Relationships: Pressure, Volume, and Temperature
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Samuel E. Hall, Jr. Chicago H. S. for Ag Sciences
11325 W. 111th Street
Chicago, Illinois 60655
Grade Level: Sophomore chemistry
Students will be able to state the relationships of pressure to volume and
pressure to temperature, and volume to temperature for a gas.
Per team: 1 small round balloon
24 inch length of string
1 meter stick
1 fire syringe Charles Law demo apparatus (available from Cenco)
4-5 inches of sewing thread
The activities are such that the students will discover the relationships
stated in the objectives above. First, Ask the students to inflate a rubber
balloon to its fullest diameter. The students should measure the circumference
of the round part of the balloon. Although you purchase round balloons, the
balloons will probably be pear shaped. We are interested in the round part.
Place the balloon into the freezer or ice chest partially filled with ice. We
need to bring the temperature of the gas in the balloon down as far as possible,
let them stand and go to the next activity; tell the students that we will come
back to this one. Don't answer any questions that they ask about the first
activity - for now they should follow directions.
The second activity should begin with a demonstration of the use of the
fire syringe. This apparatus is made of a rod with a "T" shaped handle at one
end and rubber gaskets at the other end. Another part of the apparatus is made
of a thick walled glass tube shaped like a test tube. The third part is a metal
tube fitted with a rubber foot so that the tube assembly does not slip from the
table in use. Just above the rubber foot is a viewing window so that you can
observe what is happening in the glass tube. The other end is open to receive
the glass "test tube" like part. Show the students that the thread will burn;
use a match or "bic" lighter. Then insert a fresh length of thread into the
glass tube - insert the rod - place this assembly into the metal tube. Hold the
metal tube vertically on a flat surface with one hand and with the other hand
hold the plunger. Push down with a reasonable amount of force. Ask the
students to observe the view window on the metal tube. Do not mention the fact
that the gas in the tube is compressed. Give one fire syringe to each group and
ask them to record the groups' observations.
Let's go get those balloons. Measurements of circumference should be made
as soon as possible after they come out of the cooler. Record observations.
Make comparisons of circumference before and after cooling.
Relative to the balloons, I want students to realize that as the temperature
decreased - the volume of the balloon also decreased. Use "V" and "T" to mean
volume and temperature respectfully. V/T (read volume divided by temperature)
of the initial situation is equal to V/T of the second situation (after
cooling). Discuss relationships; V/T = c ("c" is constant); V/T = V'/T'.
Relative to the fire syringe, I want students to realize that as the pressure
increased the temperature also increased. P/T = c and P/T = P'/T'.
All of the above should take approximately 40 minutes. On the next day have the
students perform a pressure - volume activity.