23.1  Fresh Water (continued)

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. Key Concept  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.
 

Modeling the Water Cycle

Procedure  Lab Safety Icons Lab Safety Icons Lab Safety Icons

  1. Place 100 mL of water in a 250-mL beaker. Cover the beaker with an evaporating dish or a large watch glass.
  2. Put the beaker on a hot plate. CAUTION: Be careful not to spill the water. Fill the evaporating dish with ice.
  3. 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 

  1. 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?
  2. Using Models   What is the effect of the ice in your model? What part of the water cycle does the ice help to model?
  3. 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.

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.

Completing the Cycle A single molecule of water moves between the surface and atmosphere many times. About 90 percent of the water that evaporates from the oceans falls back into the oceans. Most precipitation that falls on land quickly returns to the atmosphere through evaporation and transpiration. Much of the remaining water seeps into the soil as groundwater or becomes frozen in glaciers. A glacier is a large mass of moving ice and snow on land. Some water that falls on land flows into streams before it returns to the ocean.
 
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