Kids, let's try to take advantage of the cold weather here in the Midwest. You know that a backyard pond or lake in winter can be a magical place. It is also filled with many scientific wonders. These bodies of water freeze from the top down, and they do so for two reasons. The top is closer to the cold air for one, but mostly it's because water has the amazing property of getting lighter as it freezes. That's why ice floats. In liquid water, the H2O molecules can pack together very tightly and randomly (without a regular structure). In ice, the molecules form hexagonal (six-sided) crystals, like a tiny honeycomb. This arrangement forces the molecules farther apart than in liquid, which is why ice is less dense and therefore lighter than water. The ice crystals are so dominant that they work hard to force out gases and impurities. If you look at the edge of a pond that is solidly frozen, you should see clear ice with some trapped bubbles. The ice might look dark, even black, but that is the dark water underneath. You might see plant life and even fish (like perch) underneath. They'll all look magnified because of the ice.
Please note: All chemicals and experiments can entail an element of risk, and no experiments should be performed without proper adult supervision.
Try this way to visualize the internal crystal structure of the ice. Buy two polarizing filter sheets from a science supply or hobby store. These are similar to what is used in sunglasses to block certain wavelengths of light. On a cold day, look for puddles on your street or in your yard that have formed a skin of ice. Pick up a piece of the ice and press it between the two polarizing sheets. This part is important - make sure the two sheets are at right angles to each other. Now hold the "ice sandwich" up to the light and observe.
Have you heard grinding noises at the edge of a frozen lake? It's from expansion and contraction of the ice along cracks and fractures formed between different layers. You need AT LEAST four inches of solid ice to walk on, so don't walk onto a frozen lake or pond until an adult has measured the thickness. Despite the grinding noises, ice can be incredibly strong. In some places lakes freeze over a foot thick, which is enough for small vehicles. While strong, ice is also flexible. No vehicle can go faster than 10-20 mph over ice, because waves can form in the flexible ice sheet and this stress can break it apart.
Here's a fun fact. You know that polar bears live on ice and snow, and that they are white, right? Wrong. A polar bear's fur is actually made up of transparent, hollow hairs. Each hair works like a tiny fiber optic tube, and channels the sun's heat to the bear's black skin, helping it to stay warm. The clear hair reflects light, just like ice and snow, which makes them all appear white. This is all "cool" stuff, isn't it?
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Kathleen Carrado Gregar, PhD, Argonne National Labs
[email protected]
January 2002
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Reference: Tom Connor and Martin Jeffries, Scientific American Explorations magazine (www.explorations.org), Winter 2002, p. 28.
P.S. Check out an exhibit called "Animals of the Ice Age" through 5/20/02 at the SciTech Museum in Aurora, IL.
