Evaporation is another way of separating water from chemicals dissolved in it. It works in the same way as distillation, except that evaporation is usually used when it is the substances in the water that are needed, not the water itself. The water is usually allowed to drift away as steam. In other words, distillation is used to obtain the solvent, while evaporation is used to obtain the solute.
Evaporation happens when a liquid turns into a gas. It can be easily visualized when rain puddles “disappear” on a hot day or when wet clothes dry in the sun. In these examples, the liquid water is not actually vanishing—it is evaporating into a gas, called water vapor.
Evaporation happens on a global scale. Alongside condensation and precipitation, evaporation is one of the three main steps in the Earth’s water cycle. Evaporation accounts for 90 percent of the moisture in the Earth’s atmosphere; the other 10 percent is due to plant transpiration.
Substances can exist in three main states: solid, liquid, and gas. Evaporation is just one way a substance, like water, can change between these states. Melting and freezing are two other ways. When liquid water reaches a low enough temperature, it freezes and becomes a solid—ice. When solid water is exposed to enough heat, it will melt and return to a liquid. As that liquid water is further heated, it evaporates and becomes a gas—water vapor.
These changes between states (melting, freezing, and evaporating) happen because as the temperature either increases or decreases, the molecules in a substance begin to speed up or slow down. In a solid, the molecules are tightly packed and only vibrate against each other. In a liquid, the molecules move freely, but stay close together. In a gas, they move around wildly and have a great deal of space between them.
In the water cycle, evaporation occurs when sunlight warms the surface of the water. The heat from the sun makes the water molecules move faster and faster, until they move so fast they escape as a gas. Once evaporated, a molecule of water vapor spends about ten days in the air.
As water vapor rises higher in the atmosphere, it begins to cool back down. When it is cool enough, the water vapor condenses and returns to liquid water. These water droplets eventually gather to form clouds and precipitation.
Evaporation from the oceans is vital to the production of fresh water. Because more than 70 percent of the Earth’s surface is covered by oceans, they are the major source of water in the atmosphere. When that water evaporates, the salt is left behind. The fresh-water vapor then condenses into clouds, many of which drift over land. Precipitation from those clouds fills lakes, rivers, and streams with fresh water.
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