Desalination of seawater takes place every day as the sun heats seawater to the point of evaporation. When water evaporates into vapour, it doesn't carry salt from the ocean with it, and the water falls as fresh rainwater. This process of saltwater desalination via the sun is observable through a number of different science projects.
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This project, recommended by Science Fair Adventure, requires only a little water, a mug and a bowl, salt, a rock and some cling film to demonstrate the sun's capacity to desalinate water. First, mix a little water and salt (or, use straight seawater) in the mug and pour this mixture into the bowl. Rinse and dry the mug and place it mouth-up in the bowl. There should be about an inch of salty water in the bowl. Cover the bowl and mug with cling film and set it in a warm, very sunny place. Put the rock in the middle of the cling film so that the wrap is bowed over the mouth of the cup. Wait several hours until there is a drinkable amount of water in the mug. Taste it -- it shouldn't taste salty.
Big Bottle Desalination Apparatus
This project, suggested by Science Buddies, also harnesses the power of the sun to desalinate salty water. Lay a large, clear four-sided plastic bottle on its side. Cut the upper corner/edge off of the bottle so you now have bottle with three sides and a wide mouth. Later, you will cover the opening with cling film. Poke a hole in the plastic of the bottle below the wrap. A bottle cap with a straw poked through the plastic will serve as a condensation collector. Place this inside the large bottle with the straw passing through the side of the bottle. This straw should pass through the side of the large plastic bottle to drain into a cup or another bottle. Place salty water in the large bottle and cover the open side of the bottle with cling film. The water will evaporate under the sun's rays, drip into the collection device and flow into the cup at the other end of the straw. Use a quarter to weight the cling film so that condensation will roll into the bottle-cap collection device. Construct five more of these desalinators. Now you can run a series of experiments desalinating salty water through each device and compare the results between the different devices. Tweak different variables such as the amount of sun each apparatus receives or the initial salinity of water to observe changes in the resulting, freshwater condensate.
Solar Oven Desalination Experiment
This project, used in the California State Science Fair, tests different modes of desalinating water. First, construct six desalination devices constructed of plastic water bottles, cling film, water and water-collection cups as above. Build a solar oven using a cardboard box lined with aluminium foil. Place two of the desalinators in the solar oven, place two in the sun and place two in the shade. Run the experiment for six days taking measurements of the ambient temperature, water temperature and oven temperature thrice daily. At the end of six days, test the condensate collected from each desalination device for salinity and for water conductivity.
Parabolic Solar Reflector Method
This project, suggested by Science Project Ideas, requires building a simple parabolic solar reflector. Line a mixing bowl with aluminium foil. Make sure the shiny side is facing out and set this in the sun. This is your parabolic solar reflector-- place a water condensation collecting device (made with a plastic bottle filled with salty water and a drain made of a bottle cap with a straw poked through one side to catch the condensation. Channel it into a cup or other container) inside the parabolic reflector and wait until some condensation begins to collect in the water cup. This water should be salt-free. Try this with different liquids to see what happens or at different times of day to judge the effect of the sun's strength on desalinating water.
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