Audio Conductivity Tester

Introduction

Most students know that touching “live” electrical components with wet hands is dangerous. Therefore they may assume that all water conducts electricity. By using a simple conductivity tester that produces an audible sound, these demonstrations will help students “come to their senses” and understand what is needed to produce an electric current.

Concepts

• Electrical conductivity
• Electrolytes versus non-electrolytes
• Ions

Materials

Safety Precautions

Copper(II) sulfate is a skin and respiratory irritant and is moderately toxic by ingestion and inhalation. Household ammonia is slightly toxic by ingestion and inhalation; both the liquid and vapor are very irritating, especially to the eyes. A 9-V battery has a low current and is considered safe. Do not use household current. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory. Follow all laboratory safety guidelines. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. All waste solutions used in the demonstrations may be disposed of down the drain with a large excess of water according to Flinn Suggested Disposal Method #26b.

Prelab Preparation

Audio Conductivity Tester Assembly

1. Obtain the piezo buzzer, three Fahnestock clips, 16-gauge copper wire, a battery clip and a 9-V battery.
2. Using a wire cutter, cut two 4-cm pieces of the 16-gauge copper wire for the electrodes.
3. Using wire strippers, remove about 1 cm of insulation from the end of each wire lead of the buzzer and the battery clip.
4. Using one Fahnestock clip, connect the red (+) wire from the battery clip to the red (+) wire of the buzzer (see Figure 1).
5. Using another Fahnestock clip, connect the black wire of the battery clip to one copper electrode.
6. Using the third Fahnestock clip, connect the black wire of the buzzer to the second copper electrode (see Figure 1).
7. Connect the battery clip to the battery.
8. Briefly touch the two electrodes together. A loud beep should sound. If no sound is heard, check the connections and try again. Make sure the battery works. Caution: The battery, wires and electrodes will get hot with prolonged use. Unclip the battery when not in use.

Procedure

Demonstration A. Tap, Distilled and Salt Water

1. Place the bottom halves of two Petri dishes on an overhead projector stage.
2. Add enough distilled water to one dish to just cover the bottom.
3. Repeat step 2 with tap water in a second dish.

4. Place the audio conductivity tester on the projector stage and connect the battery.
5. Turn on the projector.
6. Tell the students to listen carefully as the two electrodes are inserted into the dish with tap water. Depending on the water in your area, a faint to moderate sound may be heard.
7. Rinse the electrodes with distilled water (a washing bottle and a beaker work well) and pat dry with a paper towel.
8. Insert the two electrodes into the dish with distilled water. No sound should emit from the buzzer.
9. Remove the electrodes and add a few crystals of sodium chloride (about 0.1–0.2 g) in the center of the dish of distilled water. Do not stir the water (see Figure 2).
10. Place the electrodes about 1 cm apart with the salt crystals in between. The buzzer will sound.
11. Remove the electrodes from the dish and disconnect the battery.
12. Rinse the electrodes with distilled water and pat dry with a paper towel.

Demonstration B. Concentration of Electrolyte

1. Place the bottom half of a clean Petri dish on an overhead projector stage.
2. Pour in enough distilled water to just cover the bottom of the dish.
3. Place the audio conductivity tester on the projector stage and connect the battery.
4. Turn on the projector.
5. Add a few crystals of copper(II) sulfate (about 0.1–0.2 g) to the center of the dish.
6. Add a few drops of household ammonia directly on top of the CuSO₄ crystals. The ammonia intensifies the color of the CuSO₄ solution, making it more visible on the projector screen. Note: If the CuSO₄ solution is difficult to see on the screen, dim or turn off the classroom lights.
7. Place the electrodes about 2 cm apart with the CuSO₄ crystals in between. Do not disturb the water.
8. Students will observe a faint outline of the copper sulfate solution around the crystals. This will become darker as more copper sulfate dissolves (see Figure 3). As the copper sulfate solution spreads out toward the electrodes, the buzzer will sound faintly and then gradually get louder as the concentration of ions increases.
9. Remove the electrodes from the dish and disconnect the battery.
10. Rinse the electrodes with distilled water and pat dry with a paper towel.

Demonstration C. Acids and Bases

1. Place the tops of three clean Petri dishes on an overhead projector stage.
2. Place the audio conductivity tester on the projector stage and connect the battery.
3. Turn on the projector.
4. Obtain the dropping bottle of ammonia solution. Add 10–15 drops of ammonia to one dish, making a small puddle.
5. Tell the students to listen carefully as you insert the two electrodes into the ammonia. Note the volume of sound produced.
6. Rinse the electrodes with distilled water and pat dry with a paper towel.
7. Obtain the dropping bottle of vinegar. Add 10–15 drops of vinegar to a second dish.
8. Repeat steps 5 and 6 with the vinegar.
9. Add 10 drops of vinegar to the third dish.
10. Add 10 drops of ammonia to the puddle of vinegar in the same dish.
11. Repeat steps 5 and 6 with the combined vinegar-ammonia solution. A much louder and higher pitched sound will be produced.
12. Reinsert the electrodes into each solution if necessary to reinforce the differences in the buzzer tone for each solution. Be sure to rinse the electrodes between solutions.
13. Remove the electrodes from the dish and disconnect the battery.
14. Rinse the electrodes with distilled water and pat dry with a paper towel.

