Key concepts
Water cycle
Evaporation
Pollution

From National Science Education Standards: Changes in environments

Introduction
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!

Background
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.

Materials
•    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

Preparation
•    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.

Procedure
•    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.

6 Comments

1. 1. rahulbhuju 12:02 AM 6/1/11

   Instead of placing just a plastic,it's better to use black plastic or polythene bags.As black absorbs more heat than any other colors so the process goes as fast as it could...

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2. 2. skip_kovacs 12:53 AM 6/3/11

   In "Preparation", I think you should nestle the empty glass so it is BELOW the rim of the mixing bowl. If it is even with the rim the marble will not be able to drop the level of the plastic film low enough to have the condensed water drip into the glass without the film also sealing itself against the glass's rim.
   
   And I disagree with ruhulbhuju in the second comment: if this is a teaching exercise, we should use clear film to see the evaporation/condensation process, even if it is less efficient than it would be with black film.

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3. 3. Steven Brown 11:53 PM 6/23/11

   Better yet, use sunlight to crack water into hydrogen and oxygen, making a fuel that stores solar energy for later use. As for how to contain and transport hydrogen, there is a wonderful atom called carbon that can bond to four hydrogen atoms, creating a gas called methane for which a distribution network already exists.

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4. 4. Steven Brown in reply to Steven Brown 03:56 AM 6/24/11

   The use of carbon in this manner can be carbon neutral, because carbon dioxide can be reacted with hydrogen, in a reaction powered by sunlight, to make methane.

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5. 5. clevitan 01:06 AM 7/28/11

   This is also a Boy Scout survival technique. Instead, dig a hole in the ground (replaces the bowl) and put lush vegetation in it (replaces mud).
   To disagree with rahulbhuju above, the plastic sheet has to be clear- it must be cooler than the mud, since it is the condenser, and the mud has to be as hot as possible.

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6. 6. syedasrarahmed 08:28 AM 1/28/12

   The diagram explains quite better.

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