This Week's Experiment - #250 Water, Oil and Ice, Oh My
This week, we are going to explore the world of density. This is a very
useful science, important for many things from boats to hot air balloons.
You will need:
a tall, clear drinking glass
cooking oil
ice
Fill the glass almost full of cooking oil. I found that cheap, vegetable oil
worked very well. Canola oil did not work at all, as it was not dense
enough. Place the glass of oil on a flat surface and then gently add an ice
cube. The ice should float. If it does not, try using a different kind of
oil.
Now comes the interesting part. Oil floats on top of water and ice floats on
top of oil. What will happen when the ice begins to melt? Watch a minute or
two and you will see.
What happens? As the ice begins to melt, you will see a drop of water
hanging from the bottom of the ice cube. As the drop grows, the ice cube
will float lower, as it is being weighted down by the denser water. Finally,
the drop gets large enough to pull free of the ice and it slowly sinks to the
bottom of the glass.
Water is a strange chemical. Most liquids get smaller when they freeze,
which means the solid form is denser. When water freezes, it gets larger.
It still weighs the same, but it takes up more space, which means it is less
dense. That is why ice floats in water, and in the oil. If something is
denser than water (or in this case, oil), it sinks. If it is less dense than
the liquid, it floats. As the ice melts, the water takes up less space,
becoming denser, and the denser drops of water sink to the bottom of the
glass.
After the ice had all melted, I tried reversing the process by putting the
glass of oil and water in the freezer. I was hoping that I would find a lump
of ice floating on top of the oil. Instead, the lump of ice was at the
bottom. As the water began to freeze, the surface tension of the water was
strong enough to keep the ice from rising up through the oil. Soon, it froze
to the side of the glass and then was firmly trapped at the bottom. I did
manage to gently warm the glass and get the ice to come free and float to the
top. If you try that, be sure to warm the glass slowly, so it does not
crack. Why would it crack? For that, you will either have to do some
research or wait until next week's experiment.
From Robert Krampf's Science Education Company
PO Box 60982
Jacksonville, FL 32236-0982
904-388-6381
krampf@aol.com
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