High School Earth Science/Water on Earth

Water is a simple compound, made of two atoms of hydrogen and one atom of oxygen bonded together. More than any other substance on the Earth, water is important to life and has remarkable properties. Without water, life could probably not even exist on Earth. When looking at Earth from space, the abundance of water on Earth becomes obvious—see Figure 13.1. On land, water is also common: it swirls and meanders through streams, falls from the sky, freezes into snow flakes, and even makes up most of you and me. In this chapter, we'll look at the distribution of water on Earth, and also examine some of its unique properties.

Figure 13.1: Earth, the "Blue Marble", can be seen in this photograph to be mostly covered with liquid water.

Lesson Objectives

• Describe how water is distributed on Earth.
• Describe what powers the water cycle and how water moves through this cycle.

Distribution of Earth's Water

As Figure 13.1 makes clear, water is the most abundant substance on the Earth's surface. About 71% of the Earth's surface is covered with water, most of which is found in the oceans. In fact, 97% of Earth's water, nearly all of it, is in the Earth's oceans. This means that just 3% of Earth's water is fresh water, water with low concentrations of salts (Figure 13.2). Most freshwater is found as ice in the vast glaciers of Greenland and the immense ice sheets of Antarctica. That leaves just 0.6% of Earth's water that is freshwater that humans can easily use. Most liquid freshwater is found under the Earth's surface as groundwater, while the rest is found in lakes, rivers, and streams, and water vapor in the sky.

Figure 13.2: Earth's water is mostly in the oceans. Fresh water is only 3% of all the Earth's water, and most of that is in the form of ice.

Three States of Water

Water is a special substance. It is abundant on Earth and frequently appears as a gas, liquid, and solid. It is one of the few substances on Earth that is frequently found in all three phases of matter. Moreover, it can readily cycle through the globe: the same molecule can travel through many different regions on Earth.

Part of the reason that water is unique is because of its melting point and boiling point. Under normal atmospheric conditions, water freezes at 0°C (32°F) and boils at 100°C (212°F). Because of our Earth's position in the solar system, Earth’s temperature varies from far below the melting point of water to well above that melting point. Even though water does not boil at normal temperatures, it often becomes gaseous water vapor by evaporating. All this means that we frequently see water in its three phases on Earth (See Figures 13.3, 13.4, and 13.5).

Figure 13.3: Solid ice floating amidst liquid water. This image shows what an iceberg might look like if you could see both above and below the surface.

Figure 13.4: Liquid water.

Figure 13.5: Water vapor is invisible to our eyes. However, we can see the clouds that form when water vapor condenses.

The Water Cycle

The water on Earth moves about the Earth in what is known as the water cycle (Figure 13.6). Because it is a cycle, there truly is no beginning and no end. The very same water molecule found in your glass of water today has probably been on the Earth for billions of years. It may have been in a glacier or far below the ground. It may have been high up in the atmosphere and deep in the belly of a dinosaur. Who knows where it will end up today, when you're done with it!

Figure 13.6: Water on Earth is constantly in motion.

Let's study Figure 13.6 for a moment. The Sun, many millions of kilometers away, provides the energy which drives the water cycle. Since the ocean holds most of the Earth's water, let's begin there. As you can see in the illustration, water in the ocean evaporates as water vapor into the air. The salt in the ocean does not evaporate with the water, however, so the water vapor is fresh. Some of the invisible water vapor in the air condenses to form liquid droplets in clouds. The clouds are blown about the globe by wind. As the water particles in the clouds collide and grow, they fall from the sky as precipitation. Precipitation can occur in forms such as rain, sleet, hail, and snow. Sometimes precipitation falls right back into the ocean. Other times, however, it falls onto the solid earth as freshwater.

That freshwater, now on the Earth, may be found in a solid form as snow or ice. Some of it goes directly back into the air to form water vapor and clouds again. However, most of this solid water sits atop mountains and slowly melts over time to provide a steady flow of freshwater to streams, rivers, and lakes below. Some of that water enters the Earth's groundwater, seeping below the surface through pores in the ground. This water can form aquifers that store freshwater for centuries. Alternatively, it may come to the surface through springs or find its way back to the oceans.

When water falls from the sky is rain it form streams and rivers that flow downward to oceans and lakes. People use these natural resources as their source of water. They also create canals, aqueducts, dams, and wells to direct water to living areas to meet their needs (Figure 13.7). Sometimes, our manipulation or pollution of water greatly affects other species. Many scientists are seeking better ways of using Earth's water in a sustainable and efficient way.

Figure 13.7: Hoover Dam on the Colorado River.

Obviously, people are not the only creatures that rely on water. Plants and animals also depend on this vital resource. Plants play an important role in the water cycle because they release large amounts of water vapor into the air from their leaves. This process of transpiration moves liquid water from plants into the air. You can see transpiration in action if you cover a few leaves on a plant with a plastic bag. Within a few hours, water vapor released from the leaves will have condensed onto the surface of the bag.

Lesson Summary

• Earth's surface is mostly water covered. Most of that water is in our oceans, leaving only 3% freshwater.
• Water exists on Earth in all three phases: solid, liquid, and gas.
• The water cycle moves water from the hydrosphere to the atmosphere to the land and back again.
• The major processes of the water cycle include evaporation and transpiration, condensation, precipitation and return to the oceans via runoff and groundwater supplies.

Review Questions

1. About what percent of the Earth's water is fresh water?
2. About what percent of all of Earth's water is found in groundwater, streams, lakes, and rivers?
3. Explain the following statement: The water on other planets is present in a different form than on Earth.
4. What powers the water cycle?
5. In what state would water be found at 130°C? What state would water be at -45°C?
6. Define the words condensation and evaporation.
7. Summarize the water cycle.
8. Why do you think the atmosphere is so important to the water cycle?
9. Suppose the sun grew much stronger in intensity. How would this affect the water cycle?

Vocabulary

aquifer

A layer of rock, sand, or gravel that holds large amounts of groundwater. Humans often use aquifers as sources of freshwater.

condense

To turn from a gas to a liquid.

freshwater

Water with a low concentration of salts, which can be consumed and used by humans.

groundwater

Water that is found beneath the Earth's surface, between soil or rock particles.

precipitation

Water that falls to the Earth from the sky. Precipitation usually takes the form of rain, but can also occur as snow, sleet, or hail.

transpiration

The release of water vapor into the air through the leaves of plants; sometimes called evapotranspiration.

water cycle

The cycle through which water moves around the Earth, changing both its phase (between solid to liquid to gas) and its location (in the oceans, in clouds, in streams and lakes, and in groundwater).

water vapor

Water in the form of a gas. Water vapor is invisible to humans; when we see clouds, we actually are seeing liquid water in the clouds.

Points to Consider

• How does precipitation affect the topography of the Earth?
• What natural disasters are caused by the water cycle?
• How might pollution affect creatures far from the source of the pollution?
• How might building dams disrupt the natural water cycle?
• If the temperature of the Earth increases through global warming, how might the water cycle be altered?