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This Week's Experiment - #273 OJ Ice Crystals
This week's experiment comes from some of my wife's creativity. Our hotel
has breakfast every morning, and she has been putting some extra orange juice
in our freezer each morning. By evening, it is nicely frozen and ready to be
eaten. Besides being delicious, it also offers a great lesson on the science
of freezing. To try this, you will need:
two Styrofoam or plastic cups
water
orange juice
a freezer
a spoon
Fill one of the cups with water and the other with orange juice. Put both in
the freezer, in a place where they will not be disturbed. Wait a few hours
to give both time to freeze. If you really want to observe things, check
them every 15 minutes, so you can see which freezes first. Remember that
opening the freezer door so much will slow things down a bit.
Once they are both frozen, remove them and take a close look. Do you see
anything different? The water looks like regular ice, which of course, it
is. The orange juice is frozen, but it has a sticky sort of syrup on top
that is not frozen.
Let both sit for about 5 minutes. Take a spoon and see if you can scrape off
some of the regular ice. Then try the same thing with the frozen juice. Did
you notice a difference? It was probably easier to scrape away some of the
juice. Taste the scrapings from the juice. Yum!
Continue scraping periodically at the frozen juice and the regular ice. As
they begin to warm a bit, you will notice more difference. Not only does the
juice scrape away easier, but it begins to come apart into small, flat
crystals. At this point, I began to ignore the regular ice, spending most
of my time examining and tasting the frozen juice.
Why are they so different? The key is the syrup that was on top of the
juice. Dissolving substances like sugar in water changes its freezing point,
so that it must be colder before it will freeze. The more sugar you
dissolve, the colder it must get before it freezes. As the juice gets cold
enough, some of the water in it begins to freeze. Interestingly, these ice
crystals are water ice, not frozen juice.
As these tiny crystals grow, the liquid around them becomes more and more
concentrated. The amount of dissolved material remains the same, but there
is less and less water as it is removed to become part of the ice. As the
crystals grow, this thick syrup is trapped between them. This makes the
frozen juice very granular and lets the crystals come loose when you scrape
at it.
The same process happens in the ocean, where saltwater freezes into sea ice.
The crystals connect H2O molecules together, pulling them away from the
substances dissolved in them. The ice from frozen sea water is still
somewhat salty, just as the frozen juice is still sweet. The salt is not
part of the crystal, but is trapped between. Still, the ice contains less
salt than the surrounding sea water.
When you froze the pure water, the crystals grew into each other and
interlocked to form one solid mass. There was nothing to separate the
crystals so they all merged. As the ice gets warm, its surface melts, but
the interior remains a solid mass. It does not come apart into a granular
mass of crystals.
If you like popsicles, you probably have noticed a syrupy coating on the
surface. This is the same as the syrup you saw on the surface of the frozen
juice. Of course, if you don't have any popsicles, you need to go get some
in order to complete the experiment. To be scientific, it might be a good
idea to try several different flavors, just to compare the results.
From Robert Krampf's Science Education Company
PO Box 60982
Jacksonville, FL 32236-0982
904-388-6381
krampf@aol.com
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