Experiment

The Earth’s surface is mostly covered with water. Almost all of that water is in the oceans.

But ocean water is salty, and we can’t drink salt water. We need fresh water. The problem: Only 2.5% of the water on Earth is fresh water. Fresh water is found in lakes, rivers, and underground reservoirs.

Salt water

Fresh water

Some parts of the world have few freshwater resources, but are located near the sea. What if we could turn salt water into fresh water? We can do this, through a process called desalination. The term desalination means "taking the salt out."

There are different desalination methods. One process uses the energy of the Sun to cause the water to evaporate. This water vapor is then condensed back into liquid water. Since the salt is left behind when water evaporates, the result is fresh water. This process can also be used to produce clean fresh water from water that is not suitable for drinking.

Here’s a project you can do. You can create a device called a solar still. The Sun provides the energy. Still is a short form of the word distill, which is the process of heating a liquid to vapor and then condensing the vapor back to a more purified liquid.

As you do this project, think about ways to design and build a more efficient system for water desalination and purification using the principle of evaporation and condensation.

Our Experiment

Materials

• Salt
• Drinking water
• A small cup. It is best to use a fairly heavy ceramic cup.
• A glass bowl that is larger than the cup.
• Plastic wrap
• A small rock or other weight

What We Did

1. We washed the bowl and cup to make sure they were clean enough to drink from. 2. We put drinking water in the bowl to a depth of about 2 cm (0.79 in). 3. We added some salt, enough so that the water tasted salty. Note that a few grains of salt may remain visible at the bottom of the bowl. 4. We put the cup inside the bowl, right in the center. 5. We put the bowl in a sunny place at midday. 6. We covered the bowl with plastic wrap and pressed it tightly around the rim of the bowl.

7. Then we placed a small rock on the plastic wrap so that it was right above the cup.The rock causes the plastic wrap to stretch a bit so that it is lower just above the cup than at the sides. This diagram shows how this would look in a cutaway view from the side.

8. After a while we saw droplets of water start to form on the underside of the plastic wrap. As the water droplets became larger, they began to flow down the inner surface of the plastic wrap toward the center. There they met other droplets coming from other directions. As the droplets met and joined, they became heavier and dropped into the cup below.

9. After a few hours we rolled back the plastic wrap. Some water was now in the cup. We tasted this water; it was fresh, not salty. We had succeeded in removing the salt to get fresh water from salt water!

How Our Solar Still Works

The water in the bowl is warmed by the Sun and evaporates, producing water vapor. The plastic wrap cover creates an enclosed space. As in a greenhouse, it gets quite warm inside. Since the water vapor cannot escape the humidity rises toward 100%.

One side of the plastic wrap cover is in contact with the outside air, which is cooler than the air inside the covered bowl. When the humid air inside touches this cooler surface, water vapor condenses on the plastic wrap, to form the water droplets that we see. Those droplets fall into the cup below.

The water that falls into the cup is free of salt. This occurs because only the water evaporates—the salt is left behind. This is the process of desalination.

This process in our solar still is a miniature version of the water cycle that occurs around the world.

Water evaporates from oceans, lakes, and streams. It condenses into clouds and returns to Earth as rain, snow, and other forms of precipitation. During the water cycle, condensation occurs when humid air rises to higher, cooler altitudes. Because cooler air holds less water vapor, some of the moisture condenses into clouds.

Most of this evaporated water comes from the oceans, since they cover most of the Earth’s surface. The salt remains in the ocean as the water evaporates. The resulting clouds, rain, and snow are fresh water. This is the water that goes into freshwater lakes and rivers. It is also the water that goes into the ground to underground reservoirs. This gives us drinking water from wells.

Design Your Own Solar Still

• Our solar still produces only a small amount of fresh water. Can you make a larger device that will produce more fresh water?
• We have to open our still to add more salt water and to remove the fresh water. Can you design a solar still that has an inflow for the salt water and an outflow for the fresh water?

How Does Nature Do It?
		Learn how fog beetles make use of scarce water source in the desert. Find out how scientists are learning to harness the energy of the Sun in many ways.

More Things to Try

• Add some food coloring to the salt water in the bowl. What color do you think the fresh water will be? Why? Try it!
• Instead of salt water, place in the bowl many thin slices of tomato, melon, or other fruits and vegetables that contain a lot of water. Do you think this will produce fresh water? What will happen to the fruit or vegetable? Try it!

Here’s a web site with many good ideas about solar stills:
Understanding Solar Stills

You can also use Google to search on "solar still" for additional informative web sites.