Fog and Condensation Nuclei

Cloud droplets are, for the most part, dew which forms on tiny dust and salt crystals which are blown by the winds . These particles are called condensation nuclei and are so small that they can be seen only under an electron-microscope with a magnification of several thousand times. The most efficient condensation nuclei result from the evaporation of water from tiny droplets in the spray from the ocean; however, other condensation nuclei are the result of combustion, even if no smoke is visible.

illustration of Fog and Condensation NucleiTake a gallon glass bottle and paint the back half black or tape a piece of black paper to it. Insert a short piece of fire polished glass tubing to a one hole stopper. Fasten a 6 to 10" piece of rubber tubing to it. Fill enough warm water into the jug to cover the bottom with 1/4" of water and insert the stopper into the bottle. Let it sit until the humidity in the bottle is near 100% or until dew forms on the sides of the bottle.

Darken the room or perform the experiment in a dark corner. When you are ready to demonstrate the bottle, shine a strong light beam from the side through the jug, while you look toward the black background. Using a slide projector as the light source seems to make the cloud formation most visible.

Slosh the water around the jug to clear away dew that might have formed on the inside. Withdraw some air out of the bottle through the tubing, and pinch the tubing shut. Observe what takes place in the jug. (There may be a thin fog in the bottle; however, if there is none do not be surprised.) Make a mental note of the density of the fog and of the size of the droplets.

Light a match and hold the tip of its flame to the end of the rubber tubing. Let the fumes from the flame flow into the jug. Now withdraw air from the jug and observe. ( The fog or cloud should be quite thick and noticeable from the back of the room.) Let the air back into the jug and you will see the fog disappear. Repeat the experiment and you will note that the nuclei are still in the jug, producing fog, as soon as you withdraw some air.

Most of the nuclei, but not all, can be removed from the bottle by letting it stand for many hours, during which the nuclei diffuse to and adhere to the wall of the bottle, as well as slowly fall into the water surface. Vigorous shaking of the bottle will remove some of the nuclei so the demonstration may be usable the next hour.

Material: One gallon glass jug

One-hole stopper

Glass or plastic tubing

Six to 10 inch rubber tubing to fit the eye dropper

Rubber band, black paint or piece of black paper

Bright light source (slide projector)