Pressure
The Crushing Can Experiment
We are surrounded by air. The air is always pushing against us. Luckily, we are full of liquids and gases that stop us being squashed. If we didn't have them in us, what would happen? The crushing can shows us what might occur.
A can is filled with tap water...
Before...
The can is heated up over a Bunsen burner, with the lid off. When steam is seen coming out of the can, it is removed from the burner and the lid screwed on tightly.
During...
The can is then allowed to cool down...
After...
What's happened?
When the can is heated it fills up with steam. The steam pushes all the air out of the can. When the lid is screwed on and the can begins to cool, the steam in the can condenses back to form liquid water. This forms a vacuum in the can. This means that the pressure inside the can is much less than the pressure on the outside of the can, so it is crushed.
The Cartesian Diver
This is a nice trick, which involves changes in pressure.
Take a bottle and fill it to the brim with water. We used 250ml conical flasks and rubber bungs for our investigation. Get a plastic pen cap and attach a couple of paper clips - this needs trial and error, too many paperclips means the cap will sink, too few and the trick won't work!
Put the weighted pen into the bottle. Now screw the top onto the bottle. You may get a little leakage, but don't worry! You'll only mess the carpet up.
Watch what happens to your diver as you screw the cap on further and then unscrew it.
Click here to see a diver in action.
What happens?
The pen top is full of air. As you screw the top down on the bottle the water gets squeezed. The only place for the water to go is into the pen cap. This means the cap gets denser and therefore sinks.