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Earth Science Genre Comprehension Skill Text Features Science Content Nonfiction Sequence Labels Maps Diagrams Glossary Water on Earth Scott Foresman Science 5.7 ì<(sk$m)=bdjdeh< +^-Ä-U-Ä-U ISBN 0-328-13934-3 by Anna Padilla

Vocabulary aquifer condensation evaporation Water on Earth by Anna Padilla precipitation reservoir salinity sleet sublimation water table Illustrations: 4, 6, 12, 14, 16, 18 Studio Liddell; 23 Bob Kayganich Photographs: Every effort has been made to secure permission and provide appropriate credit for photographic material. The publisher deeply regrets any omission and pledges to correct errors called to its attention in subsequent editions. Unless otherwise acknowledged, all photographs are the property of Scott Foresman, a division of Pearson Education. Photo locators denoted as follows: Top (T), Center (C), Bottom (B), Left (L), Right (R) Background (Bkgd) Opener: Raymond Gehman/NGS Image Collection; Title Page: Tom Stewart/Corbis; 2 David Pu u/corbis; 9 (T) Cosmo Condina/Getty Images, (B) Paul Linse/Corbis; 11 Tom Stewart/Corbis; 17 (TR) Graig Tuttle/Corbis, (CR) Gary W. Carter/Visuals Unlimited, 20 Dick Ruhl; 21 (TL) Patti Murray/Animals Animals/Earth Scenes, (TL) Stephen Ingram/Animals Animals/Earth Scenes, (CL) Eastcott/Momatiuk/Animals Animals/Earth Scenes, (BL) Brian Cosgrove/ DK Images, (BL) James A. Sugar/Corbis; 22 (BL) Jeff Daly/Visuals Unlimited; 23 (BL, BC) Henryk T. Kaiser/Index Stock Imagery, (BC) Frans Lemmens/Getty Images, (BR) Jim W. Grace/Photo Researchers, Inc. ISBN: 0-328-13934-3 Copyright Pearson Education, Inc. All Rights Reserved. Printed in the United States of America. This publication is protected by Copyright and permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permissions, write to: Permissions Department, Scott Foresman, 1900 East Lake Avenue, Glenview, Illinois 60025. 3 4 5 6 7 8 9 10 V010 13 12 11 10 09 08 07 06 05

How can the oceans be described? The Hydrosphere Most of Earth is covered with water. All of this water is very important to humans. It gives us a way to travel and a good source of food. More than half of the people in the United States live within 80 kilometers of an ocean. All of Earth s waters together are called the hydrosphere. Ocean water makes up most of the hydrosphere. The Pacific is the largest ocean. It is also the deepest. In its deepest spot, it is more than 11,000 meters deep. After the Pacific, the largest oceans are the Atlantic, the Indian, and the Arctic. If you look at a globe, you will see that all of the world s oceans are connected. The oceans are all a little different from each other. Some oceans have more storms than others. They have different average temperatures and different amounts of salt. The levels of the ocean s surfaces, or sea levels, are also a bit different. 2 3

Salinity What is the difference between lake water and ocean water? If you taste them both, you will notice that ocean water is very salty. Salinity is a measure of how salty water is. If you drink too much ocean water, it can make you sick. The oceans get their salt from rivers. Rivers dissolve tiny amounts of salt from rocks and soil. They carry this salt to the ocean. There, some of the water evaporates, leaving its salt behind. Ocean water is more salty in some places than others. In warm areas, ocean water has high salinity, because water there evaporates quickly. Places where rivers flow into oceans have low salinity. Different salinities help make the ocean s currents. Saltier water is heavier than less salty water, so it sinks. This sinking can cause currents. Most currents are caused by winds. Temperature The temperature of ocean water is different in different places. Distance from the equator affects water temperature. Water near the equator is usually very warm. Water near the North and South Poles is usually very cold. Oceans currents also affect water temperature. They can carry warm water toward the poles. They can also carry cold water toward the equator. Look at the map below to see how these currents move. Map Legend warm current cold current Ocean Resources We get many products from the ocean. We get tuna and other fish to eat. We can also get salt from ocean water. This is done by letting the water flow into shallow ponds. The water evaporates. The salt is left behind. We can drink ocean water if the salt is taken out. This way of getting drinking water costs a lot of money. So it is not very common. This map shows currents on the surface of the water. Currents below the surface move in different ways. 4 5

