This 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
- 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.