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Physical
Evaporation
Evaporation is the change of a liquid into a gas at its surface, as when water turns into vapor and rises into the air.
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The idea
Evaporation is a kind of vaporization that happens at the surface of a liquid as it turns into a gas. Molecules in a liquid constantly bump into each other and pass energy back and forth. When a molecule near the surface gains enough energy, it escapes into the air as a gas. At any moment, only a fraction of the molecules have enough energy to escape.
Because the escaping molecules carry energy away, the liquid left behind gets cooler, an effect called evaporative cooling. This is why sweat cools your skin on a hot day. High humidity slows evaporation, because the air already holds a lot of water vapor. In a closed container, a liquid keeps evaporating until the air above it is saturated.
Evaporation is an essential part of the water cycle. Energy from the sun evaporates water from oceans, lakes, damp soil and other sources. The water vapor can then rise and form clouds. Scientists who study water call evaporation and transpiration, the water given off through plant leaves, by one name: evapotranspiration.
Check yourself
Question 1 of 2
Which of these conditions slows down the evaporation of water?
Composition
what it's made of
- Liquid Surface — Evaporation happens where the liquid meets the air.
- Molecular Energy — A molecule must gain enough energy from collisions to break free.
- Heat Source — Sunlight supplies the energy that evaporates water from oceans, lakes and soil.
- Humidity — Water vapor already in the air slows evaporation down.
- Water Vapor — The invisible gas that forms and rises, and can later make clouds.
Connection
how it relates
Under Water Cycle
Change
how and why it evolves
- Past
- Long before anyone knew about molecules, people used evaporation to collect salt from seawater and to dry food in the sun.
- Now
- Today scientists measure evaporation and evapotranspiration to track water supplies, plan irrigation and forecast weather.
- Next
- Warmer temperatures will likely speed up evaporation in many places, which can dry out soils and add moisture to storms.