The three main states of matter are solid, liquid and gas. A transition between states of matter is a phase change. Two well-known examples of phase changes are evaporation, where a liquid is turned into a gas and condensation, where a gas is converted into a liquid. Examples of these phase changes can be seen in the home and within the Earth's atmosphere.
Evaporation of Water in the Home
Evaporation is the process that occurs when the surface of a liquid is converted into a gas. This is a distinct process from boiling which involves a formation of bubbles within the bulk of the liquid that rise to the surface and are ejected as gas. There are numerous examples of water evaporating within the home. For example, evaporation occurs when a glass of water is left out overnight and the water level is found to drop. Another example can be seen when the "sweat" on a very cold glass of water evaporates as the glass warms up.
Evaporation of Water in the Atmosphere
Evaporation of water is an extremely important process within the atmosphere. Water molecules evaporate from the surface water and rise in the atmosphere, eventually forming clouds. The evaporation of surface water is a key part of the water cycle that allows the continuous movement of water between the Earth and the Earth's atmosphere.
Condensation of Water in the Home
Condensation of water occurs when water vapor within the air cools enough in order to change into the liquid state. This occurs because the movement of the water molecules becomes sufficiently small that attractive, intermolecular forces can pull them together. A good example of condensation often occurs in the home during winter time, when water droplets form on the surfaces of cold windows.
Condensation of Water in the Atmosphere
There are numerous examples of condensation within the atmosphere. The most obvious is in cloud formation. After water molecules have evaporated, they rise within the atmosphere. As the water molecules rise within the lower atmosphere, both the temperature and pressure decrease. The change in one or both of these variables can lead to condensation. However in the simplest case, cooling to a critical point (Dew point) leads to the air within the atmosphere becomes saturated with water vapor. Further cooling beyond this point leads to the condensation of water molecules. Tiny droplets of water coalesce forming larger and larger droplets. Eventually, the mass of the droplets becomes sufficient for gravity to pull them to the ground. This phenomena is known as precipitation or rain.