WHAT YOU DO:
1. Put a little bit of water in the can (just enough to hear it
swish around as you shake the can). Also, put enough water in
the basin so that when the can is later turned upside down into
the basin, it completely covers the lip of the can.
2. Start heating the can by placing it on your heat source. Use
your tongs to hold it, and be sure you have on an eye covering
such as safety glasses.
As the can heats, you'll notice the liquid water
changing phase into water vapor. You can tell by the steam escaping
through the opening at the top of the can.
(The steam is very hot – so keep clear!!)
3. When nearly all the water liquid is changed
to water vapor, quickly turn the can upside down
into the basin filled with water. (Have the basin near by so you
can do this quickly)
4. As the can is placed into the basin's water,
the can crushes!
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HOW IT WORKS:
Remember how the can was open at the top until we turned it upside
down into the water? This opening at the top is important because
it allows the AIR PRESSURE in the room to be the same as the air
pressure inside the can. If we have the same pressure both inside
and outside the can, then those forces created by those pressures
are equal. As a result, the can will NEITHER crush nor expand.
As the can was heating, the liquid water boiled away and was changed
to steam (water vapor). Steam moves around much quicker, and takes
up more space than liquid water (about 1000 times the amount of
space as the same amount of liquid water). If the can were sealed
at the top, the steam would create a higher pressure inside the
can – forcing the can to explode! (That’s why you don’t
throw pressurized containers into fires!) But since the can was
open at the top, the pressure inside and outside the can continued
to be equal.
Once most of the water was changed to steam, the can was turned
upside down into the water basin – submerging the lip of the
can. This does 2 things:
1) It seals off the top of the can – allowing there to be
a pressure difference between the inside and outside of the can
2) When the water vapor contacts the water liquid, it causes the
vapor to change back to liquid very rapidly!
The steam that once took up lots
of space has now changed phase into liquid water, which takes up
just a little space. Since this phase change occurs so quickly,
the inside of the can suddenly has some space with virtually nothing
in it (it’s sealed at the opening now by the water in the
basin). Space with nothing in it is called a vacuum. (Outer space
is a vacuum – there’s no air and few particles). A vacuum
has very low pressure, and a perfect vacuum has no pressure. We
don’t have a perfect vacuum, so the space inside the can with
nothing in it has a very low pressure.
Since
the outside pressure (air pressure in the room) never changed,
we now have a difference between the pressure outside the
can and the pressure inside the can. Which pressure is greater?
It’s the pressure outside
the can in the room, which can now crush the can because the
can has a low pressure inside it!The force on the inner and
outer surface of the can was no longer equal. The greater
force essentially won and the can was crushed! |
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HERE’S SOMETHING TO
THINK ABOUT:
How come the can partially filled up with water when it was crushed?
Next Experiment: BALLOON
IN A BOTTLE |
