So Much Pressure!

    March 2017:

    As a kid doing dishes (does anyone do those anymore?), I was always fascinated when I had a glass filled with water and if you put a small plate on top of it and tipped it over, the water would stay in the glass. Back then it never made sense to me how this happened.


    - Paper towel
    - Styrofoam peanut
    - Clear plastic cup (you can use glass but there is the chance that it might break)
    - Sink or tub of water
    - Piece of cardboard large enough to cover the glass opening (or paper plate)


    Push the paper towel to the bottom of the glass and push the cup, upside down, into the water. Does the paper towel get wet? Why not?

    Remove the towel and place the peanut on top of the water. Push the open end of the glass down over the peanut. Did the top of the peanut get wet? Why not?

    Place the cup in the water so that it fills and turn it upside down. Lift the glass slowly. Is it tough to pull out of the water? Did the water stay in the cup until the lip of the cup broke the surface of the water allowing the air in?

    Fill the cup with water and put the cardboard on top of the glass and holding the cardboard on, turn the glass over and let go of the cardboard. Did the cardboard stay on? What happens if you don’t fill the cup all the way and do the experiment again? Try filling the cup to different levels.

    What’s happening?

    Why didn’t the paper towel get wet? You may have noticed that the water level in the glass actually got pushed down relative to the water in the sink/tub. What pushed the water down? It was the air that was trapped in the glass. Although we can’t see air, we know it is there because we see its effects around us. We need it to breathe and we can see it move the leaves in the trees when the wind (moving air) is blowing. This also explains why the top of the peanut did not get wet. The air pushed the water down.

    Why did the water stay in the cup till it broke the surface? Air molecules have mass and they are moving very fast hitting everything around us (they are travelling at about 1000 miles/hr!). The pressure that the air pushes on everything is about 15 pounds per square inch (about the weight of a bowling ball pushing on every square inch of your body!). That’s a lot of pressure pushing on us as well as the surface of the water in the tub/sink, and that pressure is what is keeping the water in the glass. If you were to poke a hole in the bottom of the glass, the water would not stay in the glass; you’ve done this experiment before when you trapped a liquid in a straw with your finger and then released your finger from the end of the straw to let the liquid flow out.

    Why didn’t the cardboard fall off? Because the air molecules are travelling in all directions they are also pushing up on things as well – like the cardboard. A typical cup opening might be 10 in2 so it has 10 in2 x 15 lbs/in2 = 150 lbs of pressure pushing up on the cardboard, and so it would require 150 pounds of water above the cardboard pushing down on it in order for the cardboard to come off. That would require a very tall glass!


    To view all past “ChemShorts for Kids”, go to:

    Paul Brandt