A classic demonstration, cloud in a bottle
Even though we don't see them, water molecules are in the air all around us. These airborne water molecules are called water vapor. When the molecules are bouncing around in the atmosphere, they don't normally stick together.
Pumping air into the bottle forces the molecules to squeeze together or compress. Releasing the pressure by removing the stopper allows the air to expand and by doing this, the temperture of the air becomes cooler. This cooling process allows the molecules to stick together, or condense, more easily to form tiny water droplets. These groups of tiny water droplets are what makes a cloud.
Clouds on Earth form when warm, moist air rises and its pressure decreases. The air then expands and cools. As the temperature drops below the dew point, clouds form. Tiny, invisible particles in the air such as pollution, smoke or dust provide the nucleus on which the water molecules can attach and collect.
Source: Exploratorium website, National Hands-on Science Institute and Steve Spangler Science .
Aren't lucky enough to have a 10-gallon glass bottle? No worries! We can change the world with a basic 2-liter bottle.
