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Hypothesis testing

  • Page ID
    405987
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    Safety and hazardous waste

    At the beginning of this lab, you will observe an experiment (a "magic trick") without knowing the substances it contains. This experiment will change every year, with different safety considerations. Over the semester, we have handled acids and bases, flammable substances, toxic substances, gases, high and low temperatures, etc. Observe all precautions you observed for previous labs. If there are any special considerations, your professor will announce them at the beginning of the lab. Do not discard any of the solutions until you have received instructions from your professor.

    Lab organization

    We will explore a "magic trick" that is based on chemical reactions. You can use all of the concepts you learned in GenChem1 and GenChem2 to try to explain what is going on. You will work in three large groups which you can organize as you wish. Your group will present twice during the lab, first announcing your initial hypothesis and observations supporting or rejecting it (about 1 hour into the lab). You will again report out at the end of the lab, presenting your updated hypothesis. Individually, your job is to write a short report stating the hypothesis. This year, we will look at the blue bottle experiment, where a blue color appears (the dye methylene blue) and then disappears again.

    Part 1

    Your group will receive 25 mL of the magic solution in a screw-cap 50 mL vial. The solution contains 0.5 mol/L sodium hydroxide, a strong base you should only handle while wearing goggles. To start the trick, you shake the closed vial. Before you do so, make sure everyone is wearing their goggles, and the lid is screwed on finger-tight. After you tried the magic trick, consider the following questions:

    1. What type of reactions might the "bluing" and "debluing" reactions be? We learned about three types of reactions in GenChem1: Acid/base, redox, and precipitation. What kind of experiment/observation could help you decide?
    2. What role could the shaking step have in starting the reaction? Could you do something other than shaking to get the same effect? What kind of experiment/observation could help you decide?

    You will have various equipment available (pH meter, pH paper, capped vials with a size between 1 mL and 50 mL, a magnetic stirrer and stir bar, a vortex mixer). Design experiments you can do with the given 25 mL of magic solution. Discuss the experiments with your instructor to make sure you are aware of all the safety considerations. Once you get the go-ahead, do the experiments and record your observations.

    Once you are done, prepare a short presentation where you share the experiments you did and your conclusions from them with the entire class. This is a "chalk talk", so you just talk and use the white board, if necessary.

    Part 2

    For this part, you will have the reagents available separately as three solutions A, B, and C. Solution A contains the strong base. This allows you to leave out reagents or change their concentration. For this part, there are three questions you should address:

    1. Which reactants are necessary for debluing, and which are necessary for bluing?
    2. Choose one of the following  
      a. How does lowering the concentration of components in solution A added change things? Is the content of B a reactant, a catalyst, or neither?
      b. How does lowering the concentration of components in solution B added change things? Is the content of B a reactant, a catalyst, or neither?
      c. How does length of shaking/mixing change things? Does the shaking add a reactant, an catalyst, or neither?
    3.  What is your model of the bluing and of the debluing reaction? Which reagents do they require?
    4.  How fast is the debluing reaction compared to the bluing reaction?

    First, repeat the magic trick, using the three solutions (mix 8 parts of A with 1 part of B and 1 part of C). Then decide on a set of experiments to address the questions. Discuss the experiments with your instructor to make sure you are aware of all the safety considerations. Once you get the go-ahead, do the experiments and record your observations.

    Once you are done, prepare a short presentation where you share the experiments you did and your conclusions from them with the entire class. This is a "chalk talk", so you just talk and use the white board, if necessary.

    Part 3

    You can do the same magic trick by mixing solution A (the strong base) with blue sports drink. Try mixing 15 mL of the blue sports drink with 5 mL of 2 mol/L NaOH in a 50 mL vial (under the hood), cap the vial and swirl to see what happens to the color. Then, shake and observe. What ingredients in the sports drink correspond to the ingredients in solution B and C, making this variation of the trick possible?

    Asking for reagents, supplies and equipment

    In your exploration, you might suggest an experiment for which you need certain materials not available to you. If there is something you have used in previous experiments (thermometer, pipette pump, etc), your instructor might make it available to you. You can also ask for anything else, but don't be disappointed if your request is declined. All experimenters, even in highly funded research labs, will have real world limitations (too expensive, too dangerous, tool has not been invented yet, limited amount of sample) determining which experiments they can and which they can't pursue.

    Cleanup

    Discard reaction mixes in the container provided under the hood. As you rinse the tubes, remember that you are still handling 0.5 mol/L NaOH. This is a strong base, and you have to protect your eyes even while cleaning up. As a mental exercise while rinsing, try to estimate the pH of the reaction mix without a calculator (hint: the concentration is in between 0.1 mol/L and 1 mol/L).


    Hypothesis testing is shared under a not declared license and was authored, remixed, and/or curated by LibreTexts.

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