Can Cruncher

Schaumburg, Carl                      Von Stueben MSC
                                      1-312-509-5000
                          

Objective

     Purpose is to show what happens to the pressure inside a container of steam when 
it is cooled and how the pressure affects the boiling point of a liquid. 

Apparatus Needed

Empty aluminum popcans and sinks or trays full of water. 
Ditto cans and rubber stoppers to fit. 
Filter flasks, short pieces of rubber tubing on sidearm, thermometers inserted in 
stoppers to fit flasks, ring stands, rings, wire gauzes, bunsen burners, bunsen 
clamps, pinch clamps, and boiling chips. 

Recommended Strategy

    Start by having the class hold popcans in clamps.  Class is directed to add 
various amounts of water (1 to 10 mL) to the can.  Boil, and place can in a sink full 
of water.  The class is told, "Be ready to discuss what happens, think of an 
explanation for what happens, and tell what amount of water works best." At least 
some of the cans should be crushed.  Discuss which techniques worked best and what 
happened inside the can.  Talk about what pressure is, and how cooling the steam 
affects pressure inside the can. 
    Sitting all by itself clamped in a ring stand on the desk, have a stoppered ditto 
can which already has been filled with steam (by boiling water inside) before the 
class came in.  As it cools in the air it should also be crushed.  A few drops of 
water on the outside may be necessary to help it a bit.  Have the class explain what 
must have happened. With another ditto can have the class recreate what happened in 
the can.  (A silent lecture could be effective here.) 
    Working in groups, final part uses the filter flask.  Add enough water so the 
bulb on the thermometer is under the surface. Add a few boiling chips, stopper with 
the thermometer, clamp on the ring and ring stand, and heat until enough steam has 
been produced to push out all the air.  TURN OFF THE BURNER, and using the pinch 
clamp close off the tubing on the sidearm.  "Watch the temperature!"  It may go up.  
If it does be ready to release extra pressure with the pinch clamp, if the 
temperature goes to 104 degrees Celsius.  Using the clamp as a handle, remove the 
flask from the stand and cool in cold water.  "Watch the temperature!"  Of course it 
goes down. but point out that the water is still boiling.  "What is the lowest 
temperature at which you can get the water to boil?" 
    Relate to the two previous parts.  When you cool the flask filled with steam the 
pressure goes down inside. "In spite of being cooled the water inside the flask 
continues to boil.  Why?"  Discussion should lead to the concept that boiling 
temperature depends on pressure.  Ask, "Where in your car is boiling at different 
pressures used?"  The radiator.  "Where in cooking is pressure used?" The pressure 
cooker.  "Why do people in Denver need special cooking instructions? Who will have a 
harder hard boiled egg, someone boiling an egg in Denver or someone on the seashore 
if they both heat the same amount of time.  How does Folgers make freeze dried 
coffee?"