Model

There is always water vapor in the air, but the individual water molecules are so tiny that you can't see them. Water in the air is invisible unless it forms tiny water droplets (fog) or bigger ones (rain) or freezes (snow crystals). Here's one way to picture it.

Look at the three boxes below from about arm's length away. What do you see in each box?

Now look at the three boxes up close. What do you see in each box?

Each box has 20 dots in it. When they are in large groups (Box A) it is like rain droplets, and you can see them. When they are in small groups (Box B) it is like clouds or fog. When they are spread out (Box C), it is like water vapor. The individual molecules are invisible because they are so small.

Imagine that you can zoom in on water molecules above the surface of the ocean. Use the dots at the top of this picture to zoom in on the water. When you zoom all the way in, you can see water molecules.

Here is a model of water in the air. The white circles represent water molecules. The blue circles represent air molecules.

A water molecule all by itself is water vapor. It is too small to see.

A bunch of water molecules close together is a water droplet. If the droplets are big enough (many billions of molecules), they can be seen as fog or mist.

When you have zoomed all the way in click here to go to the next step.

(To go back to using the zoom tool, click here)

Using the drawing tools, draw a red square around a molecule that is water vapor (gas) on the picture above.

Draw a blue square around liquid water in a water droplet.

Now you are going to run a model that shows water. To see the model, click here

Heat up
Cool down

Describe what you notice and what the molecules do.

Raise the temperature of the molecules by adding heat. What changes do you see? (Click here to take a picture of the model)

When the model is heated, notice that molecules leave the droplet and fly off by themselves. This is how water molecules leave a liquid and go into the air. This is called evaporationEvaporation is the process where water leaves the liquid state and becomes a gas, which is called water vapor. Water vapor is invisible. The water molecules are not connected to each other.. Let's explore this more carefully.

Reset the model and run it again. Count how many molecules are not in the droplet.

Reset the model and run it again. Add heat by clicking on the "Heat the molecules" buttton until the temperature bar is about in the midddle. Wait for ten seconds. Then count how many molecules are not in the droplet.

Reset the model and run it again. Add heat by clicking on the "Heat the molecules" buttton until the temperature bar is near the top. Wait for ten seconds. Then count how many molecules are not in the droplet.

Reset the model and run it again. Remove heat by clicking on the "Cool the molecules" buttton until the temperature bar is near the bottom. Wait for 20 seconds. Do the molecules form back into droplets?

When the model is cooled, notice that molecules form back into droplets. This is how water molecules come out of the air and form liquid water. This is called condensationCondensation is the process where water vapor becomes a liquid, such as a water droplet or rain or wetness on a surface. Water molecules in a liquid are close to each other, but they can slide around..

Describe how the temperature changes the droplet.

How does this model help explain why you see more condensation above hot water than above cold water?