Evaporation — Definition, Water Cycle, & Examples
What is evaporation?
Evaporation is a type of vaporization in which a substance changes phase from liquid to gas at a temperature below its boiling point. Evaporation occurs when molecules in the liquid phase of a substance gain enough kinetic energy, often from sunlight, to move apart and enter the atmosphere as a gas.
What happens during evaporation?
During evaporation, individual molecules of a liquid gain enough kinetic energy to leave the liquid state and enter the atmosphere in its gas phase as a vapor.
Evaporation is a far slower process of vaporization than boiling, the formation of gaseous bubbles below the liquid’s surface. Water vapor from an evaporating liquid has the same temperature as its surrounding atmosphere, unlike steam from boiling, which is much warmer than the ambient temperature.
The process of evaporation removes heat from the environment, which is why sweat evaporating from skin cools humans down.
Why does evaporation happen?
Evaporation occurs because the molecules of a substance obey the laws of thermodynamics. Areas of high-temperature transfer energy to areas of low temperature, and molecules move from areas of high pressure to areas of low pressure.
In contrast to boiling, evaporation occurs slowly, as energy enters the liquid from some source such as sunlight or warm room air. This can happen with a hot beverage as it cools and its molecules rise from the liquid, or it can occur if you leave a water glass unattended on a window sill.
When the surface of a liquid is exposed, the sun’s thermal energy enters the liquid, excites the molecules, and increases their kinetic energy.
With more kinetic energy, the molecules move farther apart than as a liquid. Some of the molecules gain enough energy to move out of the liquid and enter the atmosphere as gas molecules.
Evaporation of liquid water into water vapor occurs starting at , or , water’s melting point. Still, the evaporation rate (the rate at which a substance spontaneously vaporizes) is extremely slow at such low temperatures. As temperatures climb, water’s evaporation rate increases.
The higher a substance’s boiling point, the lower its rate of evaporation. Evaporation of water occurs more quickly in areas with dry air, when the relative humidity is low, or when there is a significant temperature difference between the liquid and the atmosphere.
Evaporation in the water cycle
The evaporation of water is a vital part of the water cycle, also known as the hydrologic cycle, on Earth. This cycle of condensation, precipitation, transpiration by plants, and evaporation continuously replenishes our freshwater supply.
Without the step of evaporation, Earth’s waters would either soak into the ground or remain in lakes, rivers, oceans, and streams. Plants would be unable to move water through their systems and out of their leaves. Clouds would disappear, and rain, snow, sleet, and hail would no longer fall from the sky back to Earth’s surface.
The most significant amount of water evaporated each day on Earth is from the oceans. The vast surface area of the oceans allows a large amount of water molecules to evaporate into a gaseous state and eventually form clouds.
Evaporation examples
Water is not the only liquid substance to evaporate. The higher a liquid’s vapor pressure, the faster its evaporation rate. Substances with high vapor pressure are volatile, such as:
Gasoline
Acetone
Ammonia
Isopropyl alcohol (rubbing alcohol)
Dichloromethane
Diethyl ether
If a substance is safe for skin contact, a simple test of a liquid’s volatility is to place a very small amount on the skin. The cooling sensation from rubbing alcohol is due to the alcohol’s volatility.
Three examples of vaporization through evaporation are seawater evaporating into the atmosphere to form clouds, a volatile liquid like gasoline forming gaseous and highly flammable vapors around a gas station, and damp laundry drying on a clothesline.
Evaporative cooling is an effective way to cool a home in a dry environment like many parts of the western United States. Evaporative coolers pull in dry, warm air and pass it through a wet pad. The water on the pad evaporates, which cools the air that is then pumped through the house.
Evaporation also has industrial uses. Power-production plants use tall towers to allow hot water to evaporate into the atmosphere instead of releasing it into rivers or lakes, where hot water would harm the ecosystem.
Evaporation quiz
See if you have a handle on evaporation by answering these questions.
What is evaporation, in your own words?
What does water turn into when it evaporates?
Water can evaporate at any temperature; please explain how this is possible with an example.
Please give three examples of evaporation in everyday life.
What are the four stages of the water cycle on Earth?
What is volatility?
The rate at which a substance vaporizes is called ________.
Describe Earth without evaporation.
Check your answers with ours below.
Perhaps in your own words, you said evaporation was the change of a substance from liquid to gas phase.
Water turns into invisible water vapor when it evaporates; when it boils, it turns to invisible steam, which condenses and cools as water vapor.
Water can evaporate at any temperature if the pressure differences between the atmosphere and the liquid are great enough. An example is water boiling away instantly in the extremely low pressure of outer space.
Three examples of evaporation in our everyday life are lakes evaporating to form clouds, a pot of heated water evaporating in a kitchen, and a volatile liquid like acetone evaporating to leave behind nail polish. You may have mentioned other examples in your answer.
The four stages of the water cycle on Earth are the condensation of clouds, precipitation of liquid or solid water, transpiration of water from plants, and evaporation of water vapor from oceans, lakes, rivers, and streams.
Volatility is the tendency of a liquid to vaporize quickly under normal conditions. Gasoline, ammonia, and rubbing alcohol are all volatile liquids.
The rate at which a substance vaporizes is called its evaporation rate.
Earth without evaporation would mean cloudless skies; flooded lakes, streams, rivers, and oceans; plants unable to shed water from their leaves, and no more precipitation (snow, sleet, hail, rain).