Overview

In this lesson from the Peace Corps, students in grades 3-8 learn how to generate fresh water from seawater, using the power of the sun and the natural processes of evaporation and condensation. They study the water challenges in Cape Verde and the advantages of distilling water through a simple solar still. They then build and evaluate their own working model.

Objectives

• Students will describe how water can change form through the processes of evaporation and condensation
• Students will explain how a solar still functions to purify water
• Students will discuss how water distillation can benefit human communities

Framework and Standards

Enduring Understandings

• Access to freshwater is essential for human survival
• Freshwater is not equally accessible in all areas of the world
• Knowledge of scientific principles, such as the water cycle, can be beneficial for solving problems

Science Standards

• Content Standard A: Science as Inquiry
• Content Standard B: Physical Science
• Content Standard E: Science and Technology
• Content Standard F: Science in Personal and Social Perspectives

Background Information

The water cycle

• U.S. Geological Survey — Water Science for Schools
• NASA Earth – The Water Cycle
• NASA Earth Observatory — The Water Cycle
• Related Publication: Bringing Water from Sol to Soul

About Cape Verde

• Peace Corps — Where Do Volunteers Go? — Cape Verde
• Peace Corps — World Wise Schools — Water in Africa
• CIA – The World Factbook — Cape Verde

Materials

• Markers
• World map

for each group of 3-5 students:

• Large bowl
• Short glass or cup
• Tape
• Plastic wrap
• Small rock
• Pitcher of water
• Salt
• Spoon
• Liquid measuring cup

Procedure

Day 1 — Introduction

1. Looking at a globe, discuss: What covers most of the earth? Can we drink all of the water that covers earth? Why not? Explain that most bodies of water on earth contain saltwater, and few bodies of water (lakes, rivers) contain fresh water. Less than 1% of water on earth is usable by humans.

2. Ask students to brainstorm all the places they can think of on earth where there is water. Prompt students to think about less obvious sources (e.g. not in liquid form). Is the water in these places freshwater or saltwater? [Possible lists: Saltwater—Oceans, seas; Freshwater—Lakes, rivers, glaciers, atmosphere/clouds, inside plants/animals; Both—Groundwater (can be saline or fresh) ]

3. Explain that many people get their drinking water from underground. Groundwater is usually pure and good to drink. Near the ocean, however, it can contain too much salt. This can be a problem for people in areas with little access to freshwater.

4. Tell students they are going to see how people in Cape Verde are dealing with this problem. Find Cape Verde on a map; provide some information about the culture and geography. The Source of Our Water: Cape Verde. Introduce Nicholas Hanson and Brian Newhouse as Peace Corps Volunteers serving in Cape Verde. If students are unfamiliar with Peace Corps, share some background information (http://www.peacecorps.gov/ ).

5. [3/2019 – the above video was made by Nick and Brian, so may be similar to the Peace Corps video, Bringing Water from Sol to Soul., that was part of the original activity uploaded in 2011.] Then discuss:

• What problem were Nicholas and Brian trying to solve? [remove salt from the groundwater so it would be drinkable]
• What did they do to solve the problem? [built a solar still to distill the saltwater into freshwater]
• How did the solar still work? [The sun heated up the saltwater, causing the water to evaporate, but leaving the salt behind. The evaporated water condensed on a plastic sheet, and turned back into a liquid form containing no salt]

6. Now students are ready to practice distilling saltwater into freshwater. To prepare, teachers should read the activity instructions from PBS Zoom, then view a short demonstration video. Teachers should also determine ahead of time the best placement for the stills in a protected, sunny location. Before students begin, ask them to make predictions such as:

• What do you think will happen?
• Will this solar still be able to turn saltwater into freshwater? If so, how?
• How much freshwater do you think will be produced?
• Where do you think the salt will go?
• How long do you think it will take to distill the water?
• What would make it go faster?

7. Divide students into small groups of 3-5, supplying each group with the materials. Demonstrate how to set up the model solar still.

Activity: Solar Still Construction

(from PBS LearningMedia Zoom activities)

A solar still distills, or purifies, water by turning saltwater into fresh water using the sun and gravity. Rays from the sun heat up the salty water in the bowl. When the water gets warm, it evaporates and becomes a gas. When the gas rises and hits the plastic wrap, it turns back into water droplets. Eventually, gravity makes the water droplets roll down the plastic wrap towards the rock. Then the water droplets slide off the plastic wrap into the glass. The salt doesn’t evaporate, so it gets left behind in the bowl. Water evaporates in the same way from lakes, rivers, and oceans. The water heats up, turns into a gas, and then condenses to fall back down as rain.

1. Add salt to fresh water, stirring until the salt dissolves.

2. Pour about two inches of saltwater in a large bowl.

3. Take an empty glass and place it into the bowl. The top of the glass should be shorter than the top of the bowl but higher than the saltwater.

4. Place plastic wrap over the top of the bowl, taping the edges to make sure the seal is tight.

5. Place a weight, such as a rock, in the center of the plastic wrap, suspended over the empty glass. The rock will weigh the plastic down and help the water collect.

6. Carefully transport each solar still to a designated area, preferably outside, in a protected location that receives several hours of strong sunlight.

Day 2: Evaluation

1. Have students retrieve their stills, removing the plastic wrap and observing the water that has collected inside the cup. What happened? How accurate were their predictions?

2. Help students measure the amounts of distilled water collected in the cups and taste the water to see if it still contains any salt.

3. Ask students to posit explanations about what happened. How did the water in the cup become purified? Discuss the processes of evaporation and condensation, and their roles in the distillation process.

4. Refer back to Nicholas and Brian’s project in Cape Verde. How did the ability to distill water impact the people in their community? Why is distillation useful to people with limited access to freshwater?