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Teacher Information: Chemistry

Teacher Information: Chemistry. This section contains notes on activities, teacher version of the dissolved oxygen lab, answer keys, and resources used to create these lessons. CHEMISTRY WORKSHEET FOR SOLUTIONS – KEY 1. All are homogeneous solutions, except for the mushroom soup.

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Teacher Information: Chemistry

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  1. Teacher Information: Chemistry This section contains notes on activities, teacher version of the dissolved oxygen lab, answer keys, and resources used to create these lessons.

  2. CHEMISTRY WORKSHEET FOR SOLUTIONS – KEY 1. All are homogeneous solutions, except for the mushroom soup. 2. Carbon dioxide is not very soluble in water. (Think about a soda can. The CO2 must be forced into the solution, and escapes upon opening!) Ammonia, on the other hand, is a useful substance in solution (glass cleaner), and must therefore be more soluble, and stay in solution more easily than carbon dioxide. References will vary. 3. Because water is polar, and the oil-based enamel is not, the water will not dissolve and loosen the oil-based paint from the brush. 4. RULE OF THUMB: As temperature increases, solubility increases. EXCEPTIONS: HCl (g), NH3 (g), SO2 (g). 5. The additional solute would simply settle to the bottom of the solution. A saturated solution cannot “hold” any more solute unless the temperature is raised. 6. A supersaturated solution can be prepared by heating the solvent (usually to its boiling point), and dissolving as much solute as it will “hold”. Then, you allow the solution to cool SLOWLY, so that the solute does not precipitate out of the solution.

  3. WATER ACTIVITIES: INSTRUCTOR NOTES These activities are designed to be a supplement/replacement for the formal Dissolved Oxygen lab. If you feel your students are not up to the challenge of the titration, or if you are using this for middle school students, these activities may be enough (qualitative rather than quantitative). These activities may be a starting place, where your students can study the rubric, and write a formal lab report based on the rubric requirements. Use the rubric supplied for the dissolved oxygen lab.

  4. The water samples may be collected by your students, or you may collect them yourself. If you do not have access to a fresh water source that you believe will be suitable for these activities, you may prepare solutions for your students. If you choose to prepare the solutions, be sure to vary the concentrations of the variables you are testing for. Some suggestions are: • For dissolved oxygen: • 1. Heat a sample to drive off as much DO as possible. • 2. Cool and stir a sample to maximize DO. • 3. Use a sample of tap water, which is aerated as it comes out of the tap. Seal it in an air-tight container to minimize the amount of DO lost to the atmosphere. • For pH: • 1. Mix a solution with a small amount of 0.01 M HCl or other acid (acetic works well). • 2. Mix a solution with a small amount of bleach or window cleaner. • For chloride count: • 1. Mix a salt solution (the NaCl ionizes, releasing Cl- ions into the water). Vary the amount of salt so that there may be levels of below and above 250 ppm.

  5. The questions given are just suggestions, and you may choose to do any questions that you feel are relevant for your needs.

  6. WATER ACTIVITIES: ACTIVITY 1: Dissolved Oxygen (Qualitative) OBJECTIVE: To determine the dissolved oxygen content, an index of water quality, in a body of water. MATERIALS: Methylene blue indicator solution, clear bottles, test tubes, and measuring spoons. LEVEL: Any, but excellent for beginning science students (5-8). TIME: 1 hour for water collection.* 1 hour to test for dissolved oxygen. * Instructor may collect samples up to 48 hours before activity, and make samples available to students.

  7. PROCEDURE: 1. Collect water samples, or obtain them from your instructor. 2. Pour about 2 ounces in a test tube. 3. Add ½ ounce of methylene blue indicator solution. 4. In lab notebook (or other designated place), record the amount of time that it takes the solution to change from dark blue to very light blue or clear. The faster the color change, the less oxygen is present, and the more carbon dioxide and bacteria. 5. Repeat steps 2—4 for two additional samples.

  8. QUESTIONS: 1. What is the average time it took for the color change? 2. If your sample(s) contained high levels of CO2, what factors that we have discussed could account for this? 3. Write a two to three paragraph description of what you would look for in your sampling place that would tell you whether you could expect high levels of dissolved oxygen or high levels of CO2.

