How would you get water to drink if you were stranded on a desert island, surrounded by salty ocean? If you happened to have a big leaf and a few concave (bowl-like) items- maybe a big shell and a smaller shell, you could make a solar water purifier to collect drinking water.
Even if you’re not on a desert island, it’s fun to make your own solar water-purifier using a big big bowl, a small bowl, some plastic wrap and a marble or a pebble. You’ll also need salt and food coloring to “contaminate” the water you’ll be purifying. This experiment works best on a hot, sunny day since it harnesses the power of the sun’s rays to clean the water.
Put the small bowl inside the big bowl. The top of the small bowl must be lower than the top of the big bowl.
In a separate container, mix together water, salt and food coloring until you have a mixture too salty to drink. Pour the water into the big bowl, making sure the liquid stays outside of your small bowl, since you’ll be collecting clean water in the small bowl.
Loosely cover the top of the bowl with a single piece of plastic wrap. Place a pebble or marble in the center of the plastic wrap and adjust the wrap so that there is a dip directly above the small collection bowl. Try to seal the plastic wrap around the edges of the bowl as well as you can.
Now, place the bowl in the sunlight. The sun’s ultraviolet rays will go through the plastic wrap and into the colored water, where they’ll be absorbed and re-released as heat energy. Since the heat can’t escape back out through the plastic wrap, the air and water in the bowl heat up.
The warm temperature helps water molecules on the surface evaporate, or escape, into the air in the bowl. When they collide with the plastic wrap, they encounter a cooler surface since the air outside the bowl is not as warm. This causes them to condense, or form droplets, on the plastic wrap. When the droplets get big enough, gravity pulls them to the lowest part of the plastic wrap (the dip) and they drip into the collection container.
When you’ve collected enough purified water (be patient- it can take a day or two), you can taste the water to see how your purifier worked. Be sure to wipe of the bottom of the collection bowl before you pour it out so you don’t contaminate your clean water!
A variation on the experiment is to add vinegar to the water, purify it, and check the pH of your starting and purified water using litmus paper. You can make your own litmus paper using red cabbage!