Hey people,

I'm teaching my fourth graders a unit on the water cycle next week. I did a bit of research while writing this lesson, but there were a lot of different explanations out there and I wanted to make sure I had the best ones.

I've checked my facts, but I want to make sure I'm not feeding the kids any outdated or incorrect information since our school district doesn't see fit to replace the kids' textbooks until they show actual signs of decomposing.



Where is all the water?

Show the students a globe and explain that seventy percent of the Earth’s surface is water. Ask students to brainstorm different places on the earth where water can be found (naturally) and write them on the board or in their science journals.

Ask the students if their parents have ever told them not to waste water. If there’s so much water on the earth, why should we worry about wasting it?

Show students a liter bottle full of water labeled “all the water in the world.” Empty the following amounts into separate jars to represent the relative amounts.

Source Amount Percent of Total

Oceans 4 cups 97%
Icecaps/Glaciers 1.5 tablespoons 2.1%
Groundwater ½ tablespoon 0.6%
Lakes and rivers 2 drops 0.1%
Atmosphere ½ drop 0.001%



What is Water Anyway?


Water is made up of tiny particles called molecules that are always moving. These molecules can exist in three states depending on the temperature.

Show students examples of liquid water, frozen water, and gaseous water.

When you’re in a hot room, do you like having a lot of people crowded around you? Water molecules are kind of the same way. They don’t like being close together when it’s warm. Heat (usually from the sun) causes the water molecules in a puddle to move farther and farther apart. There’s a lot of extra room in the open sky, and water molecules don’t weigh very much because they’re even smaller than specks of dust. Because they’re so light, they can float right up into the air to where all that empty space is waiting for them. This is called evaporation. Evaporated water molecules move around in the air as a kind of gas known as water vapor.


When a cup of water evaporates, is the water still in the room? If so, why can’t we see it?

Tell the students that you are going to perform a magic trick for them. Dump a tiny amount of flour on your hand and make sure everyone gets a good look. Then blow on the flour as hard as possible. Did the flour really disappear? What happened to it? When students conclude that the flour is no longer visible because the particles aren’t as close together as they were, explain that we can’t see water vapor for the same reason. The heated water molecules are so far apart, they can’t be seen.

Another thing students can learn from this demonstration is that heat isn’t the only thing that causes evaporation. Wind can blow water molecules right off the surface of lakes, rivers, and oceans and into the air. That’s why puddles evaporate more quickly on hot, windy days, and why blow dryers are so useful when you want to dry your hair in a hurry!

To prove this point, put a glass of water near the window, where it can easily be reached by heat and sunlight. Place a second glass of water in a dark corner of the room and compare them throughout the week.


Can evaporated water molecules ever come back together again, or do they just float around in the sky by themselves forever?

If water just stayed in the air after it evaporated, eventually all the oceans and rivers and lakes would dry up. So we know that all those water molecules floating around in the air must have some way to come together again.


What makes the water molecules come back together?

Condensation is the opposite of evaporation. We know that air heated by the sun causes water molecules to move farther apart, but what happens to all that warm air after the sun goes down, or when the seasons change? You and I don’t like being together in a hot crowded room, but as soon as it gets cold, we all want to get closer together! Water molecules start to move closer together when it’s cold, too. When we look up in the sky and see clouds, we know that condensation has occurred. During condensation, the air becomes cold enough that water molecules don’t mind being crowded. Sometimes, two water molecules get so close, they bump into one another and stick together. Soon more and more water molecules join together until they make a little cluster. When we look at a cloud, what we’re actually seeing is sunlight shining on all those tiny clusters of water molecules!

Your body is also full of water molecules that are constantly being released into the open air from pores in your skin. Even the inside of your mouth is full of water vapor! You can breathe onto your hand and feel how moist it is for yourself—the dampness is a result of the water vapor collecting on your skin. During the winter, the water molecules from inside your body want to stick together as soon as you breathe them out into the cold air. When we see our breath during the winter, what we’re actually seeing are little clouds, which sometimes stick to the car windshield and make it really hard to see where you’re going.


If cold air causes water molecules to stick together again, why does water condense and form clouds in the bathroom when you’re taking a hot shower?

Have you ever been in a situation where you wanted to get out of a hot, crowded room, but all the doors and windows were shut? Even though you felt like you were cooking and really wanted to get away from all those people, you didn’t have a choice.

Humidity is measured by the number of water molecules in the air. Sometimes when heated water molecules are trapped in a closed space, such as a bathroom or a pot of boiling water, there isn’t enough room left for them to spread apart. As more and more water molecules are released into the air through evaporation, the space becomes more and more humid. If the number of water molecules in the air continues to increase, they eventually start getting squished together, just like people in a crowded rock and roll concert, and a visible cloud of steam is formed.

So the absence of heat isn't the only thing that causes water molecules to stick together again. Humidity plays an important role in condensation, too!


What happens if a floating water molecule touches something that’s not another water molecule?

Sometimes condensation can take place very close to the ground. When it’s foggy outside, we are walking around inside a cloud that formed in the exact same way as the clouds in the sky do. But when these little clusters of condensed water start to form somewhere close to the ground, chances are they’ll run into something that’s not another molecule, like a leaf or a rock. Splat. Eventually enough molecules will join the droplet to form a dewdrop. That’s why we see dew on the ground during cold mornings, and if it’s cold enough, the dew freezes and becomes frost. The dew point is the exact temperature at which dew starts to form.


What happens to the water droplets that are too far up in the sky to make contact with plants and rocks and things?

Even many miles up in the air, there are bits of dust and pollen floating around waiting for water molecules and water droplets to condense around them. When enough water collects around the object, it gets too heavy to float, and goes falling back to the ground as rain. Snowflakes are formed when water vapor freezes during condensation.


But what happens to the water molecules once they fall back to the ground?

They take another trip through the water cycle, which we will cover in depth next time!



Closure: We learned a lot about water today. Let’s see how much you can remember. Review guided notes with the students.

Where is the earth’s water? (in the oceans, glaciers, underground, lakes and rivers, and the atmosphere)

What are the three states in which water exists? (solid, liquid, gas)

What is evaporation? (when liquid water becomes water vapor)

What two things cause evaporation? (heat and wind)

What is water called when it is a gas? (water vapor)

What is condensation? (when water vapor becomes liquid water)

What two things cause condensation? (cold and humidity)

What is the dew point? (The exact temperature at which dew is formed)

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