For this week's experiment, we
are going to make a ball of oil. Don't worry, this is not nearly as
messy as it sounds. You will need:
* a clear container. I used a
tall, thin bud vase, but you can use a drinking glass, etc. * water * rubbing alcohol * cooking oil * a drinking straw
Fill the container 1/2 full of
water. Tilt the glass and SLOWLY and CAREFULLY pour alcohol along the
side of the glass to fill it near the top. Place the glass on a
flat surface. Looking from the side, it will seem that the water and
alcohol have mixed, but unless you were in too much of a hurry, they have
not.
Dip the end of the drinking straw
into the cooking oil. Place your finger over the top end of the
straw and lift it out of the oil. One or more drops of oil will stay in
the end of the straw. Hold the straw over the glass of water and
alcohol, near the surface. Remove your finger and the oil will drop
into the glass. Each drop of oil will sink about halfway through
the liquid and then stop, forming a round ball of oil that floats
there. If you make several drops, they will each form a ball and the
balls usually do not join unless forced together or touched by something
like the end of the straw.
What is going on? Several
different scientific principles are at work here. First, the alcohol is less
dense than the water. One cubic inch of alcohol weighs less than one
cubic inch of water. This lets the alcohol float on top of the
water, forming two layers of liquid.
The oil is denser than the
alcohol, so it sinks through that layer. The oil is less dense than the
water, so it floats on that layer. This causes the oil to float in
the middle of the glass.
OK, so now we know why the oil is
in the middle. Now, why does it make a ball? The oil will not mix
with the water or the alcohol. Water and alcohol are both polar
liquids. That means that one end of the molecule has a slight
positive charge and the other end has a slight negative charge. Oil is a
non-polar liquid, and it does not have these charges on the ends.
Polar liquids will mix with polar liquids. If you stir the liquid
in your glass, the water and alcohol will mix. Non-polar liquids mix
with non-polar liquids. If you stir two different kinds of oil
together, they will mix. Polar and non- polar liquids will not mix, so
the oil remains separate from the water and alcohol.
The oil forms a ball because of
pressure. The liquid around it is pushing inwards equally in every
direction. The oil is pushing outwards equally in every
direction. This equal pressure causes the oil to form a ball. If your oil
ball is large enough, You may see it start to flatten out. This is
because gravity is pulling downwards, which means that the pressure is
not equal in all directions and the ball is deformed. The larger the
oil ball, the more it will flatten.
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