DO MOLECULES EXIST AND MOVE? [VIII-2]

The problem. In previous experimental work we have discovered that gases mix rapidly and completely. A light gas like hydrogen will not float on a heavy gas, such as carbon dioxide, but will scatter itself throughout the carbon dioxide in a short time. Many such instances of diffusion come to our attention in everyday life. The theory which is

used to explain such peculiar events assumes that the gases are composed of tiny particles called molecules. Air, under ordinary conditions, at sea level contains about thirty to forty million billion molecules per cubic centimeter. The average distance that a molecule travels between two collisions is approximately the two hundred and fifty thousandth part of an inch. If we cannot see the molecules, how do we know that they exist or move?

FIG. 18

Materials needed. A ring stand, one ring, wire gauze with asbestos center, burette clamp, Bunsen burner, matches, and sealed vacuum tube containing some broken glass (blue) and mercury.

What to do. 1. After the class has examined the vacuum tube assemble the apparatus as shown in the figure. The vacuum tube containing the mercury and broken glass should be set in a vertical position in the middle of the asbestos, as shown.

2. The burette clamp should be placed near the top of the tube so that almost all the tube above the mercury will be visible.

3. Light the burner, and heat the asbestos slowly at first. Watch the mercury and glass.

4. At the close of the work remove the Bunsen burner and watch the glass.

Questions and records. Why should the glass tube have a heavy wall? What fraction of the tube is filled with mercury? Is the mercury a solid, liquid, or gas? How long must the tube be heated before action begins? Why? Describe the behavior of the broken glass. Do the big pieces act like the small pieces? Why does the glass move? What collects on the inside glass wall? Why? Do molecules exist and move?

Draw a diagram of the tube, wire gauze, and Bunsen burner and print the names of the parts on the drawing. Write a paragraph which gives your answer to the title of this problem.

Reading references and optional problems. Read sections 103 to 107 of the text.

Examine an old tungsten lamp. Is the glass clean inside? If not, how was anything deposited on it? In what respects does the process of boiling water resemble this experiment?

IS THERE ANY UNFILLED SPACE IN A LIQUID?

[VIII-3]

The problem. If you have been studying the molecular theory, you have learned that liquids and other substances are believed to be composed of molecules with some space between them. If that is true, when two substances are mixed the molecules of one might slip between the molecules of the other in such a way that they would occupy less space when mixed than if unmixed. In the following experiment, note the volumes carefully.

Materials needed. A piece of glass tubing three eighths of an inch in diameter and about eighteen inches long, with stoppers to fit, or a long test tube; alcohol; water.

What to do. Stopper one end of the glass tube and fill it half full of water. Incline the tube, and carefully pour in alcohol so that it will mix with the water as little as possible. If the water is cold and the alcohol warm it will be easier to prevent mixing. When the tube is filled to the top, close the end with a stopper or with the thumb and invert the tube. After the alcohol and water are well mixed bring the tube into the upright position again and note the volume.

Questions and records. Does the mixture of alcohol and water occupy as much space as the two substances occupied before they were mixed? If not, how do you explain the change in volume?

Write an account of the experiment and your conclusions.

Reading references and optional problems. Read section 108.

Fill a tall, narrow jar with water. Drop into it a piece of blue vitriol and close the jar with a stopper; then allow the jar to stand undisturbed for several days. Note whether the blue vitriol goes upward in the solution. Explain.

RELATION OF AIR TO THE CANDLE FLAME [IX-1]

The problem. It has already been suggested that air is not composed of a single substance. We have found that water vapor is often present in considerable amount. In the Bunsen burner, in the gas-stove burner, and in the gas lamp, air is always mixed with the gas. Flames from such mixtures are very hot and have the advantage of depositing no soot. In order to obtain more information about the other gases in the air and about the part they play in burning, in breathing, and in plant growth, we may begin with the simple candle flame.

Materials needed. A

large tallow or paraffin candle, matches, Bunsen

burner, glass tubing four

LIME WATER

inches long (jet tube), test-tube holder, wide

mouth bottle, and lime

water solution.

FIG. 19

What to do. 1. Light the candle (fig. 19). Find three layers, or zones, in the flame. The inner zone is dark, the middle zone is bright, and the outer zone is a pale-blue layer over the middle zone. Make a full-size drawing of the three zones and color them. Label the parts.

2. Hold a piece of white paper stretched horizontally between the hands, and quickly thrust it into the flame in such a position that the center of the paper is just above the wick. When the paper begins to char, remove it quickly. Examine for the heating effect of the different zones. Make a drawing of the charred paper.

3. Hold a match or small splinter of wood across the flame; examine as in 2, noting very carefully to determine whether all the match within the flame is burned.