Changes of State
If you take a hot shower in a closed bathroom, the mirror is likely to "fog" up. The "fog" consists of tiny droplets of water that form on the cool surface of the mirror. Why does this happen? Some of the hot water from the shower evaporates, so the air in the bathroom contains a lot of water vapor. When the water vapor contacts cooler surfaces, such as the mirror, it cools and loses energy. The cooler water particles no longer have enough energy to overcome the forces of attraction between them. They come together and form droplets of liquid water.
The process in which a gas changes to a liquid is called condensation. Other examples of condensation are shown in Figure below. A gas condenses when it is cooled below its boiling point. At what temperature does water vapor condense?
Water vapor condenses to form liquid water in each of the examples pictured here.