The Water Cycle
Water moves among the oceans, atmosphere, and land in a cycle that covers the entire surface of Earth and even extends below the ground. This continuous movement of water, called the water cycle, is shown in Figure 2. 
The water cycle is made up of several processes, including evaporation, transpiration, condensation, precipitation, and the eventual return of flowing water to the ocean. The energy of sunlight and the force of gravity power the water cycle.
Evaporation
Solar energy causes water on Earth's surface to evaporate. Recall that evaporation is the process that changes a liquid into a gas. Sunlight heats water and causes it to change into water vapor in the air. This warm, moist air expands and rises, carrying the water vapor higher into the atmosphere. Most evaporation occurs over the ocean and large lakes. Smaller amounts of water evaporate from streams, ponds, and other sources. Some water vapor also enters the atmosphere when it is released from the leaves of trees and other plants in a process called transpiration.
Condensation
Water vapor does not stay in the atmosphere for long. As warm air rises, it expands and cools. Because colder air can hold less water vapor than warm air, condensation occurs. In condensation, water vapor forms droplets on small particles in the air. At lower temperatures, ice crystals form. Billions of these droplets or ice crystals make up clouds.
Precipitation
When the water droplets or ice crystals in clouds get too heavy, gravity causes them to fall to the ground as precipitation—rain, snow, sleet, hail, or freezing rain. Worldwide, the rates of evaporation and precipitation are balanced.
Quick Lab
Modeling the Water Cycle
Procedure 
Place 100 mL of water in a 250-mL beaker. Cover the beaker with an evaporating dish or a large watch glass.
Put the beaker on a hot plate. CAUTION Be careful not to spill the water. Fill the evaporating dish with ice.
Turn the hot plate on to medium. Observe the contents of the beaker for 10 minutes. Record your observations. Be sure to turn off the hot plate.
Analyze and Conclude
Using Models What source of energy drives the water cycle in your model? What energy source does this correspond to in Earth's water cycle?
Using Models What is the effect of the ice in your model? What part of the water cycle does the ice help to model?
Predicting As water evaporates, what will happen to the concentration of any substances dissolved in the water? How does evaporation affect the concentration of salts in the ocean?
Figure 2 Water is constantly moving through the water cycle. Interpreting Diagrams In which step of the water cycle is liquid water converted to water vapor?
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