therefore more dense or less dense than cold air? (Think about whether air rises or sinks when it is heated.)