by the Boom

The KINETIC THEORY states that molecules are in motion, have elastic collisions, and the warmer they are the faster they move.

(These demonstrations need a full period to perform).

1. GAS PRESSURE. Inflate a balloon and ask why it expands as air is added. If it is just because the balloon must make room for the air, then why is the air so very compressible? A sand-filled balloon would not be compressible. Ah, there must be much space between the molecules, but then why doesn't the balloon squeeze 'em tightly together? Hmmmm, the molecules must be moving and going BAM BAM BIFF BIFF and pushing each other apart and the balloon too.

2. EXPANSION UPON HEATING. Use the standard ball and ring device to show expansion of solids. A flask fitted with a one-hole stopper and a piece of glass tube, and filled completely with colored water, will show liquid expansion (a Galileo thermometer). A balloon immersed in hot water will show gaseous expansion.

Note: Be sure to point out that the molecules themselves do not expand, but it's the impact of their collisions which push them further apart to cause expansion.

Then there is the famous discussion on whether a hole in a metal plate gets larger or smaller when the plate is heated. It gets larger along with the rest of the plate since molecules are pushing each other farther apart in all directions.

3. DIFFUSION. At the beginning of the period, fill a large beaker (ie 3000ml) with water and, when the water is still, add a couple of drops of food coloring. Later in the period the color will have diffused throughout the water. Ah the intermingling of moving molecules.

For gaseous diffusion, pour a few ml of 15 Molar ammonium hydroxide onto the floor in the middle of the room. The students will soon notice that the ammonia has diffused through the air.

4. OSMOSIS. Diffusion through a porous membrane can be shown either with the usual biological methods, or with the unglazed porous cup made for this purpose obtainable from scientific catalogs. Insert a one-hole rubber stopper in the cup with a glass tube about 50cm long. Hold the cup up with the tube sticking into a beaker of colored water (the diffusion beaker above is ideal). Then invert a 1000ml beaker over the cup which you are holding with your hand. Then with the other hand, put a piece of rubber tubing onto the gas jet, start the gas, and hold the end up inside the beaker containing the cup and your first hand. After a few seconds, you may turn off the gas jet. Inasmuch as the low density methane will diffuse into the cup faster than the higher density air will diffuse out, an osmotic pressure is built up shown by the bubbles coming out in the water below.

Then, when the bubbles stop, remove the upper beaker. Now the gas inside the cup will be diffusing out faster than the air outside will diffuse in, the osmotic pressure inside is less than atmospheric, and the water in the beaker below will dramatically be pushed up the glass tubing. Neat!

5. MORE DIFFUSION. Wearing your goggles, pour a couple of ml of 12 M hydrochloric acid into a collecting bottle. Likewise pour a couple of ml of 15 M ammonium hydroxide into another collecting bottle. Cover both bottles with glass plates. Carefully shake the bottles to encourage some evaporation of the liquids. Point out that the HCl fumes from the acid are twice as dense as the ammonia fumes from the ammonium hydroxide. Next remove the glass plates, invert the ammonia bottle, and place its mouth on top of the mouth of the HCl bottle. The white fumes (ammonium chloride) formed show the meeting of the gases. Even though gravity would have the gases remain apart, due their densities, there is diffusion in both directions. Those molecules are moving!

5. HEAT CONDUCTION. Place drops of candle wax on a bar of metal (the standard heat conduction demo bars are nice). Heat one end and see the progress of heat conduction by the melting of the wax drops.

6. VAPOR PRESSURE. Toy steam engines are great for this (there is usually a student that can bring one in). Or make a steam generator with a thick walled flask, a one-hole stopper with a glass bend. Boil the water and shoot the steam on a pinwheel. If you have a Hiero's Steam Ball, use it. Steam power is neat!

7. BROWNIAN MOTION. Try the Biology department for the apparatus for this. Otherwise, describe it (cop out time). It is usually shown in your films anyway. But it does need a microscope or microprojector.

Return to Demos Page