Pressure and Volume of Gases

Sister Stephanie Blaszczynski  Resurrection High School
                               7500 W. Talcott Avenue
                               Chicago, IL 60631
                               312-775-6616

Objectives:

 1. To understand the relationship between pressure and volume in gases (Boyle's 
    Law). 
 2. To understand the workings of a manometer and a barometer.
 3. To define pressure and establish units for pressure.
 4. To elicit examples of effects of pressure and volume changes in everyday 
    life.

Materials needed:

 1. Imploding can: empty soda can, water, hot plate, container of ice water,
    tongs to transfer hot can.
 2. Pop corn and pop corn popper.
 3. Water manometer: 1.5 m of plastic tubing, meter stick, balloon, tape, 
    vertical support, small pipette and water with food coloring in it to make 
    it more visible.  Test tube to hold a column of water when inverted in 
    beaker of water and mercury manometer to make comparisons if available. 
 4. Boyle's Law Apparatus consisting of inverted syringe with tight cap to hold 
    a volume of air and a platform to hold about 10 identical books or weights. 
 5. Potato gun.

Strategy:

 1. Have pop corn popper popping as class enters.
 2. Heat small amount of water to boiling in an empty soda can and then plunge 
    the can, opening down, into ice water to cause immediate implosion of can.
 3. Elicit from students 3 measurable variables: temperature, pressure and 
    volume involved in demonstrations of pop corn and crushed pop can. 
 4. Review units of measurement for temperature and volume.
 5. Define pressure and model this with students pushing on opposite sides of 
    doors with equal and unequal pressures.
 6. Blow up a balloon and elicit explanations of why the balloon is getting 
    larger so that students understand that more air means more hits or more 
    pressure to push the balloon out more. 
 7. Fill a test tube completely with water and invert it in a beaker of water to 
    show that the water column will stay in the test tube due to air pressure 
    pushing on the surface of the water in the beaker. 
 8. Describe the workings of a barometer and introduce units for measuring 
    pressure.  Explain that air pressure in the form of many hits on the surface 
    of the mercury tries to push the mercury in the empty space at the top.  
    Compare a mercury barometer which needs to be about 1 meter tall and a water 
    barometer which would need to be more that 10 meters tall since the density 
    of mercury is 13.6 times that of water.  Use overhead transparencies. 
 9. Make a water manometer to measure how much greater balloon pressure is 
    compared to atmospheric pressure.  Repeat the experiment using a mercury 
    manometer if one is available. 
10. Have groups of students use simple Boyle's Law apparatus.  Make data tables 
    of pressure measured in books placed on the platform and corresponding 
    volume measurements of air column in syringe and graph the results to arrive 
    at an inverse relationship. 
11. Elicit a statement of Boyle's Law from students.
12. Use a potato gun and have students explain the principle of how it works.
13. Elicit other practical examples of pressure-volume relationships including:
    Cartesian divers and Scuba diving
    Weather balloons expanding as they travel to higher altitudes
    Need for opening windows in a tornado
    "Popping" of ears on a plane or when going up in a fast elevator
    Packing tennis balls in pressurized cans
    Pop corn popping
14. Review the concept of increased temperature causing more rapid motion of 
    molecules.  More rapid motion of molecules causes more "hits" on the 
    container, meaning a greater pressure.  Have students explain the collapse 
    of the soda can in the opening demonstration.  
15. Assign homework to research how pop corn pops using concepts learned in 
    class. 

References:

"Water Manometer". ChemMatters. October, 1985.
"Potato Gun". Weird Science Chemistry West. 1986.
"Pop Corn". ChemMatters. October, 1984.