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Making a Solar Still

Imagine for a moment that you are stranded in the desert, and running short on water. Or imagine that you are in a boat far out at sea, surrounded by salt water, but no fresh water to drink. Is there any way you could use some science to get something to drink? Of course there is!

For this experiment, you will need:

  • a large, wide bowl
  • a small glass or jar, shorter than the bowl
  • clear, plastic wrap
  • salt
  • water
  • a coin
  • tape

If you can't dig a solar still outside, like in the video, you can make one inside. In this case, we are going to pretend we are adrift in the ocean, and need to change salt water into fresh water.

Place about an inch of water into the large bowl. Add a couple of spoons of salt. Stir and then taste the water. Salty! That will be our ocean water. Carefully, place the small glass in the center of the bowl. Be careful not to get any salt water into the glass. Stretch a sheet of plastic wrap across the top of the bowl. If the plastic does not cover the entire top of the bowl, use two or more pieces. They must fit tightly over the bowl and you may want to use some tape to hold the pieces in place.

The plastic should not touch the glass in the center of the bowl. Finally, place a coin in the center of the plastic wrap, just over the glass. It should form a depression in the plastic sheet, just over the center of the glass, but the plastic should still not touch the glass. If it does, then try a shorter glass. Carefully, place the entire setup in direct sunlight and wait. If you don't have a sunny place, put the bowl under a bright lamp, as close to the bulb as you can without it touching.

After a short while, you will see that water drops are beginning to form on the plastic wrap. As more and more water collects on the plastic, it will begin to run down the plastic towards the coin. There, it will drip into the glass. After you collect a little water in the glass, open the plastic and remove the glass. Carefully taste the water in the glass. Is there any salt? No!

You have just made distilled water. Sunlight shines through the plastic and heats the water. This causes the water to evaporate. As the humidity in the bowl increases, the water begins to condense on the plastic. Only the water evaporates. The salt is left behind in the bowl. If you ever find yourself stranded on a lifeboat in the middle of the ocean, remember this experiment. It might come in handy.