Where is fresh water found? Fresh Water Less than 3 100 of Earth s water is fresh water. Humans need fresh water to drink. We also use it for cooking, cleaning, and watering crops. Almost all of Earth s fresh water starts as rain or snow. Some of this water soaks into the ground. Some collects in rivers and lakes. Some is frozen in ice sheets and glaciers. Some places have plenty of fresh water. Other places do not have enough for the people who live there. No place has unlimited fresh water. People must be careful not to use too much. Water should be used wisely. Groundwater Rain or melted snow that sinks into the ground is called groundwater. As it sinks, this water fills spaces between small bits of soil and rock. It keeps sinking until it reaches something it cannot pass through, such as rock or clay. A layer of this material will cause groundwater to flow sideways instead of down. An aquifer is a layer of rock and soil that contains groundwater. Many people get their water from wells drilled into an aquifer. The top level of groundwater in an aquifer is the water table. The level of a water table changes. It rises when water is added by rain or melting snow. It falls when a long time passes without rain. The level will also fall when water is pumped out through wells faster than it is replaced. 6 7

Rivers Rivers, streams, and lakes are surface waters. They are formed by groundwater, rainwater, and melting snow. Water from rain and melting snow flows downhill. It flows in small streams. These streams join to form larger streams and rivers. Groundwater also seeps into rivers. Each river collects water from a certain area of land. That area is the river s watershed. Pollution in a watershed s soil will flow into the river. The river can carry the pollution far away. This can affect ecosystems wherever the river flows. Lakes Lakes form when water collects in a low spot. This happens when water flows into a place surrounded by higher land. Sometimes people build dams to make new lakes. These artificial lakes are called reservoirs. Reservoirs are often created as a place to collect fresh water. When you look at a lake, the water doesn t seem to be going anywhere. But water is leaving lakes all the time. It flows into rivers, seeps into the ground, and evaporates into the air. 8 9

Ice About 7 10 of Earth s fresh water is frozen. This frozen water is difficult for people to use for drinking. It is too far away from most cities. Much of Earth s ice is near the poles. Thick ice sheets cover much of Greenland, which is near the North Pole. They also cover much of Antarctica. These ice sheets can be several kilometers thick. The North Pole is covered with ice as well. But there is no land there, so the ice floats on the ocean. Some glaciers and ice sheets are smaller than others. Glaciers that form in high mountain valleys are called valley glaciers. These glaciers flow downhill slowly. They crush and move the rocks beneath them, changing the shape of the land. Glaciers and ice sheets form when more snow falls than melts. The weight of the new snow presses down on the old snow. After many years, the snow at the bottom is squeezed into ice. Icebergs are large chunks of ice floating in the ocean. They form when pieces of ice sheets or glaciers break off and fall into the water. Icebergs can be huge. One iceberg was twice the size of the state of Rhode Island! Ice is not salty. When ocean water freezes, the salt gets pushed out. This makes the ocean saltier where new ice is forming. 10 11

Getting Water to Homes How do you get your water? You might just walk to the sink and turn on the tap. But it takes a complex system to get the water to your sink. People usually get their water from lakes, rivers, or the ground. But any of these sources might be polluted. Chemicals from farming, parking lots, and even lawns get washed into water supplies by rain. Water must be treated before we use it. Getting water to homes and businesses is not easy. So we should be careful not to waste water. We should use no more than we really need. There are many easy ways to save water. You can run the faucet only as long as you need it. You can take shorter showers. And you can water lawns and gardens in the evening. That way, less water will be evaporated by the Sun. If a faucet leaks, it should be fixed. One dripping faucet can waste gallons of water every day. Lakes and rivers are important sources of drinking water. Average Daily Water Use (Per Person) Toilet Flush Laundry Shower/ Bath Other 71 L 57 L 49 L 38 L 12 13

The drawing on these two pages shows the steps in water treatment. First, water is pumped from the source to a treatment plant. The source may be a well or it may be a lake. As the water enters the plant, screens remove large objects as water flows through them. Next, small bits of dirt floating in the water are removed. To do this, chemicals are added to the water. The chemicals form tiny, sticky particles. Bits of dirt get stuck to the chemical particles. The growing particles get heavier and sink to the bottom of a tank. Next, the water is passed through filters. The filters are often made of layers of sand and gravel. Smaller particles still in the water do not pass through the filters. Finally, other chemicals are added to the water. These chemicals kill germs. Many treatment plants use a chemical called chlorine. Fluoride may be added as well. It helps your teeth resist decay. The treated water is pumped to the top of a water tower. Water Treatment Plant Screens keep out large objects. Gravity pulls water down from the tank. Chemicals are added to stick to pollutants. Heavy pollutants fall to the bottom of the settling tank. The water and chemicals are mixed. Water flows through sand to filter out small particles. Pumping station Chlorine, fluoride, or other chemicals may be added at the end of treatment. Water flows through pipes to homes and businesses in the community. 14 15

