High School Biology-Chemistry SMILE Meeting
09 September 2003
Notes Prepared by Porter Johnson

Ben Stark [IIT, biology]       Calculating the Oxygen Content of Air
Ben
showed us a simple method to calculate the amount of oxygen in air, which also demonstrates the need for oxygen in air to support combustion.  He placed a candle upright into a shallow dish containing a little water, and put an inverted beaker over the candle, in such a way that the mouth of the beaker was completely underwater.  He marked the initial water level of the beaker, and determined V1, the volume of air in the beaker, as the total volume of the beaker, VT, minus the volume of water initially in the beaker, minus VC, the volume of the candle above the water level.  He obtained V1 = 310 ml. He then removed the beaker, lit the candle, and replaced the beaker.  As the candle burned, the level of water inside the beaker gradually rose.  After the candle flame went out, he again measured the volume of air in the beaker, obtaining V2 = 287 ml.   He then calculated the ratio V2 / V1 = 0.93.  Ben next used the perfect gas law, P1V1 = n1 RT1 and  P2V2 = n2 RT2   along with the fact that the pressure and temperature should be about the same before and after:  P2= P1  and T2= T1.  Thus, n2 / n1 = V2 / V1 = 0.93.  Therefore, in the process of consumption there has been a 7% loss in the number of moles of gas.  How come?

In burning wax, a hydrocarbon with a string of CH2 monomer units, the basic (approximate) chemical reaction is

2 C H2 (wax) + 3 02 (gas) ® 2 C02 (gas) + 2 H20 (liquid)
In other words, we convert 3 molecules of oxygen gas into 2 molecules of carbon dioxide gas.  So that the reduction in the number of oxygen molecules is three times the net reduction in the number of gas molecules. Thus, we estimate that 21 % of the molecules initially in the air were oxygen molecules expended in the process of combustion. This result is amazingly accurate!

Note: One must measure the volumes of the beaker, water, and candle carefully both before and after the candle burns to get precise results.

A breath of fresh air for us all!  Thanks, Ben!

Pat Riley [Lincoln Park HS, chemistry]        Recognizing Physical Properties: Comparing and Contrasting
Pat
began by holding up two 500 ml beakers, one which was 50% filled with a red liquid, and the other 80% filled with a red liquid [red food coloring in water]. She asked us to find as many similarities as possible between to two liquids [same color, same smell, same density, pour the same way --- same viscosity, etc.].  The only difference appeared to be in volumes in the beakers.

Pat then replaced the 80% beaker with one that was 50% filled with a clear liquid [water without food coloring].  The similarities were in the amount of liquid, the weight, and the viscosity.  The only difference appeared to be in the color.

Pat then took two beakers about 25% filled with yellow liquids [water with food coloring and Karo® Syrup, respectively].  This time the colors and volumes were the same, but the density and viscosity were different.

Pat then asked us to compared the red beaker containing red liquid with a red ball.  The only evident similarity was in the color, whereas the states [liquid versus solid] were different, there were different volumes, and the liquid would not bounce off the table.

Pat then asked us to compare two red balls. The color, size, and state were the same, but the balls did not bounce in exactly the same way.

Pat then asked us to compare a red ball with a steel ball.  They were both balls of about the same size, but with different masses, densities, colors, and bouncabilities.

We then compared a large cube with a small one.  Finally, Pat held up a roll of tape, and asked us to list its physical properties:  hollow, strong, wide, sticky, black, moveable, cylindrical, etc.

Pat also distributed a handout sheet containing the following questions:

1. Study the contents of two beakers.  How are they similar?  Different?  List the similarities and differences.

2. Same questions, with different beakers

3. Same questions, with different objects.

4. Same questions, with different objects.

5. Same questions, with different objects.

6. What did you use to tell whether the objects were similar or different?  Did these properties change as you made your observations?

7. To test your knowledge, list 5 physical properties for the object being displayed.

Good exercises in observation!  Thanks, Pat!

Theresa Donatello and Erma Lee will make presentations at our next meeting, 23 September 2003. See you there!
Notes taken by Ben Stark.