VC000

An Experimental Tour: A Collaborative Investigation

J.Chem.Ed.  Vol. 76 No. 9 September 1999, Revised May 17, 2000

 

The Case

 

Susan was cleaning out her kitchen cabinets when she found several interesting samples.

·        In her laundry cabinet, she found an old shirt that she then washed only to find that an ink pen had been in its pocket. Though the pen had black ink, the shirt pocket now had several colors of stain.

·        Under her sink, in the cleaning supplies, a bottle of household ammonia had been stored near a bottle of muratic acid whose cap had not been appropriately tightened. The outside surfaces of several containers in their vicinity had become coated with a white powdery substance. She was afraid to touch it, so she put on gloves and discarded the containers and their contents.

·        In her pantry, she found a box with unknown con­tents. Her children had been playing "house" and had destroyed the label sufficiently to make it unreadable. It contained a white crystalline powder that was odorless. She knew it was either powdered sugar or cornstarch. Not being certain, she discarded it, as well.

After working for a few hours, Susan decided to relax with a can of soda. She usually drank diet soda, but she only had regular soda in stock so she settled for it instead.

While reflecting on her days activities, she pondered whether she should be storing all the cleaning agents together under the sink. Where had that white powder come from? Thinking of white powders, she considered the unlabeled box. Her imagination started churning out scenarios whereby its content was neither sugar nor corn starch, but some potentially toxic substance! It scared her to think that her children had been playing with it and she really wasn't 100% certain that it was safe. She wondered how she could have deciphered its contents. She wondered how the shirt had become stained with so many colors when only black ink had been in the pen. Were there other pens in the pocket that got lost in the machine and would show up on the next washday? She wondered about diet and regular soda, how much sugar was she drinking in that 12‑oz. can? Is the sugar the reason that her mind was now racing with all these questions? She decided her "rest" period had not turned out to be that restful, and went back to work, resolved to be more careful about storage of household materials and access to them.

 

Experiments and Questions

 

Be sure to wear your safety glasses and follow the guidelines for caution at each experimental station. All groups do Experiment 1 either first or second, then rotate through the remaining experiments, and return to finish Experiment 1.

 

Experiment 1

 

I.     Experimental

1.      Obtain two TLC strips (e.g., Eastman Kodak TLC silica‑coated film) that measure about 2.5 X 10 cm each.

2.       Spot water‑soluble* inks on a pencil line about I cm from the bottom of two strips. The spots should be small, about 2 mm in diameter.

3.      Set the TLC strips into a medium‑sized beaker (approximately 200 mL) containing about 10 mL of solvent," making certain that the sampling line is above the level of the liquid solvent. Cover the beaker with aluminum foil.

4.       While waiting for the solvent front to move to within 1.0 cm from the top, and move on to experiments 2-4, but periodically (about every 20 minutes) check the progress of the solvent front.

5.      Remove the plate when the solvent has ascended to within about 1.0 cm of the top of the plate, remove the plate from the beaker and mark the upper limit of the solvent front with a pencil. Allow the plate to dry and then circle each of the spots.

6.      Calculate the Rf value for each spot. A sample is shown below to illustrate the measurements.

 

 Rf = sample migration distance/solvent migration distance.

 

7.      Compare Rf values of comparable spots.

 

 

II.   Included in your experiments are examples that illustrate the following chemical concepts. Use your observations of your experiment to:

 

1.      Differentiate between solutions, mixtures and compounds.

2.      Differentiate between precision and accuracy.

 

III. When Susan washed the shirt with a black ink pen, she produced a stain with several colors. Suggest an explanation why the stain apparently changed from black to multicolored.

 

Experiment 2

 

Describe everything you can about following samples.