  9. ACTIVITY 2: pH count (measure of acidity and alkalinity of a solution). OBJECTIVES: To determine the pH of water. MATERIALS: pH paper** and color chart 3 water samples, collected or obtained from instructor Glass stir rod LEVEL: All TIME: 1 hour for collection (or obtain from instructor) 30 minutes for analysis

  10. PROCEDURE: • Obtain 3 water samples from different sources, or three different samples from your instructor. • 2. Place stir rod into your first sample, and put a drop of the water on a piece of pH paper. • 3.Compare the color of the pH paper to the color chart, and determine the pH of the sample. • 4. Record the results in your lab notebook, or other designated place. • 5. Repeat steps 2—3 for the remaining 2 samples.

  11. QUESTIONS: • Were the samples extremely acidic or alkaline? What could be a possible explanation for this? • 2. What does pH tell us about water? • 3. Are organisms able to live in water with pH values ranging from 0.0 to 14.0 or do they have specific requirements? • (Explore on the internet or in the text book to answer these questions) • ** Blue litmus (pH) paper turns red if the solution is acidic, and blue if basic. Red litmus (pH) paper turns blue if basic and red if acidic. Both will show no color change if substance tested is neutral.

  12. ACTIVITY 3: Chloride count OBJECTIVE: To test for high concentrations of chlorides which make water unsuitable for industrial, agricultural, and municipal use. MATERIALS: 3 water samples (collected or from instructor) Measuring cup (small, usually 10 – 20 mL) Large vial Silver nitrate solution (0.1 M) A combination of potassium chromate, sodium bicarbonate, and silica obtained from a water test kit. (C-2 powder) LEVEL: All TIME: 1 hour to collect samples (or obtain from instructor) 30 minutes for analysis

  13. PROCEDURE: 1. Fill the measuring cup to the 5 mL mark with water to be tested. 2. Empty the water into the vial. 3. Empty one pillow of C-2 powder (contains potassium chromate, sodium bicarbonate, and silica) into the water and shake gently to dissolve the powder. 4. Add silver nitrate drop-wise. Count the drops as you add them, and swirl the vial after each drop. 5. Continue to add drops until the water changes to orange. 6. Multiply the number of drops by 50 to get the chloride content in parts per million (ppm). If your sample has less than 250 ppm of chloride, the water may be safe to drink (DO NOT DRINK THESE SAMPLES!!!!!!!) 7. Repeat steps 1—6 for two more samples.

  14. ANALYSIS: Write 2 to 3 paragraphs discussing what conditions might lead to high chloride levels in water. Do we have these conditions in our area? Use whatever reference sources you feel necessary.

  15. DISSOLVED OXYGEN LAB: INSTRUCTOR VERSION

  16. DISSOLVED OXYGEN LAB: INSTRUCTOR VERSION INSTRUCTOR NOTES: If you do not have access to EXCEL, you may want to have your students manually graph their results. If you choose, you may have your students prepare the solutions needed for this lab. You may use stoichiometric equations to determine the amount of dissolved oxygen in your samples. You could use this lab as a separate activity from the rest of the module, and merely as an addition to your regular unit on water and its properties. I use it when I teach my students about solubility of gases. Answers to some of the questions will depend upon the conditions of your sampling area. The rubric in the Chemistry section is meant for this lab, but you may use your own grading method.

  17. INTRODUCTION: Dissolved oxygen (DO) is essential for the maintenance of healthy lakes and rivers. The presence of oxygen in water is a positive sign. Animals that live in an aquatic environment need oxygen to survive and plants need the carbon dioxide the animals produce, in other words a cyclic relationship. Some aquatic organisms, game fish like trout, need a medium to high level of dissolved oxygen to survive. Others, like carp and catfish, do very well in water with a low dissolved oxygen level. Dissolved oxygen in water comes mainly from atmospheric oxygen, which becomes mixed with the water due to wave action and tumbling of water in streams and rivers. Another source of dissolved oxygen in water comes from the algae, microorganisms both animal like and plant like, and aquatic plant life. When plants photosynthesize, they give off oxygen as a by-product. The level of dissolved oxygen can fluctuate from very low to very high on a daily basis. These fluctuations are normal in bodies of water with very extensive plant growth. The levels of DO rise from the morning through the afternoon, as the plants are undergoing photosynthesis. The DO levels reach a peak in late afternoon. As night falls, the DO levels begin to decrease, as the plants discontinue photosynthesis. The plants and other aquatic organisms are still respiring, however, and continue to use up the DO in the water. The lowest level of DO occurs just before dawn. All of these factors, as well as the temperature of the water, the movement of the water, buildup of organic waste, sewage, etc., must be taken into consideration when sampling water for testing of DO levels.

