READ: Changes of State

1. Evaporation

The clothes on this clothesline are soaking wet, but before too long they will be completely dry. How does this happen? Where does the water go? And what factors might help the clothes on the line dry more quickly?

From Liquid to Gas Without Boiling

Evaporation explains why clothes dry on a clothesline. Evaporation is the process in which a liquid changes to a gas without becoming hot enough to boil. It occurs when individual liquid particles at the exposed surface of the liquid absorb just enough energy to overcome the force of attraction with other liquid particles. If the surface particles are moving in the right direction, they will pull away from the liquid and move into the air. This is illustrated in the Figure below.

Factors that Affect the Rate of Evaporation

Many factors influence how quickly a liquid evaporates. They include:

• temperature of the liquid. A cup of hot water will evaporate more quickly than a cup of cold water.
• exposed surface area of the liquid. The same amount of water will evaporate more quickly in a wide shallow bowl than in a tall narrow glass.
• presence or absence of other substances in the liquid. Pure water will evaporate more quickly than salt water.
• air movement. Clothes on a clothesline will dry more quickly on a windy day than on a still day.
• concentration of the evaporating substance in the air. Clothes will dry more quickly when air contains little water vapor.

CK-12 Foundation, Physical Science. http://creativecommons.org/licenses/by-nc-sa/3.0/