Chem 122 Laboratory, Spring 1996      Name________________________________

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Experiment 3: Temperature and Molecular Motion

1. Inspect the digital thermometers and try their switches. What part of the probe is the temperature sensitive part?

2. Measure the room temperature and record both in both Celsius (deg. C) and Fahrenheit (deg. F) in the table below.

|                    | Degrees C  |  Degrees F |
|  room temperature  |            |            |
|  fingers           |            |            |
|  friction heating  |            |            |
|  boiling water     |            |            |
|  tap water         |            |            |
|  ice-water slush   |            |            |
|  ice-salt slush    |            |            |

3. Squeeze the probe between your fingers to warm it slightly and record that temperature in both deg. C and the deg. F in the table.

4. Push the tip of the probe into the small whole in the cork stopper and grasp the cork between your fingers. Wait for the temperature to stabilize, then twist the cork back and forth quickly several times to produce a little friction. Watch the temperature reading and when it has reached a maximum, record the temperature in both deg. C and the deg. F in the table above.

Try to provide a explanation of the mechanism that is operating here - why should there be a connection between the friction of the cork against the probe tip and its temperature?

5. a. Place a half-full flask of water on your hot plate and turn it up to a gentle boil. What are the bubbles composed of?

b. Record the temperature of the boiling water and record the temperature in both deg. C and the deg. F in the table above.

c. Turn off the hot plate and allow the water to stop boiling. Place a cool glass surface over the mouth of the flask of hot water. What happens? How does the water get to the cool surface?

d. Predict what effect the temperature of the water in the flask would have on the rate at which condensation will accumulate on the cool surface. Why?

6. a. Check the room temperature again, just in case it has changed since the last time you measured it. Half fill a small flask with tap water and measure its temperature. Record the temperature in both deg. C and the deg. F in the table above. If you let the flask of water sit in the room for a long time (covered to prevent evaporation), would you expect the water temperature to become the same as the room temperature, or below or above room temperature? Why?

b. If you take the probe out of the water, how would you predict the temperature reading to change?

c. Now remove the probe from the water and holding it in the air, taking care not to touch or wipe the tip of the probe, carefully observe the temperature. What did you observe? Compare your observation to your prediction.

d. Have your lab partner fan the probe with a piece of folded paper. Does fanning the probe effect the temperature reading?

e. For purposes of comparison, do the following: Dry the tip of the probe, wait for the temperature to stabilize, and again fan the probe and observe the temperature. What did you observe? If the results are not clear, repeat steps a - e in the portion of the experiment.

For an even more dramatic demonstration of this effect, soak a small piece of paper towel in tap water and drape it over the end of the temperature probe. Fan the wet paper towel and observe the temperature.

f. The phenomena that is being observed here is called "evaporative cooling". List other examples or applications of evaporative cooling you may have experienced.

g. Talk it over with your lab partner and attempt to construct a explanation for the mechanism of evaporative cooling at the molecular level.

h. In general, based on your common experience, what effect does temperature have on the rate at which things dry? Why?

i. Describe how all of the above observations and experiences have made a connection between temperature and molecular motion.

7. a. Fill a styrofoam cup with shaved ice from the ice bucket. Insert the probe into the ice, wait for the temperature to stabilize, then record the temperature in both deg. C and the deg. F in the table.

b. Add some tap water to the cup of ice until the water level is about half filled. Wait for the temperature to stabilize, then observe the temperature. Add some more ice to the cup, wait for the temperature to stabilize, then observe the temperature again. Does the relative amount of water and ice in the cup have a large effect on the temperature?

c. Ice tends to float to the top, as you know. Do you observe any significant difference in temperature if you position the tip of the probe in the water layer in the bottom or in the icy layer at the top?

Don't discard your ice-water mixture; set it aside for later use.

8. Place some ice in a glass beaker and set it aside for a few minutes. Observe the accumulation of water on the outside of the beaker. Where does that water come from? How could you prove that that water did not simply soak through the glass from the inside?

9. a. Take two styrofoam cups and place 50 mL (50 grams) of tap water in each. Measure and record the temperature of the water in each cup. Into one cup pour 10 grams (10 mL) of ice water from the ice-water mixture that you prepared earlier. In the other cup place 10 grams of ice. Wait until the temperature has stabilized in both cups and record their temperatures. Which cup was cooled to the greater extent. How can you explain this, in light of the fact that the same weight of ice water and ice was added to the two cups and that the ice water and ice had essentially the same temperature?

10. a. Everyone knows that salt is used to melt ice on roadways in the winter. How does that work? How does salt melt ice?

b. Take a cup of shaved ice in a styrofoam cup, check its temperature, and add a roughly equal quantity of table salt. Stir and observe. If all the ice melts, add a little more ice. What happens to the temperature?

How does this observation reconcile with your explanation in (a)?

c. What is the coldest temperature you can produce by mixing salt and ice?

d. Both the Celsius and the Fahrenheit temperature scales are operationally defined on the basis of two reference points, one "cold" and one "hot", which are arbitrarily defined as zero and 100 "degrees", respectively. Everyone knows that the reference points for the Celsius scale are the freezing and boiling temperatures of pure water at normal atmospheric pressure, which are easily reproducible conditions. The 0deg. and 100deg. reference points for the Fahrenheit temperature scale were also originally chosen to be conveniently reproducible temperatures, but they turned out not to be so absolute and precise as the freezing and boiling temperatures of water. Can you guess what the original reference points for the Fahrenheit temperature scale were?

11. Take your table of temperatures and use your graphing calculator or a computer plotting program (I recommend Cricket Graph, which is very easy to use) to prepare a scatter (x-y) plot of the Celsius (deg. C) versus the Fahrenheit (deg. F) temperatures of the various systems you measured. Fit these data to a straight line using the built-in linear least squares curve fit feature. What is the equation of the best-fit line?

Attach a print out of this graph, if you have a print account.

Compare this to the equation that your textbook gives (or that you may have memorized) for the conversion of Celsius to/from Fahrenheit temperatures.

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