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Grade Levels: 6-8
Introduction: We all know that ocean water tastes salty because it contains many dissolved salts of different elements. These salts are similar to the salt that we use to season our food. In the water, these salts are separated into ions of chloride, sodium, sulfur, magnesium, and calcium. These ions are small particles of these elements that can carry electricity. For this reason, it is dangerous to handle electrical appliances in the water, because the water solutions can conduct electricity.
In this activity, students will investigate a property of water called conductivity. Conductivity is the ability of salt-water solutions to conduct electricity. To study the conductivity of salt water, the students will use the TI calculator and the CBL (Calculator Based Laboratory) in conjunction with a conductivity probe.
Learner Objectives:
- The student will be able to describe some of the components of seawater
- The student will be able to describe how the components of seawater change the properties of the water.
- The student will be able to explain how salt dissolved in water can cause water to conduct electricity.
Florida Sunshine State Standards: Science: SC.A.1.3.1,GLE 1, SC.H.1.3.2, GLE 1; SC.H.1.3.5, GLE(s) 1, 2, 3; Math: MA.D.1.3.1 MA.D.1.3.2
Competency-Based Curriculum: Science: M/J1-I.1.A, I.2.A, I.3.A, I.8.A, I.1.B, I.2.B, IV.1.A, M/J2-I.3.A, M/J2-I.8.B, I.1.B, I.2.B, M/J3-I.3.A, I.8.A, I.1.B, I.2.B, III.2.A
Materials:
CBL System 100-mL beaker
TI Graphing Calculator distilled water
Vernier Conductivity Probe stirring rod
Vernier adapter cable utility clamp
TI-Graph Link ring stand
sodium chloride solution (1.0 M (5.85 g of NaCl per 100 ml))
Figure 1
Activity Procedures:
- Obtain and wear goggles.
- Get 40 mL of distilled water in a clean 100-mL beaker.
- Assemble the conductivity probe, utility clamp, and ring stand as shown in Figure 1. Be sure the probe is clean and dry before beginning the experiment.
- Prepare the conductivity probe for data collection.
• Plug the conductivity probe into the adapter cable in Channel 1 of the CBL.
• Set the selection switch on the amplifier box of the probe to the 0-2000 µS range.
• Use the link cable to connect the CBL System to the TI Graphing Calculator. Firmly press in the cable ends.
- Turn on the CBL unit and the calculator. Start the CHEMBIO program and proceed to the MAIN MENU.
- Set up the calculator and CBL for a conductivity probe and a calibration of 0 to 2000 µS.
• Select SET UP PROBES from the MAIN MENU.
• Enter Ò1Ó as the number of probes.
• Select CONDUCTIVITY from the SELECT PROBE menu.
• Enter Ò1Ó as the channel number.
• Select USE STORED from the CALIBRATION menu.
• Select H 0-2000 MICS from the CONDUCTIVITY menu.
- Set up the calculator and CBL for data collection.
• Select COLLECT DATA from the MAIN MENU.
• Select TRIGGER/PROMPT from the DATA COLLECTION menu. Follow the directions on the calculator screen to allow the system to warm up, then press “Enter”.
- Before adding any salt solution:
• Carefully raise the beaker and its contents up around the conductivity probe until the hole near the probe end is completely submerged in the solution being tested. Important: Since the two electrodes are positioned on either side of the hole, this part of the probe must be completely submerged as shown in Figure 1.
• Monitor the conductivity of the distilled water displayed on the CBL screen for 4-5 seconds (the unit of conductivity is the microsiemens, µS).
• Press “Trigger”on the CBL, and then enter “0” (the volume, in drops). The conductivity and volume values have now been saved for the first trial. This gives the conductivity of the water before any salt solution is added.
• Lower the beaker away from the probes.
• Record the conductivity value in your data table (round to the nearest 1 µS).
- You are now ready to begin adding salt solution.
• Select MORE DATA.
• Add 1 drop of salt solution to the distilled water. Stir to ensure thorough mixing.
• Carefully raise the beaker and its contents up around the conductivity probe until the hole near the probe end is completely submerged in the solution being tested.
• Briefly swirl the beaker contents. Monitor the conductivity of the solution for 4-5 seconds.
• Press “Trigger”, and then enter “1” (the volume, in drops). The conductivity and volume values have now been saved for the second trial.
• Lower the beaker away from the probes.
• Record the conductivity value in your data table.
- Repeat the Step 9 procedure, entering “2” this time. Record the conductivity value in your data table.
- Continue this procedure, adding 1-drop portions of salt solution, measuring conductivity, and entering the total number of drops added until a total of 10 drops have been added.
- Select STOP AND GRAPH from the DATA COLLECTION menu when you have finished collecting data. Examine the data points along the displayed graph of conductivity vs. volume. As you move the cursor right or left, the volume (X) and conductivity (Y) values of each data point are displayed below the graph. Confirm the conductivity and volume data pairs you recorded in your data table.
- If available, use the TI-Graph Link cable and program to transfer the graph of conductivity vs. volume to a Macintosh or IBM-compatible computer. Print a copy of the graph.
- Dispose of the beaker contents as directed by your teacher.
Data Table:
| DROPS |
CONDUCTIVITY (µS) |
| 0 |
|
| 1 |
|
| 2 |
|
| 3 |
|
| 4 |
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| 5 |
|
| 6 |
|
| 7 |
|
| 8 |
|
| 9 |
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| 10 |
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Processing the Data:
If the graph link and computer is not available, plot a graph of the conductivity vs. drops data on graph paper. Label both axes and show correct units on either the printed or the hand-made graph. Label tickmarks with the numerical values they represent.
Assessment:
The students should prepare a lab report, which should include all major parts of a lab report plus the graph and the answers to the following questions:
- Describe the appearance of your graph. What does this indicate? Explain.
- Describe the change in conductivity as the concentration of the NaCl solution was increased by the addition of the drops. What kind of relationship does there appear to be between conductivity and concentration?
- How do you think that the ability to conduct electricity will change if you use other types of salts?
- What differences do you observe between the prepared salt solution and real seawater?
Activity Extensions:
- Obtain seawater and water from other sources and try a similar experiment with these samples instead of the prepared NaCl solution.
- Compare your results and those of other obtained when samples from more than one source are tested.
- Interdisciplinary: Design a poster for swimming pool or the beach explaining the dangers of swimming during lightning storms (Art, Physical Education).
Home Learning Activity:
Allow students to bring in samples of water from their homes, the ocean, and surrounding areas to measure and compare their conductivity in the classroom.
Vocabulary: ions, chloride, sodium, magnesium, calcium, salt
References:
Holmquist, D.D., Randall, J., Volz, D.L., (1995) Chemistry with CBL, Portland, OR; Vernier Software.
Glencoe (2000) Science Voyages, Westerville, OH; Glencoe/Mc Graw Hill.
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