Conducting Solutions. Evaluation copy

Transcription

1 Conducting Solutions Computer 5 In this experiment, you will study the electrical conductivity of water and various water solutions. A solution can contain molecules, ions, or both. Some substances, such as sucrose (C 12 H 22 O 11 ) and glucose (C 6 H 12 O 6 ), dissolve to give a solution containing mostly molecules. An equation representing the dissolving of sucrose (table sugar) in water is: C 12 H 22 O 11 (s) C 12 H 22 O 11 (aq) where (s) refers to a solid substance and (aq) refers to a substance dissolved in water. Other substances, such as calcium chloride (CaCl 2 ), dissolve in water to produce a solution containing mostly ions. An equation is: CaCl 2 (s) Ca 2 + (aq) + 2 Cl (aq) Calcium ions are necessary for muscle contraction, mitochondrial activity, bone formation, and many other metabolic processes. Organisms may obtain minerals such as calcium from their water supply, since ions dissolve in water. You will determine conductivity of the solutions using a computer-interfaced Conductivity Probe. The unit of conductivity in this experiment is microsiemens per centimeter, or µs/cm. OBJECTIVES In this experiment, you will Write equations for the dissolving of substances in water. Use a Conductivity Probe to test the electrical conductivity of solutions. Determine whether molecules or ions are responsible for electrical conductivity of solutions. Evaluation copy Figure 1 Biology with Vernier 5-1

2 Computer 5 MATERIALS computer Vernier computer interface Vernier Conductivity Probe Logger Pro sodium chloride, NaCl, solution calcium chloride, CaCl 2, solution aluminum chloride, AlCl 3, solution ring stand ethanol, C 2 H 6 O, solution sucrose, C 12 H 22 O 11, solution glucose, C 6 H 12 O 6, solution stream or lake water ocean water (optional) various foods in solution distilled water utility clamp PRE-LAB EXERCISES Many of the materials you will be using today are found in common household items. A list of common names or uses can be found below: Sodium chloride, NaCl Calcium chloride, CaCl 2 Acetic acid, CH 3 COOH Ethanol, C 2 H 6 O Fructose, C 6 H 12 O 6 Sucrose, C 12 H 22 O 11 Glucose, C 6 H 12 O 6 Common household salt Used to pickle cucumbers, or to help concrete cure in cold weather Vinegar Found in gasoline or in alcoholic beverages. Usually obtained from yeast fermentation Fruit sugar Table sugar, beet or cane sugar Corn or blood sugar 1. An equation representing the dissolving of sucrose in water is: C 12 H 22 O 11 (s) C 12 H 22 O 11 (aq) Like solid sucrose, the substances glucose, C 6 H 12 O 6 (s), and ethanol, C 2 H 6 O(l), dissolve in water to yield solutions containing mostly molecules. Write equations showing the dissolving of these two liquids in water in Table An equation showing the dissolving of CaCl 2 in water is: CaCl 2 (s) Ca 2+ (aq) + 2 Cl (aq) Like CaCl 2, the substances NaCl and AlCl 3 dissolve in water to give solutions containing mostly ions. Write equations showing the dissolving of these two solids in water in Table 2. PROCEDURE 1. Obtain and wear goggles. 2. Secure the Conductivity Probe with the ring stand and utility clamp as shown in Figure Connect the Conductivity Probe to the computer interface. Check to be sure the Conductivity Probe is set to 0 20,000 µs/cm. 4. Prepare the computer for data collection by opening the file 05 Conducting Solutions from the Biology with Vernier folder of Logger Pro. 5-2 Biology with Vernier

3 Conducting Solutions 5. Test the conductivity of each solution listed in the data table. You can do the tests in any sequence. a. Carefully raise each vial 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. b. Briefly swirl the beaker contents. Once the conductivity reading in the meter has stabilized, record the value in Table 3. c. Before testing the next solution, clean the electrodes by surrounding them with a 250 ml beaker and rinse them with distilled water from a wash bottle. Blot the outside of the probe end dry using a tissue. It is not necessary to dry the inside of the hole near the probe end. DATA Table 1 C 6 H 12 O 6 (s) C 2 H 6 O(l) Table 2 NaCl(s) AlCl 3 (s) Table 3 Solution Material Conductivity (µs/cm) 1 Distilled water 2 Sodium chloride, NaCl 3 Calcium chloride, CaCl 2 4 Aluminum chloride, AlCl 3 5 Ethanol, C 2 H 6 O 6 Sucrose, C 12 H 22 O 11 7 Glucose, C 6 H 12 O 6 8 Tap water 9 Stream water 10 Ocean water Biology with Vernier 5-3

4 Computer 5 QUESTIONS 1. Which solutions conduct electricity best, those containing mostly ions or those containing mostly molecules? 2. Does distilled water conduct electricity well? Explain. 3. Does tap water conduct electricity? Account for this observation. 4. Consider the conductivity readings for the NaCl, CaCl 2, and AlCl 3 solutions. What trend do you observe? Account for this trend. 5. How does the conductivity of ocean water compare to pond or stream water? How can you account for this? 6. Which foods in solution conducted electricity well? How can you account for this? 7. Suggest three other substances whose water solutions would conduct electricity well. Explain how you decided on your choices. EXTENSIONS 1. Test your predictions for Question 7 above. 5-4 Biology with Vernier

5 Vernier Lab Safety Instructions Disclaimer THIS IS AN EVALUATION COPY OF THE VERNIER STUDENT LAB. This copy does not include: Safety information Essential instructor background information Directions for preparing solutions Important tips for successfully doing these labs The complete Biology with Vernier lab manual includes 31 labs and essential teacher information. The full lab book is available for purchase at: Vernier Software & Technology S.W. Millikan Way Beaverton, OR Toll Free (888) (503) FAX (503)