I Experimental. Use samples (a small quantity for each test) of sugar, cornstarch, benzoic acid, and carbon to conduct the following:

1.   Examine each for its physical properties.                                 

2.      Test the solubility of each in water and in hexane.

3.      Burn the sugar in a crucible over a Bunsen burner and describe the product(s).

4.      Put a few drops of concentrated sulfuric acid on each and describe the results.

 II. Included in your experiments are examples that illustrate the following chemical concepts, Use your observations of these experiments to illustrate the following:

·        Mixtures, compounds, and elements

·        Heterogeneous and homogenous mixtures

·        Solutions

·        Chemical and physical changes

III Susan found a box with unknown contents in her pantry, which she knew, was either powdered sugar or cornstarch. Investigate the chemical and physical properties of sugar and cornstarch. Suggest how Susan might have differentiated between the two.

 

Experiment 3

 

I.   Experimental

1.   Put cotton swabs on each end of a glass tube (18‑24 inches long).

2.      Carefully, under the hood, wet one with concentrated ammonium hydroxide and one with concentrated hydrochloric acid.

3.      Cover each end with a rubber septum.

4.      Observe (it will take a few minutes before you see a change).

 

II. Included in your experiments are examples that illustrate the following chemical concepts. Use your observations of your experiment to illustrate:

 

1.      Molecules are always in motion (and the amount of motion varies with state of matter).

2.      NH3(g) + HCl(g) ‑‑> NH4Cl(s)

3.      States of matter

4.      A chemical reaction is a rearrangement of atoms.

5.      Conservation laws

 

III. Susan observed that in her cleaning supplies, a bottle of household ammonia had been stored near a bottle a muratic whose cap had not been appropriately tightened. The outside surfaces of several containers in their vicinity had become coated with a white powdery substance. Investigate possible chemicals in muratic and suggest an explanation for Susan's observations.

 

Experiment 4

 

I  Experimental Use a can of diet soda and regular soda in the following experiment.

1.      Find Volume of a can of soda: Measure radius and height of a can and use the formula (vol = 3.14r2h) to calculate its volume (note that 1 cm3 = 1 cc = 1 mL, r=1/2 the can diameter and 6.28 r= circumfrrance).

2.      Weigh each can on a triple beam balance.

3.      Find the density (in g/mL) of each can using the formula D = mass/volume.

4.       Compare the densities of each to that of water:

a.  Put each can in tub of water and observe.

b. What does this suggest about densities and floating? (Dwater=1.00 g/mL)

c.  Are your observations consistent with your calculated densities? Why?

5.      Determine the sugar and Calorie content: Assuming that the difference in masses of the two cans is due to sugar content, determine the mass of sugar in the can of regular soda. If sugar contains about 4 Calories/ gram, use your calculations to determine the number of Calories Susan ingested in the can of regular soda. Compare your calculated Calorie value to that listed on the nutritional information on the can's label.

6.      Units of measurement:

a. How many cups are in a can of soda? (30 mL = 1 fluid oz., 1 c = 8 fluid

    oz.)

b. How many quarts are in a can of soda? (1 q t= 4 c)

c. How many liters are in a can of soda? (1000 mL 1 L)

 

II. Included in your experiments are examples that illustrate the following chemical concepts. Use your observations of your experiment to illustrate:

 

1.      Calculations of volume are related to distance measure­ments.

2.      Use of dimensional analysis helps you solve many chemistry problems.

3.      Density is related to mass and volume.

4.      Compare conversions when using the metric system with those using the English system of measurements.

 

III. Susan relaxed with a can of soda, Investigate the terms heat, temperature, calories and Calories. Investigate artificial sweeteners in terms of their chemical constitution and Calorie content. Utilize these in your discussion of Susan's choice of beverage.

 

After completion of the tour, and during the next class period, groups are asked to present their observations to the rest of the class, leading to class discussion of the basic concepts related to the experiments.

 

 *Water soluble inks from Pilot Razor point pens or Flair ultra fine pens work well. **A solvent that works well is a mixture of 5:5:5:2 parts 1‑butanol, ethyl acetate, absolute ethanol, and water, by volume.