Earth’s Water Cycle

Earth’s Water Cycle — page 1 — CHAPTER 1 Water Water Everywhere Water is all around us About 70 percent of Earth is covered with water Look at a photo of the planet from space All the blue parts are water That’s why Earth is sometimes called the blue planet Water is the most abundant or plentiful substance on Earth and one of the most important Water World About 97 percent of all the water on Earth is contained in five oceans the Atlantic Pacific Indian Arctic and Antarctic Ocean water is salt water Only about 3 percent of the planet’s water is fresh water That’s the kind of water that people and animals drink About two-thirds of Earth’s fresh water is not available to drink because it is frozen as ice in the Arctic and in Antarctica Just one-third of all fresh water is found in rivers and lakes and underground This adds up to only about one percent of all the world’s water being available as drinking water 4 This photo from space shows Earth’s western hemisphere It also dramatically illustrates why we call our world which is 70 percent covered with water the blue planet The clouds that swirl around the planet are also filled with water They play a major role in Earth’s weather patterns and in the water cycle

Earth’s Water Cycle — page 2 — This underground water is called groundwater Water exists as a gas in our air In this form it is called water vapor Up to four percent of our air is made of gaseous water or water vapor This amount varies from day to day and place to place Water is the only substance on Earth that naturally exists in three states solid liquid and gas Solid water takes the form of ice or snow In addition to the ice and snow around Earth’s North and South Poles glaciers and icebergs are made of solid water Liquid water is what fills our oceans lakes rivers and streams Water also soaks into the ground where gravity pulls it deeper and deeper Sea ice Arctic Ocean 5 Above This photo of an iceberg in Antarctica shows water in all three of its states liquid blue sea solid the iceberg and gas invisible water vapor in the air Inset Earth’s North Pole is covered with floating sea ice over the Arctic Ocean

Earth’s Water Cycle — page 3 — You can’t see water vapor or smell it or taste it but sometimes you can feel it On a hot muggy day it is the water vapor that makes the air feel humid clammy or damp Another way you can tell that liquid water has turned into water vapor is to hang wet laundry on the clothesline Eventually the laundry is no longer wet That’s because the liquid water has turned into vapor and escaped into the air The only time you can see water vapor is when a lot of it collects in one place and starts to cool At that point water vapor turns into steam For example when a tea kettle boils a tiny cloud of steam comes out of the spout That’s because the hot water in the kettle quickly turns to vapor collects in a small area and immediately begins to cool The steam is actually a collection of tiny water droplets floating in the air When wet laundry hanging on a clothesline dries water has gone from a liquid to a gas in the form of water vapor The steam coming out of this tea kettle is actually a tiny cloud of water droplets It forms when water vapor from within the kettle comes into contact with cooler air outside of the kettle 6

Earth’s Water Cycle — page 4 — States of Water Everything in the world is made up of atoms and molecules Atoms are individual building blocks of matter and molecules are combinations of those building blocks The Periodic Table of Elements lists 118 known types of atoms For example one of those is hydrogen represented on the periodic table as H Another is O H oxygen represented by the letter O Water might look like a pretty simple substance but it is actually made up of hydrogen and oxygen When two hydrogen atoms combine with one oxygen atom they form a molecule called H2O That is water Whether it is a solid liquid or gas this structure basically never changes Water is solid as ice when the temperature is below 32 degrees Fahrenheit 0 degrees Celsius When it’s warmer than that ice melts and becomes liquid water H 2 hydrogen atoms 1 oxygen atom 1 water molecule H2O The Periodic Table of Elements shows all the chemical elements known to exist Some elements occur naturally and others are developed in laboratories and are therefore artificial or human-made The periodic table lists them in order of increasing atomic number or the number of protons in the atom’s nucleus Hydrogen is number 1 in the table Oxygen is number 8 Periodic Table of Elements 7

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