Hands-on Activity: Saltwater Circuit
Educational Standards :
Learning Objectives (Return to Contents)
After this activity, students should be able to:
Materials List (Return to Contents)
Each group needs:
For the entire class to share:
Introduction/Motivation (Return to Contents)
(Before beginning, gather materials to conduct a classroom demonstration of a saltwater circuit, as described in the Materials List and Procedure sections. Create two saltwater concentrations, one that allows the light bulb to turn on but stay dim and another selected to allow the light bulb to be bright. Suggested concentrations: Solution A: 300 ml water and 1 gram salt. Solution B: 300 ml water and 11 grams salt. Solution A will be much dimmer than Solution B.)
(Also prepare a projector to show the attached Saltwater Circuit Presentation [PowerPoint] at the end of the Introduction/Motivation session.)
Do you think water and electricity should ever be mixed? (Answer: Usually no.) What if you could safely mix water and electricity? Can you think of any cool technologies that could come from this? (Give the students a few minutes to think.) Today, we are going to work on answering this question. In fact, we are going to join water and electricity in a special way that is safe.
Has anyone ever built any type of electrical circuit before? (Pause to give students a minute or two to think about this.) Well, today we are going to build a saltwater circuit and we are going to investigate the conductivity of saltwater. In particular, we are going to answer the question: "How does the amount of salt in a saltwater circuit affect the electric current flowing through the circuit?"
(Conduct the saltwater circuit demo.)
Our question is a scientific question, but it also has an engineering application. After all, engineering is the application of math and science to create technologies that make the world a better place. One engineering application for this science is the development of a tool to test the efficiency of a water desalination plant.
A water desalination plant is a system that takes in saltwater and produces clean drinking water. If one were to design a water desalination plant, a saltwater circuit could be incorporated as a tool to detect the presence of salt at the output of the desalination plant. If the saltwater circuit conducts electricity, then the plant did not remove a significant amount of salt, and if it does not conduct electricity, then the plant did remove a significant amount of salt from the water input.
(Show students the attached Saltwater Circuit Presentation [PowerPoint].)
Vocabulary/Definitions (Return to Contents)
Procedure (Return to Contents)
Saltwater Circuit — A saltwater circuit consists of a battery, wire, light bulb, light bulb socket, and two electrodes (see Figure 1). When the battery is connected and the electrodes are touched together we have a closed circuit and electrons flow from the positive terminal of the battery to the negative terminal of the battery. This flow causes the light bulb to light up. When the electrodes are not touching, the circuit is "open" and electrons do not flow; this is called an open circuit. In our saltwater circuit, the electrodes act as a switch.
If you submerge the electrodes in regular tap water, the light bulb does not turn on because no medium exists to transfer electrons from one side of the water to the other. But if you submerge the electrodes in saltwater, the light bulb turns on. In addition, the amount of salt in the saltwater solution influences how much current flows through the circuit, and in turn, how bright the light bulb glows.
Why Does the Saltwater Circuit Work? — An ion is an atom that has an electrical charge, either positive or negative. Salt molecules are made of sodium and chlorine. When salt enters water, the water causes the salt's sodium and chloride atoms to pull apart and make the salt crystals begin to disappear. As a result, a sodium ion and a chlorine ion are formed. The sodium ion is missing an electron, which gives it a positive change. The chlorine ion has an extra electron, which gives it a negative charge.
When an electric potential is applied, the positively-charged sodium ions are attracted to the negative pole and the negatively-charged chlorine ions are attracted to the positive pole. These ions carry the electricity through water. The essence of the above process is that an "invisible wire" is formed that allows electrons to move from ion to ion across the water.
Before the Activity
With the Students — Building the Saltwater Circuit
1. Individually wrap two large Popsicle sticks in aluminum foil (see Figure 2-left). These are your electrodes.
2. Connect one wire to each electrode using electrical tape. Make sure the bare end of the wire touches the aluminum foil (see Figure 2-left).
