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HOME EXPERIMENTS
Not all experiments require a laboratory to do them in! Here are a few experiments to try at home with a short explanation of what they show us about science. Please read all the instructions carefully before you start.
CRUSHED BY THE ATMOSPHERE
The atmosphere exerts a pressure on everything. This pressure (air pressure) is caused by the weight of the air pushing down. At sea level this pressure is 101325 Newtons per square metre. In other words, there is a weight like that of 638 kilograms pushing down on the area the size of a piece of A4 paper. That is more than half a tonne, or similar to the weight of a car! The reason everything is not crushed instantly is that this pressure pushes in all directions, so the pressure is the same under the paper as above it.
In this experiment you are going to suddenly reduce the pressure inside a drinks can and allow the huge push of the atmosphere to do its work!
Equipment list
- An empty drinks can
- Access to water
- Tongs to hold the can when it is heated (Barbecue tongs are perfect, oven gloves will not do)
- Access to a gas stove
- A washing-up bowl full of water
 Instructions
- Fill your washing-up bowl with water and place it near the gas stove.
- Put a small amount of water in the bottom of the drinks can. It should be just enough to cover the indentation at the bottom.
- Get your tongs and make sure you have a firm grip on the can.
- Now light the gas stove and hold the can directly over the flame.
- Wait until the water is boiling furiously. This will be indicated by the fact that the bubbling noise will stop getting louder and the amount of steam coming out of the can will be constant. DO NOT LOOK INTO THE CAN!
- Now, very quickly, plunge the can upside down into the water. This means you will have to turn it over, so that the mouthpiece goes in first. You must do this quickly.
- Watch as the can is squashed flat by the atmospheric pressure!
The Science bit
As long as the can is open to the air the pressure inside is the same as that outside, so the can keeps its shape. Plunging the can into the water when it is full of hot steam has two effects. The steam suddenly cools down and this cause a sudden drop in pressure. Of course, now the can's mouth is underwater, so more air cannot get in to equalise the pressure. The much larger pressure outside the can crushes it flat.
THE INDESTRUCTIBLE BALLOON
If you hold a party balloon over a candle flame, I'm sure you can work out what will happen. This experiment will make your normal party balloon resistant to a candle flame, at least for a short time.
Equipment
- Several party balloons (in case the first one goes wrong)
- A jug of water
- A candle and matches
 Instructions
- Take one of the balloons and, before you blow it up, fill it with a small quantity of water.
- Now blow up this balloon and tie it off in the normal way.
- Light your candle and hold the balloon in the flame. You must make sure that the flame is only touching the bottom of the balloon, where the water is.
- Impress friends, family and future employers by the fact that the balloon will go black in the flame, the water may even start to boil, but the balloon will not burst!
The Science Bit
Water has a very high specific heat capacity. This means that a quantity of water can absorb a lot of energy without its temperature going up too much. In this case, the water is absorb the heat from the candle flame and carrying it away from the surface of the balloon by a process called conduction. This keeps the balloon fairly cool and stops it from bursting.
All these experiments come from a website of home experiments that is now off-line. If they come back, you can find them at www.scifun.wisc.edu .
If you have tried one of these experiments and it worked really well, then please e-mail me to tell me all about it! The best write-ups will be featured on the science department homepage.
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