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Egg in a Bottle
The real experiment for this week is another classic,
but a very misunderstood one. You will even find that many books of science
experiments get this one wrong.
You will need:
Warning: This experiment uses fire, so be careful and
follow all safety procedures. Never do fire experiments when you are alone,
even if you are an adult. If you are not an adult, then at least one of the
people with you should be an adult. Think through the experiment before you try
it, to be sure that everything you do is safe.
For this experiment, we are going to put a hard boiled
egg into the bottle. First, you need a hard boiled egg. Let it cool and
carefully remove the shell. Be sure not to damage the egg, as this will keep
the experiment from working properly. If you accidentally tear your hard boiled
egg, put it in a bowl and chop it with a little mayonnaise or salad dressing.
It makes a nice snack while you are boiling another egg.
Once you get the egg peeled, then you have to put it in
the bottle. Gently place it on the top of the bottle. Not much happens. It
just sits there. If you tried to push the egg into the bottle with your
fingers, the egg would tear. We want the egg to be whole when it is inside the
bottle. How are we going to do it?
We will start by removing the egg from the top of the
bottle. CAREFULLY hold a piece of paper about one inch wide and three inches
long. While you are holding one end, use the match to light the other end. As
soon as it is burning, drop it into the bottle and quickly place the egg on
top. Watch carefully what happens.
The paper burns for a second or two. As the fire goes
out, the egg begins to move downwards into the bottle. It squeezes through the
neck and drops into the bottle. How did that happen?
Most books will tell you that the paper burned up the
oxygen, lowering the air pressure inside the bottle, and that the greater air
pressure on the outside of the bottle pushed the egg inside. They get the part
about the air pressure right, but the lowered pressure in the bottle is not due
to burning up the oxygen.
When you burn paper in oxygen, the carbon in the paper
combines with oxygen in the air to form a new gas called carbon dioxide. This
gas takes up about as much space as the oxygen, so the burned oxygen is replace
with carbon dioxide and the pressure in the bottle stays the same.
But if the egg was pushed into the bottle by air
pressure, what lowered the pressure in the bottle? The answer is temperature.
While the paper was burning, it heated the air in the bottle. The heating air
expanded and pushed out past the egg. If you put the egg on the bottle quickly
and watch carefully, you may be able to see the egg shake as the air rushes out
past it. As the flame begins to die down, the air inside the bottle cools. As
it cools, it gets smaller, reducing the pressure in the bottle. The egg acts as
a cork, sealing the top of the bottle to keep air from coming in to balance the
pressure. Instead, the higher outside pressure pushes the egg into the bottle.
Now, how do we get the egg out of the bottle? One way
is to turn the bottle upside down, with the egg resting in the neck of the
bottle. Put your mouth against the mouth of the bottle and blow as hard as you
can. This increases the pressure inside the bottle. Quickly move the
bottle away from your mouth and the egg should pop out. Rinse off the carbon
from the burned paper and your egg is now ready for snack time.
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