  18. INTRODUCTION, CONTINUED In this lab, you will take numerous samples of fresh water to test for dissolved oxygen, using the Winkler method. You will then be asked to speculate on the causes of both high and low levels of DO. Ultimately, you will use EXCEL to analyze and report your data. • MATERIALS: (per group)* • River or stream water, several 500 mL samples. • Distilled water, several 500 mL samples. • Biological Oxygen Demand Bottles, 300 mL, 5. • Manganese sulfate solution, 6 mL • Alkaline iodine solution, 6 mL • Sodium thiosulfate working solution, 60 mL • Sulfuric acid, concentrated**, 6 mL • Starch solution, 6 mL • Thermometer, 1 • Burette, 1 • 100-mL graduated cylinder, 1 • 500-mL Erlenmeyer flask, 1 *Amounts listed include enough materials for several practice titrations. ** Students should not perform steps involving H2SO4. The teacher should perform all manipulations of the concentrated sulfuric acid wearing gloves and eye protection.

  19. PREPARATION OF SOLUTIONS Manganese sulfate solution Dissolve 480 g MnSO4 4H2O; 400 g MnSO4 2H2O; or 364 g MnSO4 H2O in distilled water, filter, and dilute to 1 liter. Alkaline iodide solution Dissolve 500 g NaOH (or 700 g KOH) and 135 g NaI (or 150 g KI) in distilled water and dilute to 1 liter. (CAUTION: This will get very hot) Sodium thiosulfate stock solution Dissolve 24.82 g Na2S2O3 5H2O (15.8 g anhydrous) in boiled and cooled distilled water and dilute to 100 mL. Keep refrigerated. Sodium thiosulfate working solution Just prior to titration, prepare by diluting 250 mL of stock sodium thiosulfate solution to 1000 mL. Starch solution Dissolve 5 grams in 100 mL gently boiling distilled water. Dissolve the starch bit by bit and stir.

  20. PROCEDURE 1. Obtain at least 3 samples of water from a river or stream near you. If you do not plan to sample and analyze on the same day, take the temperature of the samples IMMEDIATELY, and record the temperature, along with the sample number. Try to test the samples at the same temperature that you recorded. You may lower temperature by using an ice bath, and raise temperature by using a hot-water bath. NOTE: This will alter the results of your experiment, but, you will still obtain usable results. When heating or cooling water, stir with a magnetic stirrer, in order to simulate the movement of the water in the river 2. Carefully siphon water from the sample into a BOD (biological oxygen demand) bottle. Be sure to place the siphon at the bottom of the BOD bottle and fill the bottle until it overflows by approximately one-third of its volume. (Now you know why you got more than 1 sample!) Place the stopper in the bottle and turn it upside down to remove all of the excess water in the well around the stopper.

  21. PROCEDURE, CONTINUED 3. Remove the stopper and pipette 2 mL of manganous sulfate into the sample. Be sure to insert the tip of the pipette below the surface of the sample. 4. Pipette 2 mL of alkaline iodide into the same sample. Again, be sure to insert the tip of the pipette below the surface of the sample. 5. Stopper the BOD bottle and shake the sample by inverting the bottle several times. 6.Allow the precipitate to settle until it occupies approximately one-half of the bottle.

  22. PROCEDURE, CONTINUED 7. While waiting for the precipitate to settle, fill a burette with standardized sodium thiosulfate working solution and obtain a bottle of starch solution. 8. Remove the stopper from the BOD bottle, and have your instructor carefully pipette 2 mL of the concentrated sulfuric acid (H2SO4) into the sample. 9. Stopper the BOD bottle and shake the sample by inverting the bottle several times. The precipitate will dissolve and the sample will turn a clear yellow-gold as free I2 is formed.

  23. PROCEDURE, CONTINUED 10. Using a graduated cylinder, remove a 200-mL sample and pour it into a 500-mL Erlenmeyer flask. 11. Titrate this sample with the thiosulfate solution in your burette until a pale straw color is reached. Remember to continually swirl the solution in your Erlenmeyer flask during the titration process. Also, set the flask on a sheet of white paper to enhance the color change. 12. To help you accurately identify the endpoint of this titration, introduce a dropper-full of starch solution to the straw-colored sample. The sample should turn a purple color.

  24. PROCEDURE, CONTINUED 13. Continue to titrate drop by drop, until the purple color disappears. At this endpoint in you titration, all free iodine has been converted to sodium iodide by the addition of sodium thiosulfate. The volume of sodium thiosulfate (in mL) used to titrate your 200-mL sample is approximately equivalent to the concentration of dissolved oxygen (mg/L) in your original sample. Convert mg/L to mL/L using the information on the graph below.