3. Connect the opposite end of the wire from one electrode to one terminal of the light bulb socket. Insert the bare wire around the socket terminal and tighten with a screwdriver. Add a piece of electrical tape to secure the connection (see Figure 2-right).
4. Connect a wire to the opposite terminal of the light bulb socket. Again tighten with a screwdriver and cover with a piece of electrical tape (see Figure 2-right).
5. Use electrical tape to connect the wire from the light bulb socket to the red wire of the 9-volt battery cap (see Figure 3-left).
6. Use electrical tape to connect a wire to the black wire of the 9-volt battery cap (see Figure 3-left).
7. If using a multimeter: Connect the free wire to the negative terminal of the multimeter. Then connect the positive terminal of the multimeter to the free electrode (see Figure 3-middle).
8. If not using a multimeter: Use electrical tape to connect the free wire of the battery cap to the free electrode (see Figure 3-right).
9. Test your circuit by touching the two electrodes together. This completes the circuit, allowing electricity to flow from one terminal of the battery to the other, and illuminates the light bulb in the process. If the bulb does not light up, check your wire connections to make sure they are all secure and try again. (See Figure 4.)
With the Students — Solutions, Data Collection and Analysis
1. Hand out an activity worksheet and saltwater density card to each group.
2. Direct teams to use the information provided on the card to make three different saltwater solutions. Label the cups A, B, C, from highest to lowest salt concentration.
3. Data Collection Have students insert both electrodes in one saltwater solution (without touching electrodes) and observe how bright the light bulb becomes and record the current reading from the multimeter. (If multimeters are unavailable, making a visual observation is sufficient.) Record measurements and/or observations on the worksheets.
4. Data Analysis Rank the solutions from dimmest to brightest by visual observation.
5. (If using multimeters) Once the solutions have been ranked, have students plot the measurements vs. solution label.
6. Conclude the activity by having students complete the Reflection Worksheet, as described in the Assessment section.
Attachments (Return to Contents)
Safety Issues (Return to Contents)
Troubleshooting Tips (Return to Contents)
If the light bulb does not light up, make sure all wire connections are tight.
Investigating Questions (Return to Contents)
Does more salt in the saltwater circuit mean that light bulb will be brighter than if less salt is used? (Answer: Yes. If you increase the amount of salt in the saltwater solution, the light bulb becomes brighter.)
Do you think you can continue to add salt and make the light bulb brighter, or is there a point at which more salt does not affect the brightness of the light bulb? (Answer: Eventually, any additional salt will not cause the light bulb to become brighter. Only so much electrical current can be drawn from a battery source for a given electrical circuit.)
Assessment (Return to Contents)
Class Discussion: During the Saltwater Circuit Presentation (PowerPoint), create an environment in which students can be actively involved in the discussion.
Activity Embedded Assessment
Reflection Worksheet: In this worksheet, students answer questions about concepts learned and their participation. Review worksheets to gauge students' mastery of the subject matter.
Activity Extensions (Return to Contents)
Continue the activity by conducting the associated Water Desalination Plant activity in which students design/build/test a model desalination plant using inexpensive materials.
Activity Scaling (Return to Contents)
References (Return to Contents)
PBS Kids Go. Zoom-Saltwater Rocks. WGBH Educational Foundation. Accessed May 1, 2010. http://pbskids.org/zoom/activities/sci/saltwatertester.html
Wikipedia.org, Wikipedia Foundation Inc., Accessed May 1, 2010. (Source of vocabulary definitions, with some adaptation.) http://wikipedia.org
ContributorsJuan Ramirez Jr., Carleigh Samson, Stephanie Rivale, Denise W. Carlson
Copyright© 2009 by Regents of the University of Colorado. The contents of these digital library curricula were developed by the Integrated Teaching and Learning Program under National Science Foundation GK-12 grant no. 0338326. However, these contents do not necessarily represent the policies of the National Science Foundation, and you should not assume endorsement by the federal government.
Supporting Program (Return to Contents)Integrated Teaching and Learning Program, College of Engineering, University of Colorado at Boulder
Last Modified: March 7, 2014