Gases, Pressure, and Volume

Patricia A. Riley              Lincoln Park High School
                               2001 N. Orchard St. Mall
                               Chicago IL 60614
                               (312) 534-8130 ext. 148

Objectives:

  For students in sixth, seventh, and eighth grades 
  1.  To demonstrate the meaning of pressure. 
  2.  To demonstrate that gases exert pressure.
  3.  To demonstrate that gases take the volume of their container.
  4.  To demonstrate the relationship between the pressure of a fixed quantity 
      of gas and its volume.

Materials Needed:
  
  For class demonstration:         
  8"-16" balloons                         625-nail board
  2-liter pop bottle                      1-nail board
  small plastic pop bottle                2-nail board
  3 1-hole rubber stoppers with           3-nail board
    glass tubing inserted                 overhead projector
  2 pieces of rubber tubing that          plastic petri dish with cover
    fit the glass tubing                  small plastic beads
  3 tubing clamps                         4 small potatoes of same size

  At each team work station:
  ringstand                               syringe, 10 to 20 mL
  test tube clamp                         flat, stackable weights or textbooks
  solid rubber stopper

Strategy:

  1.  Sit on 625-nail board on top of a table where all can see; be sure to 
      swing feet.  Introduce the lesson and then jump down from table.  Hold up
      board for all to see; invite everyone to come up and sit or stand on it.
      a.  Put out the 1-nail board, 2-nail board, and 3-nail board; again invite 
          everyone to come up and sit or stand on these boards.
      b.  Ask questions such as:  Why are there no takers to sit or stand on the
          1, 2, or 3-nail boards, but plenty willing to sit on the 625-nail 
          board?  What would happen if someone actually did sit on these small 
          boards?  Why can we sit on the 625-nail board but not the others?
      c.  Lead the discussion until students mention that body weight spread 
          over many different nails is what makes the difference.  Now write the
          formula for pressure on the board:  Pressure = force/surface area.  
          Stress that surface area refers to the surface directly acted on by 
          the force.  So what is this surface:  your bottom or the nail tips?   
          Stress that a force is a push or a pull.  So what acts as the force 
          when we sit or stand on the board?  What about the space between the 
          nails?  Drop one potato onto each nail board from the same height and 
          compare the results.  


      d.  Write the formula for pressure on the board again, this time 
          substituting in the force and surface area used:  
    
               Pressure = body weight/area of nail tips
        
          Ask students to determine for which board the pressure would be 
          greatest, least, and why.  
  2.  Blow up a balloon and knot it.  Ask the following questions to stimulate 
      discussion:  Do gases, for example air, exert pressure?  How do we know?  
      Is the air in the balloon exerting pressure?  How do gases do this?
      a.  Using an overhead projector and small plastic beads inside a covered 
          petri dish, demonstrate the movement of the gas particles.
      b.  Ask students to relate this model to the nail board model and to the 
          balloon of air. 
  3.  Insert a 1-holed stopper into the mouth of a second balloon.  Blow up the
      balloon to about the same size as the first balloon and clamp it closed.  
      Uncap an empty 2-liter plastic pop bottle and pour hot water into the
      bottle and pour hot water into the bottle.  Place the bottle in a pan of 
      hot water for several minutes.  Attach a piece of rubber tubing to a 
      piece of glass tubing inserted into a 1-holed rubber stopper.  Now dump 
      out the water in the bottle and quickly insert the stopper into the
      mouth of the bottle and clamp the rubber tubing closed.  Allow the bottle 
      to cool.  Do the same with the smaller bottle.  This will create a partial 
      vacuum in each bottle.
      a.  Ask the students to observe what happens to the bottles.  Why did it 
          happen?  
      b.  Attach the glass tubing of the second balloon to the rubber tubing of 
          the 2-liter bottle.  Now remove the clamps.  Ask the students to 
          describe what happens to the balloon, to the bottle.  Why did it 
          happen?  Did the volume, pressure, shape of the air change?
      c.  Blow up the second balloon again and attach it to the smaller bottle.
      d.  What conclusions can be drawn?  Hopefully the students will say that 
          the pressure of a gas changes when its volume changes.
  4.  Let's try to test this conclusion.  Divide the students into small groups 
      of three or four.  They are to set the syringe volume at the maximum, seal
      the tip with a solid rubber stopper and support the syringe with a clamp 
      attached to a ringstand.  Tell the students to read the volume of air in 
      the syringe before adding any weights (books work best) and then to read 
      the volume after adding each weight.  They should record the total weight
      and the volume associated with it after each addition of weight.
      a.  What do they observe?
      b.  Why didn't the syringe plunger collapse?  Why didn't it fly out of the
          syringe?
      c.  What happened to the pressure of the air inside the syringe?
      d.  What's the relationship between the volume of a gas and the pressure?
      e.  The students can graph the pressure on the x-axis and the volume on 
          the y-axis.
      f.  Students should be able to state that as the pressure on a gas 
          increases, its volume decreases.