`Gases, Pressure, and VolumePatricia A. Riley              Lincoln Park High School                               2001 N. Orchard St. Mall                               Chicago IL 60614                               (312) 534-8130 ext. 148Objectives:  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 stopperStrategy:  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.         `