Note: this experiment involves fire and flammable materials, adult supervision is a must.

• Ideal gas law
• Phase changes
• Droplet formation

• 20 ounce or 1 liter plastic soda bottle
• Warm water (1-2 oz.)
• Matches

1. Add warm water to the soda bottle until the bottom is just barely covered with water.
2. With the bottle tipped so that you don’t burn yourself, ignite a match and insert the burning end of the match into the bottle. Allow the bottle to fill up with smoke.
3. When the bottle is fairly filled with smoke, or when the match goes out, remove the match and screw the cap on to close the bottle. Clouds will form when water vapor forms small-but-visible droplets around particles in the air. In this experiment, the smoke will provide these particles around which the water can form small droplets.
4. With the bottle closed, squeeze it several times. You should see a cloud form! When you squeeze the bottle, the temperature of the gas inside may temporarily increase, but the temperature will quickly equilibrate with the surroundings (recall the ideal gas law we discussed in “Atoms and Molecules,” PV = Nk_bT). When you release the squeezed bottle, the temperature inside decreases, cooling the water vapor and helping it to liquefy into droplets on the particles provided by the smoke. This is very similar to how real clouds are formed in the atmosphere!