Key concepts
Water cycle

From National Science Education Standards: Changes in environments

Have you ever seen a stream or river right after a big rainstorm? The water is often muddy—and full of leaves, sticks and other things that were washed in by the rain. Even though it's the same water that moves from sky to ground and back to the sky again, when it rains again, the rain isn't muddy or full of sticks. Why is that?

How does the water become clean again? The process involves evaporation, and we'll demonstrate that today and clean a little water while we're at it!

Water molecules are so small that you can only see them when a number of them are clumped together to form a droplet. And they are constantly on the move. After a rain, they will eventually evaporate into water vapor. These airborne water molecules get carried back up into the sky to form clouds and then more rain (or snow or hail).

But, thankfully, other things, such as big clumps of dirt, sticks or leaves, are too big to be carried up with the water molecules into the atmosphere. So this process of evaporation can help to purify water of some of the big things it might be mixed in with.

•    Mixing bowl
•    Dirt
•    Plastic wrap
•    Clear drinking glass (slightly shorter than the rim of the mixing bowl)
•    Small round marble
•    Sunny ledge or warm surface
•    Warm water

•    Fill the mixing bowl with dirt about 2/3 of the way.
•    Add warm water slowly until you have nice soupy mixture that's still firm enough to stand the glass in.
•    Nestle the empty glass upright carefully into the center until its top is even with that of the mixing bowl rim.

•    Carefully place the plastic wrap over the bowl so that it seals around the edges of the mixing bowl (so water vapor won't escape) but somewhat loose in the center of the bowl. Make sure the plastic wrap isn't touching the surface of the dirt mixture.
•    Place the bowl in a sunny (and/or quite warm) location (direct sun works best).
•    Gently put the marble in the center of the plastic wrap over the cup (the marble helps direct the water condensation into the cup). Make sure the plastic wrap doesn't seal off the cup from the surrounding dirt.
•    Check on the cup every 15 minutes or so. It might take a while for the sun (or other heat source) to do its work. But the cup should soon start collecting water after 30 minutes to an hour. What does your collected water look like?

Read on for observations, results and more resources.

Observations and results
How much water were you able to collect? You can try leaving it for longer and see how much more you can accumulate.

Not everything can be separated out from water this way. Tiny particles, such as dust or chemical pollutants, can still find their way up into the sky. In fact, raindrops form around small pieces of dust in the clouds, and polluting "acid rain" can contain chemicals from burning fossil fuels.

What are other ways you can think of to purify water?

Share your water cleaning observations and results! Leave a comment below or share your photos and feedback on Scientific American's Facebook page.

Don't drink the water, but you can use it to water plants! You can use the moist dirt for houseplants, outdoor plants or to start a new plant from seed.

More to explore
"Sour Showers: Acid Rain Returns" from Scientific American
"Warmer Climate Produces Less Rain" from Scientific American
The Water Cycle game from the Environmental Protection Agency
"What Is Acid Rain?" overview from the Environmental Protection Agency
A Drop of Water by Gordon Morrison, ages 4–8
The Water Cycle: Evaporation, Condensation & Erosion by Rebecca Harman, ages 9–12
Up next…
High Seas: What Happens When the Glaciers Melt?

What you'll need
•    Small bowl
•    Ice cubes
•    Modeling clay
•    Toothpicks
•    Warm water