Student Worksheet PDF

12743_Student1.pdf

Teacher Tips

• This kit contains enough materials to perform the demonstration seven times: 15 mL of household ammonia, 10 mL of vinegar, 2 g of copper(II) sulfate, 2 g of sodium chloride and 3 disposable Petri dishes. Enough materials to make one reusable audio conductivity tester are included: 1 piezo buzzer, 1 battery clip, 6" of 16-gauge copper wire and 3 Fahnestock clips.
• The thin wires connected to the copper electrodes may slip out of the Fahnestock clips due to their size difference. Place a small piece of electrical tape over the clips to prevent this.
• The audio conductivity tester may also be used to test whether various materials are conductors or insulators. Gather an assortment of each (e.g., coins and other metal objects, plastic items, wood pencils, pencil leads, sulfur) and have students make predictions before testing.
• Make this activity inquiry-based by setting up the materials for Demonstration A before students come into the lab. Conduct the demonstration without identifying the different solutions (do not hand out the worksheet yet). Engage the students in a lively discussion to explain how the different effects could be produced.
• Show the difference between ionic compounds and molecules by comparing salt and sugar solutions with the conductivity tester. The sucrose molecules will not dissociate in water, therefore sugar is not an electrolyte. Be sure to use deionized water, not tap water.
• For an earth science application, test the conductivity of various rocks and minerals, one characteristic used in their classification. Those with high metal content will conduct an electric current.
• If a class has a hearing-impaired student, a light emitting diode (LED) and a 1 K-ohm resistor may be added to the circuit.
• Even though the human body is a conductor, skin is not a good conductor unless wet. Demonstrate this by firmly holding the copper electrodes in each hand between a dry thumb and forefinger, then repeat with wet fingers. A faint sound should be heard when the fingers are wet. This is safe with the low voltage supplied by a 9-V battery. Do not try this with household current!
• The copper electrodes will last through many demonstrations. Additional 16-gauge wire is available from Flinn Scientific, Catalog No. C0146.
• Be sure to disconnect the battery when the tester is not in use to avoid overheating and draining the battery as well as to avoid inadvertent sounds!

Sample Data

Observations

Demonstration A. Tap, Distilled and Salt Water
Describe what happened when the electrodes were placed in each solution, respectively.

When the electrodes were placed in the tap water, the buzzer sounded; in distilled water, no sound was emitted; and in salt water, a louder sound was heard than with tap water.

A very faint sound was heard at first. As more copper sulfate dissolved, the solution spread out closer to the electrodes and the sound gradually became louder.

The tester emitted a faint sound when the electrodes were placed in the vinegar and a slightly louder sound when placed in the ammonia solution.

Answers to Questions

1. Based on your observations in Demonstration A, does pure water conduct electricity? What evidence led you to this conclusion?
   

   No, pure water does not conduct electricity. No sound was emitted from the tester when the electrodes were placed in distilled water.

2. What effect did adding sodium chloride to the distilled water have on the conductivity of the water? Explain.
   

   The conductivity of the water increased with the addition of sodium chloride. The salt dissolved in the water, separating into ions that carried the electric charges across the electrodes, completing the circuit.

3. In Demonstration B, what happened as more copper(II) sulfate dissolved in the solution?
   

   As more and more copper(II) sulfate dissolved in the solution, the sound emitted from the tester increased in volume and pitch. The more ions in solution, the greater the conductivity.

4. In Demonstration C, how did the conductivity of the combined vinegar-ammonia solution compare to each separate solution? What can you infer about the concentration of ions present in each solution?
   

   With the combined vinegar-ammonia solution the sound produced by the tester was much louder; therefore this solution had greater conductivity than the two separate solutions. The combined solutions had the greatest concentration of ions, followed by ammonia; vinegar had the least concentration of ions.

5. Based on your observations, explain what is needed to conduct an electric current in a circuit.
   

   In order for electricity to be conducted in a circuit, a voltage source is needed (provided by the battery) and an unbroken flow of charge through one or more conductors.

Discussion

Substances that conduct an electric current when dissolved or melted are called electrolytes. Electrolytes contain moveable free ions (charged atoms or molecules). The moveable ions carry the electric charge through the solution thus creating an electric current. Salt water is an electrolyte because table salt is an ionic compound that dissociates (breaks up into simpler components) in water, forming positively charged sodium ions and negatively charged chloride ions. Other electrolytes include other salt solutions, tap water, and acids and bases such as vinegar and ammonia, respectively. Solutions that do not contain moveable free ions and therefore cannot conduct an electric current are called non-electrolytes (e.g., alcohol, distilled water, sugar water). Sugar is a molecule formed by covalent bonding and does not dissociate in water.

The audio conductivity tester is an open circuit with a battery connected to a high-pitched buzzer. When an electrical conductor comes in contact with both electrodes, the circuit is completed and a sound is emitted. The piezo buzzer is very sensitive to the flow of electrons in a current. In any electric circuit, substances vary in conductivity. Good conductors will produce a high frequency tone, and substances with a lower conductivity will produce a low frequency tone. As the concentration of ions increases in a solution, the current also increases, thus increasing the pitch and volume of the buzzer tone.

Acids and bases also vary in conductivity. Some acids and bases dissociate (ionize) completely when dissolved in water. Other weaker acids and bases may ionize only partially when dissolved in water and are weak electrolytes. Vinegar and household ammonia are considered weak electrolytes. The neutralization reaction of vinegar and ammonia produces more ions, increasing the conductivity of the solution.

References

Special thanks to Bob Becker, Kirkwood High School, Kirkwood, MO, for providing the idea for this activity to Flinn Scientific. Conducting Solutions, http://scifun.chem.wisc.edu/homeexpts/CondTester/SolutionConductivity.htm (Accessed April 2008).