Illustrated Guide to Home Chemistry Experiments

312 DIY Science: Illustrated Guide to Home Chemistry Experiments

iodine solution. Stopper and shake test tube #1 to mix the

contents thoroughly.

4. Use a measuring pipette or Beral pipette and 10 mL
   graduated cylinder to transfer 4.5 mL (half the contents)
   from tube #1 to tube #2. Add 4.5 mL of water to tube #2
   and mix thoroughly.


5. Transfer 4.5 mL (half the contents) from tube #2 to tube
   #3. Add 4.5 mL of water to tube #3 and mix thoroughly.


6. Repeat this procedure to create reference comparison
   samples in tubes #4, #5, and #6. When you finish,
   you have six test tubes, each of which has half the
   concentration of iodine that is in the preceding tube. Tube
   #1 should be an intense orange color, and tube #6 a pale
   yellow-orange.

Test tube #1 contains the same concentration of iodine that
will be the starting point in the reactions in Part II. If we define
the concentration of iodine in test tube #1 as 100%, test tubes
#2 through #6 contain 50%, 25%, 12.5%, 6.25%, and 3.125%,
respectively. Place these test tubes in one of the racks. You’ll use
them later to estimate how far the reaction has proceeded in each
of the test samples.

FIGURE 17-1:

Reference samples with various concentrations of iodine

PII: RTA ETERd mINE THE EffECTS of
dIffERENT LIGHT SoURCES oN IodINE/oxALATE
SoLUTIoN SAmpLES

1. If you have not already done so, put on your splash
   goggles, gloves, and protective clothing.


2. Place a weighing paper on the balance pan and tare the
   balance to read 0.00 g. Add oxalic acid crystals until the
   balance indicates about 2.5 g. (The exact amount is not
   critical.) If you do not have a balance, use 3/4 tsp. of
   oxalic acid crystals.
   3. Transfer the oxalic acid crystals to the small beaker
   and add 25 mL of water, measured with your graduated
   cylinder. The solubility of pure oxalic acid in water is
   about 120 g/L at room temperature, so this solution is
   nearly saturated. You may need to stir the contents of
   the beaker to dissolve all of the crystals. If some crystals
   remain undissolved, you can decant the clear solution
   off to another container or warm the solution slightly to
   dissolve the few remaining crystals.
   4. Once the oxalic acid crystals have dissolved, add 25 mL
   of clear household ammonia to the beaker and stir to mix
   the solutions. The oxalic acid and aqueous ammonia react
   to form a solution of ammonium oxalate.
   5. Label six test tubes A through F, and transfer 4 mL of
   the ammonium oxalate solution to each of the six test
   tubes. (The exact amount is not critical, but keep the
   level in each of the test tubes the same.) Using 4 mL per
   test tube leaves you with about half of the ammonium
   oxalate solution unused, in case you need to repeat the
   experiment or collect additional data under different
   conditions.
   6. Wrap test tube A in aluminum foil, leaving an open flap
   of foil at the top of the test tube, through which you will
   introduce the iodine solution. The goal is to prevent the
   solution in this test tube from being exposed to any light at
   all. The contents of this test tube will serve as the control.
   7. Use the dropper or Beral pipette to transfer 10 drops of
   the iodine solution to test tube A, and immediately close
   the foil flap to prevent the contents of that test tube from
   being exposed to light. If necessary, put the test tube
   in a closed drawer or closet to protect it from exposure
   to light. (The reaction between ammonium oxalate and
   iodine produces carbon dioxide gas, so do not seal any of
   the test tubes with a cork or stopper.)
   8. Working as quickly as possible, add 10 drops of the iodine
   solution to each of the remaining five test tubes. Expose
   these five test tubes to different light sources, as follows:

• Expose test tube B to direct sunlight.


• Expose test tube C to open shade (under an open sky, but
  not in direct sunlight).


• Expose test tube D to the ambient light in your working area.


• Expose test tube E as close as possible to a fluorescent light.


• Expose test tube F close to a strong incandescent light.
  (but not close enough to be heated significantly by it).
  9. As you expose each test tube to light, record the
  beginning time. Depending upon the intensity and type of
  light to which the test tube is exposed, the reaction may
  take from a few minutes to many hours to proceed to
  completion.
  Examine each of the test tubes after 15 minutes, 30
  minutes, one hour, two hours, and four hours of exposure.

10.