Summary

In this lesson, students review important concepts of the water cycle and then design, test, and evaluate a working model of a desalination plant (a distiller). They extend their understanding of the critical need for desalination around the world as they research new and promising technologies.

Standards

NGSS and Cross-Disciplinary Extensions addressed in this lesson

Objectives

By the end of this lesson, students should be able to

• Diagram the water cycle.
• Cooperate within a team to design a working model of a desalination plant.
• Measure, record, graph, and analyze quantitative data.
• Communicate investigation findings.
• Research and describe new methods of desalination.

Chemistry Topics

This lesson supports students’ understanding of the following topics in chemistry:

• States of matter
• Solutions
• Mixture
• Separating mixtures

Time

Teacher Preparation: 30 minutes

Lesson: 3 class periods, 45 minutes each

Materials

For each group:

• Bowl or similar shaped container (capacity between 2 L and 3 L)
• Small and short cup (capacity about 100 mL; the heavier, the better)
• Clear plastic wrap (to cover the bowl)
• Small stones or weights (2)
• Salt water solution (about 1 L)
• Area outside that is exposed to the sun for several hours
• Small paper cups (one for each student)
• Graduated cylinder (about 20–30 mL in size)
• Clock or timer (optional)

Safety

• Remind students not to look directly at the sun when they are outside setting up their distillers.
• Remind students not to drink the water (unless instructed by the teacher).

Vocabulary Terms

• condensation
• evaporation
• solute
• solution
• solvent
• water cycle
• water vapor

Keywords

desalination, water cycle, model, salinity, system, distill, solar still, engineering, design

Teacher Notes

• If students already have a strong grasp of the water cycle, Explore 1 and Explain a can be skipped.
• To see what others have done to make a model desalination plant, explore one or more of the following web sites.
  
  • Solar Still
  • Solar Still Part 1: Salt Water
  • Salt Water Distiller
    
• Be aware that if there is too much water in the bowl and the cup inside is not heavy enough, the cup might float and move away from the center of the bowl. Be prepared to have students add a second rock or weight to the cup to weigh it down.
  
  
• Be prepared to mop up any water spills if the experiment is done indoors.
  
  
• If you can provide new small paper cups and feel comfortable having your students drink the fresh water their distillers will produce, let them taste this water. It will add to their engagement in the lesson. Make sure the collecting cup placed inside the distiller is sufficiently clean.
  
  
• Once students conduct the initial investigation, they may want to investigate the effects of other variables such as amount of time in the sun, location of the distiller (full sun vs. shade), or initial salinity of the water. Keep in mind, however, that all other variables, including the design of the distiller, would need to be controlled.
  
  
• Visit the following web sites for additional information about desalination.
  
  • How Desalination Works
  • What if you drink saltwater?
  • Solar Still

Lesson

Engage

1. Hold up a container of water. Ask students if they've had a drink of water today and if so, where did that water come from? A brief discussion about drinkable water will probably include the relative ease and convenience of obtaining safe and drinkable water whenever they need it. Ask Students: Have you ever been in a situation where you did not have access to drinkable water? Why did it happen? How did you feel when this happened? How would you feel if it happened?

Next, ask students if they know how people in other parts of the world get their drinking water. Guide students to consider the broader issue of the importance of safe drinkable water to all people around the world.

2. Use some of the water from the bottle to wet a sponge. Swipe the wet sponge across a blackboard or other smooth surface the students can see; it should leave a visible smear mark. Allow the smear to dry/evaporate as you ask students to carefully observe any change in the smear. (An alternative and quick way to demonstrate evaporation is to vigorously breathe on the blackboard and step away so students can observe a dark spot of moisture on the board. This is even something students could do themselves.)

Facilitate a brief discussion about the “disappearance” of the water and draw out the students’ understanding of evaporation. Can they articulate the change of state from liquid water to water vapor? What do they understand about the particle nature of matter such as water? Do they use the term “water molecule”? As key concepts are brought up, write them on the board or chart paper: water, liquid, gas, water vapor, molecule, evaporate and others. You might even want to organize these concepts in graphical form in order to show relationships among the concepts.

3. Move on to pose the dilemma of being stranded on a small island somewhere in the middle of the ocean without having any fresh water to drink. How would students survive without any water to drink? Encourage students to brainstorm possible ideas for survival. (This could be done in small groups, using Think-Pair-Share, or as a whole class. Ideas could be charted or diagramed on the board or chart paper and summarized with the class.)

4. Next pour some of the water from the bottle you have been using into a beaker or glass, add a small amount of salt and stir. Tell students you have just made a salt water solution, and that the water is called the solvent and the salt is called the solute. Write these three terms on the board. (If time and availability of materials permit, individual or pairs of students could be allowed to make their own salt water solution. Students should be cautioned not to drink their salt water.) Prompt students to make a connection between this salt water solution and ocean water, both of which contain salt. Is this salt water drinkable? Is the salt water from the ocean drinkable?

Tell students they are going to have an opportunity in this lesson to design, build, and test a device that can remove salt from water so the water is drinkable. Hold up your beaker of salt water. In theory, if they were indeed stranded on a small island, they should be able to obtain drinkable water by using such an apparatus. Why would such a device be helpful to populations around the world? What areas of the world might benefit from a device like this? (Save this beaker of salt water for later use.)

Download the Teacher Guide to view the rest of this lesson.