Earth’s Water Cycle

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

Earth’s Water Cycle — page 5 — Liquid water may become vapor in more than one way In one process liquid water heated to its boiling point of 212 F 100 C will become vapor When you watch a pot of boiling water you see bubbles forming in the water Those bubbles are water vapor surrounded by liquid water The bubbles rise to the surface of the boiling liquid and escape into the air Another way liquid water becomes water vapor is through a process called evaporation In every substance molecules are in constant motion In liquid water H2O molecules zip around and sometimes crash into each other When that happens at the water’s surface some molecules are knocked right out of the liquid and into the air Evaporation happens at the surface of every body of water be it a glass of water a puddle or an ocean It can happen at any temperature as long as the water is in liquid form The crystal-clear waters of a babbling river the salty ocean and a stagnant mud puddle All three bodies of water contain some form of salt although only the ocean has enough for us to call it a saltwater body Salty Water Everywhere There are many kinds of salt on our planet and all water contains some form of dissolved salt When we drink a glass of water from the tap though it doesn’t taste salty to us One reason we can’t taste salt is that it may not be the kind of salt most of us are used to putting on our food Also the amount of salt in fresh water is small compared to the amount of salt in seawater In fact ocean water is about 220 times as salty as lake or river water That’s a big reason it tastes so well salty Imagine all the rivers that drain into an ocean Each of those rivers contains substances that include salt When hundreds of rivers drain into an ocean all those substances go into the ocean too When water evaporates from the surface of an ocean it is mostly the water itself that evaporates Most of the salt especially the salt that isn’t dissolved into the water stays behind Millions of years ago seawater wasn’t as salty as it is today Over time the salt concentration meaning the amount of salt in the water has built up to today’s levels 8

Earth’s Water Cycle — page 6 — The warmer the water the faster the molecules move and the more quickly the water evaporates As water freezes into solid ice the molecules slow down Our Body of Water Water is all around us but it’s also inside us Every human being is mostly made of water Our water content ranges from about 60 to 75 percent depending on a person’s age gender and activity level among other things Like other living things however we all need water to survive In animals water keeps organs and body systems working properly It distributes nutrients throughout a body and helps with the digestion of food It also helps regulate or control body temperature to make sure the animal doesn’t get too hot or too cold Bodily fluids such as blood are mostly water Most plants are 90 to 95 percent water Plant cells are full of water In humans blood is made up of red blood cells which carry oxygen through the body white blood cells which help fight infections and disease platelets which help blood clot and plasma a liquid that carries nutrients and other essential substances in your blood and in which the blood’s cells and platelets are suspended More than half our blood is made of plasma which is mostly water This means that our blood is about half water So drink plenty of water it’s good for your blood White blood cells Platelets Plasma Red blood cells 9

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