Monday, October 4, 2010

Air Pressure: Bell Jar Demonstrations

BELL JAR FOR VACUUM MACHINE
If your school has a bell jar and vacuum pump, break them out!  Try some of my favorites:

- 1 -
Fill a small beaker part way with water.  Have some students touch the water - it's cool/room temperature.  Place the water in the jar and turn on the pump... the water quickly boils.  Remove the water and have students touch it again - it's still cool/room temperature!  With no air pressure, water boils at room temperature! 

 - 2 -
Fill a small balloon with just a small amount of air (just enough so there's air in there, not enough to even stretch the balloon).  Place the balloon in the jar and turn on the pump.  The balloon will expand!  Without air surrounding it, the air molecules trapped in the balloon will begin to expand, thus expanding the balloon!

 - 3 -
Place a marshmallow in the jar and turn on the pump.  The marshmallow will grow right before your eyes!  It's like the balloon above - there's air trapped in the marshmallow that expands when outside air pressure is removed.  Eventually the sugar structure will be stretched to its breaking point and the marshmallow will stop expanding.  When you return the air pressure, the marshmallow will be completely crushed - there's no structure to support/trap air molecules any more.

For some extra fun, pick up some extra Marshmallow Peeps during the holidays!

 - 4 -
Place a small dollop of shaving cream in a small beaker.  Set the beaker in the jar and turn on the pump.  The shaving cream will expand, out of the beaker and will fill the whole bell jar (be careful - turn off the pump before the shaving cream gets into the pump - that would be disastrous).  The shaving cream has air trapped inside of it.  It behands in the same manner as the marshmallow. 

You can also use whipped cream for this, but I find ath shaving cream cleans up very quickly and easily. 

**I've been told that you can do the marshmallow "trick" in one of the vacuum systems for food preservation (the kind that pulls the air out of a jar).  Not having such a system, I haven't been able to confirm, but that information came from a very reliable student.

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