BACKGROUND:

Air pressure is difficult for children to understand because it is difficult to see and feel.  However, if air's properties are made aware to children it is very easy for them to understand.  The atmosphere has its own weight or air pressure. Air pressure is lower on the top of the mountains because there is less air to weigh down from above. Cold air is heavier than warm air, which is referred to as high air pressure. The warmer the air the lower the air pressure. Air with a high moisture content is heavier than without moisture.

Galileo Galilei was the first person to show air has weight and it was his student Evangelista Torricelli who invented the barometer. He noticed that the level of mercury changes in a glass tube open to air at the base.

PROCEDURE:

1. Show students the “handboiler.”  When you hold the bottom of the handboiler you will notice that the liquid moves upward and looks like it is boiling.  However, what is happening is that you are increasing air pressure in the vacuum.  This causes the air to force the liquid (methyl alcohol) up the tube, the “boiling” is actually the rest of the air. 
     
2. The air that is all around us can act as a force.  The following experiments illustrate air pressure by doing several activities.  
   1. Since this experiment requires water, it is better to do experiments one at a time. Many of these activities should be done over the sink, over a bucket or tray, or do it outside.  Tell students how long each exercise will take.  On the students' lab sheets, the exercises are illustrated.  Use the outline below to verbally review the instructions.

   EXERCISE I 
      

   1. Fill a plastic glass 3/4 full of water.
   2. Place a piece of cardboard enough to cover the top of the glass.
   3. Hold the card tightly on top of the glass.  Turn the glass upside down.
      
      REASON:  air pressure holds the cardboard to the glass.

   EXERCISE II
     

   1. Hold a small square piece of paper against one end of a straw.
   2. Suck through the other end.  Release the paper.
        
      REASON:  air pressure holds the paper against the straw

EXERCISE III

1. Take the same straw and hold your finger over one end.
2. Push the other end straight down into a glass of water.
3. Now put straw in without finger on top.
4. After the straw is submerged put finger on the top of straw and remove from glass. 
     
   REASON:  air pressure prevents water from going into the straw and  once water is in straw it prevents water from going out

EXERCISE IV

1. Using a large pan of water, put a glass with a paper towel inside the glass and turn glass upside down into the pan.  The towel will not get wet. 
2. Put one glass filled with water upside down, put another glass not filled with water next to the other glass.  Move the glass full of air so that an edge is under the glass full of water, tipping it so that the air bubbles move up into the glass full of water.  Air will transfer from one glass to the other. 
3. You may want to refer to Air is All Around Us by F. Branley, if you would like more information.

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