  25. PROCEDURE, CONTINUED 14. Repeat steps 2 – 13 for three more samples, one at 4oC, one at room temperature, and one at 30oC. You will need to look at the room thermometer to determine the room temperature in degrees Celsius. 15. Repeat steps 2 – 13 for three samples of distilled water at 4oC, room temperature, and 30oC.

  26. ANALYSIS GRAPH Use the line to find % saturation by setting the top ‘dot’ on the water temperature, and the bottom ‘dot’ on dissolved oxygen content. Read % saturation where the line crosses the % saturation bar of the graph. Be sure to set the ‘dots’ directly on the line!

  27. QUESTIONS • What factors were present in your sampling place that would affect the amount of dissolved oxygen in your fresh water samples? (See introduction for examples) • 2. Which of the factors listed in #1 are humans able to control? Should they be controlled? Explain your answer. • How did temperature affect the amount of dissolved oxygen in your samples? • 3.At which temperature was the amount of dissolved oxygen the greatest? The lowest? Explain your answers. • 4.How does the amount of dissolved oxygen for fresh water differ from the amount of dissolved oxygen in distilled water? Explain your answer.

  28. New Mexico Content Standards and Benchmarks: • Interpret evidence to understand changes in natural and artificial systems (2.A) • Discriminate between the effects of constancy and change as properties of objects and processes (4.A) • Determine and use the appropriate type of device to measure objects in a given problem of situation (4.C) • Apply the scientific method in daily life both within and outside the school environment (5.A) • Evaluate, design, and use the most appropriate equipment, tools, techniques, and information sources to improve scientific investigations and solutions to problems (5.B)

  29. New Mexico Content Standards and Benchmarks, cont’d: • Apply explanations to their data that abide by rules of evidence, can be questioned and modified, and are based on historical and current scientific knowledge (6.B) • Use evidence to understand data and to develop consistent arguments to logically explain data (6.D) • Explain and interpret the results of investigations to teachers, peers, parents, and others (6.E) • Design and construct artificial or computer systems that simulate natural systems (14.A) • Evaluate human activities for the potential they have for increasing or decreasing environmental risks (16.B) • Develop models for tracking changes in social risk factors and natural hazards (16.C)

  30. NATIONAL SCIENCE EDUCATION STANDARDS: Content Standard B.3: Use technology and mathematics to improve investigations and communications. A variety of technologies, such as hand tools, measuring instruments, and calculators, should be an integral component of scientific investigations. The use of computers for the collection, analysis, and display of data is also a part of this standard. Mathematics plays an essential role in all aspects of an inquiry. For example, measurement is used for posing questions, formulas are used for developing explanations, and charts and graphs are used for communicating results.

  31. NATIONAL SCIENCE EDUCATION STANDARDS: Content Standard B.4:Communicate and defend a scientific argument. Students in school science programs should develop the abilities associated with accurate and effective communication. These include writing and following procedures, expressing concepts, reviewing information, summarizing data, using language appropriately, developing diagrams and charts, explaining statistical analysis, speaking clearly and logically, constructing a reasoned argument, and responding appropriately to critical comments.

  32. NATIONAL SCIENCE EDUCATION STANDARDS: Content Standard E.4: Propose designs and choose between alternative solutions. Students should demonstrate thoughtful planning for a piece of technology or technique. Students should be introduced to the roles of models and simulations in these processes. Content Standard E.5: Evaluate the solution and its consequences. Students should test any solution against the needs and criteria it was designed to meet. At this stage, new criteria not originally considered may be reviewed.

  33. CHEMISTRY RESOURCES Ebbing, Darrell D.; Wentworth, R.A.D., Introductory Chemistry, 2nd Edition, 1998, Houghton Mifflin Company, Boston, New York. Heron, J. Dudley, Ph.D; Sarquis, Jerry L, Ph.D; Schrader, Clifford L., Ph.D; Frank, David V., Ph.D; Sarquis, Mickey, M.S.; Kukla, David A., M.Ed, Heath Chemistry, 1993, D.C. Heath and Company, Lexington, Massachusetts/Toronto, Ontario. http://www.ncpublicschools.org/curriculum/science/Chemistry/solubil.htm

  34. OUR THANKS TO: Ray and Gayle Elliot Richard Braatz, Ph.D, UIUC Garrett Love, Ph.D, SHODOR Scott Lathrop, Ph.D, UIUC Angel, Susan Rhonda, Roxanne And all of the other SI2002 Mentors!

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