At-Home STEM Activities: DIY Solar Still
For New Hampshire’s April school vacation week, when none of us is actually going anywhere, we decided to focus on things we could do if we did have some sort of wild vacation adventure and ended up on a deserted island.
On Tuesday, we learned how to use the sun’s energy to cook our lunch. Today, we’ll see how we can use to sun to make water drinkable. Note that since we aren’t actually on a deserted island surrounded be sea water, we need to make our own for this activity. Use of a stove is needed to boil water—please use adult supervision for this!
It’s a well-known fact that there is a lot of water on Earth—to be exact, 75% of it’s surface is covered in water. It’s a good thing that there’s so much water, since humans need it to stay alive. But most of the water on Earth is in the oceans, and people can’t drink salt water. Of all the water on Earth, only about 2.5% is fresh water. Of this 2.5%, about 70% is frozen, so only about 30% of the fresh water on Earth is drinkable. So 30% of 3% means of all the water on Earth, only 0.75% is available for people to drink.
One way to make a homemade solar still (if you wanted a more permanent set up than what we’re going to make) Image via motherearthnews.com
Now, imagine were stuck on a deserted island in the middle of the ocean. What would we do if we can only find salt water? Or, if we could find fresh water, what if it was full of mud? We would need to clean the water, and one way to do that is with a solar still. A still is an apparatus that distills, or purifies, a liquid, in this case, water.
Let’s make our own simple solar still and use the power of the sun to turn salt water into fresh, drinkable water!
DIY Solar Still
Materials:
Water
Salt
Tea kettle or small pot
Measuring cup
Large bowl, preferably (but not necessarily) one with a dark color on the inside
Cup that’s shorter than the bowl
Plastic wrap
Rubber band
Weight (something not too heavy, we’re using a small rock)
Instructions:
1. Since we’re not actually on a deserted island, we want to start by making our own sea water. Bring some water to a boil in the tea kettle or small pot. Pour about a quarter of a cup of salt into the measuring cup, and when the water is boiling, carefully fill the measuring cup with water. Give the mixture a stir and allow some time for the water to dissolve and the salt water to cool.
Note that salt will dissolve in cooler water as well, so it isn’t absolutely necessary to use boiling water—hot water just dissolves the salt faster. If you want, you can skip the boiling step and just use hot water from your faucet.
2. Now that we have our salt water, let’s set up our solar still. Pick a place to set up your still that receives sunlight for several hours. Place the cup inside the bowl, and pour the salt water into the bottom of the bowl. Tightly cover the top of the bowl with the plastic wrap, using the rubber band to keep the plastic in place. Set the rock on top of the plastic wrap, over the cup.
3. Let your still sit in the sun for a few hours. Throughout the day, you should see fog and water droplets form on the inside of the plastic wrap.
4. After a few hours or a whole day, see how much fresh water you distilled! Take a sip to taste that you truly collected fresh water in your cup.
We left our still out in the sun for five hours, and, as you can see, we didn’t collect very much water. So a solar still isn’t the fastest method to get pure water, but it is effective. Solar stills can be used wherever the sun is shining, so they’re often kept in emergency kits in life boats and planes.
How does a solar still work?
A solar still depends on two scientific principals: evaporation and condensation. By covering the top of the bowl with plastic wrap, we create a greenhouse effect. The heat of the sunlight gets into the bowl, and then the plastic traps the warmth inside. This warmth heats up the salt water and turns it to vapor (evaporation). Since water evaporates at a much lower temperature than salt, the salt is left behind in the bowl—when you take the cup of fresh water out of the solar still, look in the bowl and you can see the all the salt left. When the water vapor hits the plastic at the top of the bowl, it condenses and forms water droplets. Since we placed a weight over the cup, the plastic wrap slopes down. The water droplets follow the slope, and drip down into the cup.
Take this activity further:
Think about the pros and cons of a solar still. We already saw that a solar still takes a long time to distill fresh water. Are there other negatives of a solar still you can think about? What about positives?
Come up with a better design. Can you figure out a solar still design that can collect more water in the same amount of time?
Figure out other ways to find fresh water on a deserted island. What other sources of fresh water can you find in nature?
