Evaporation occurs when a liquid changes its state to form a gas or vapor. Most that occurs on Earth is the change from the liquid state of water to a water vapor. Though it looks very simple, water is not only the most abundant compound on Earth, it is also very complex. The quantity of water on the Earth never changes; it simply changes form as it passes through what is known as the water cycle. Each stage entails different processes that are required to change water to a new form.
Water has three different states: liquid, vapor, and ice. Evaporation of water is the process by which it changes from liquid to a vapor. To do this, water requires one of several conditions to be in place. For a body of water such as a lake, river, or ocean to allow some of its water molecules to switch from a liquid and be released as a water vapor into the air, heat from the sun must be present to initiate the process. A change in energy or pressure is always required from some source for this process to occur.
Water molecules contained in the same body of water, such as an ocean, do not have the same level of energy. Individual molecules with higher energy will break away from other molecules when the sun delivers heat, or energy, to the entire body of water. This is how evaporation occurs in oceans. The more energy supplied, the stronger some molecules become and the easier it is for them to break their bond to the water molecules with less kinetic energy. This is why boiling water also results in the change from liquid to vapor. The heat supplied from the stove provides the energy that allows some of the water molecules to escape into the air.
Another example is human perspiration. When the human body heats up with exertion, humans sweat, then the perspiration covering the skin evaporates to cool the body down. This occurs because the high energy molecules escape into the air as a water vapor, and some of the heat used in the process escapes as well. As a result, the skin and body are cooled down.