What is the water cycle? Water in the Air Water is all around you all the time. You can t see it because it is in the form of an invisible gas in the air. The gas is called water vapor. Water vapor makes up only a small fraction of all the gases in the air. Particles of gas in the air are always moving. The pressure of the particles pushing against objects is called air pressure. The higher you go in the atmosphere, the weaker air pressure becomes. The Water Cycle Water is always moving and changing form. The water cycle is the movement of water, again and again, through the environment. Look at the picture on these two pages. It is a simple picture of the water cycle. Water often changes form as it goes through the water cycle. In evaporation, liquid water changes to water vapor. This usually happens when water is heated by the Sun. In condensation, water vapor changes to liquid water. Water vapor may turn to droplets of liquid water in clouds. This also happens when dew forms on grass. In precipitation, water falls from clouds as rain, snow, sleet, or hail. In sublimation, ice changes directly into water vapor. It doesn t melt first and become water. This is what happened when ice cubes shrink after being in a freezer for a long time. Some ice in the cubes changes to water vapor. Water vapor may freeze into ice without first becoming liquid. The ice crystals that form on surfaces are called frost. Condensation Precipitation Evaporation Runoff is water moving downhill. 16 17

Many Paths of the Water Cycle The picture below is a more detailed picture of the water cycle. All water does not cycle forever. Some water is broken down by plants as they make sugar. Some water is produced by living things as they make energy during respiration. Energy in the Water Cycle Moving all this water around takes energy. It takes energy to raise water vapor to the clouds. It takes energy to move the clouds by wind. This energy comes from the Sun. Water vapor gives off energy when it condenses. This energy is heat. It warms the air or water in the area. As water vapor rises, it may form a cloud. Very slowly, snow and ice turn into water vapor by sublimation. Rain and snow fall to Earth. Water evaporates with warm temperatures and winds. Water runs off the land into streams and rivers that flow into lakes and oceans. Some rain or melted snow soaks into the ground and becomes groundwater. Groundwater slowly moves through aquifers into rivers, lakes, and the ocean. This can take thousands of years. 18 19

How do clouds form? Temperature and Pressure Clouds form when water condenses into tiny water droplets. If the cloud forms in cold air, the water droplets may turn into ice crystals. Many clouds are made of ice, even during the summer. This is because temperatures high in the air are often much cooler than on the ground. Air pressure also affects how clouds form. Clouds often form when air moves up. In higher areas, the air pressure is lower and air expands. When it expands, it cools, forming clouds. Cirrus clouds form at high altitudes. They form more than 6,000 meters above ground. They are thin, wispy, and white. Thunderstorm clouds, called thunderheads, have rising air inside them. The bottoms of these clouds may be as low as 1,000 meters above ground. They might be as tall as 12,000 meters. Saucer-shaped clouds can form when winds blow over a mountain. Altocumulus clouds look like small, puffy balls. Their sides are white, because sunlight bounces off them. The bottoms of these clouds can look dark because sunlight may not reach them. The bottoms are 2,000 7,000 meters above ground. These are low altitude clouds. They are called stratus clouds. They cover the whole sky and block out the Sun. Stratus clouds are often seen less than 2,000 meters above ground. Fog is a cloud at ground level. It forms when air near the ground is cool. Water vapor condenses and forms a cloud near the ground. 20 21

Precipitation In the United States, most of our rain starts as snow and melts as it falls. The temperature of air that is high above ground is often freezing. If the air is cold all the way to the ground, the snowflakes will not melt. If the air is warmer near the ground, they will melt into rain. Sometimes rain turns back into ice before it hits the ground. This is called sleet. Freezing rain is water that falls as rain, but freezes when it hits the ground. Hail Formation Hail forms when very strong winds blow upward into a cloud. These winds blow raindrops into the freezing air high in the cloud. The raindrops freeze. As the frozen raindrops blow around inside the cloud, more and more water freezes and they grow. The ice becomes heavy and falls to the ground as hailstones. Most clouds over North America are made of ice crystals. Ice crystals melt as they fall through warmer air. They fall to the ground as rain. Ice crystals fall from clouds. Ice crystals melt to form raindrops as they fall through warm air. A layer of air close to the ground is colder than 0ºC. This cold air makes the ground very cold. Rainwater freezes when it lands. Ice crystals melt as they fall through a thin layer of warm air high above the ground. If raindrops fall for a long time through cold air, they freeze before they land. Frozen raindrops are called sleet. Ice crystals will fall as snow if the air between the clouds and ground has a temperature below 0ºC. Most hailstones are about the size of a pea. Some can get larger than a baseball. Rain Freezing Rain Sleet Snow 22 23

Glossary aquifer condensation evaporation precipitation reservoir salinity sleet sublimation water table the layer of soil and rock that contains groundwater the changing of a gas into a liquid the changing of a liquid into a gas water leaving clouds as rain, snow, sleet, or hail a lake formed behind a dam a measure of how salty water is frozen raindrops when ice turns into water vapor without first melting the top level of groundwater in an aquifer What did you learn? 1. Why is there sometimes warm water near the poles? 2. What are three ways that water leaves lakes? 3. What is the difference between sleet and freezing rain? 4. Many people get their fresh water from an aquifer. On your own paper, write to explain why people should not take more water from an aquifer than they need. Include details from the book to support your answer. 5. Sequence What are four steps in treating water? 24