Most recent answer: 10/22/2007
- Gail Elber (age 39)
|Gail - |
Actually, most liquids do /not/ expand when they freeze. In fact, most liquids lose volume as they freeze. This is because when you freeze a liquid, its molecules slow down. The more they slow down, the tighter the bonds are between them. And the tighter the bonds are between them, the closer together they become, usually. When the individual molecules become closer together, the substance will take up less space.
Water is one of the rare liquids that forms a crystalline solid that takes up more space than the liquid itself does. The reason for this is purely incidental (although important for many things, as you mentioned). Because of the shape of the water molecule and the angles that it forms when it bonds, the solid form of water actually ends up taking up more space than liquid. Like I said, this is /very/ unusual. There are a few other substances that behave the same way, but not many at all.
For more information, check out the answer to the question "Freezing Water" in the Solids, Liquids, and Gases - Liquids section of our answers list.
(published on 10/22/2007)
Follow-Up #1: freezing water and life
- S.A.S (age 13)
that’s a really interesting question, but I can only make a few remarks on it due to my general lack of knowledge.
First, the expansion of freezing water is not always useful for living things. If, for example, a fish froze the expanding ice crystals would break open its cells and kill it. That’s on my mind because one of my sons is in Antarctica studying the ways some fish have evolved special antifreeze molecules to survive the winter.
Second, the reason that ice forms on the top of lakes, etc. is that it’s more expanded, i.e. less dense, than liquid water. I’ve heard that many living things have adapted to life in the water under the ice. For bottom-dwelling organisms, such as plants which grow from the bottom of the lake, that might not be possible if the ice settled to the bottom. No doubt there are many other cases in which the way plants and animals live would be disrupted if ice sank rather than floating.
However, the key point is that the reason we all get along ok in a world where ice floats is that it’s always been that way, so that’s the world our ancestors evolved to survive in. If ice sank, somewhat different creatures would have evolved to fit those different conditions.
(published on 10/27/2007)
Follow-Up #2: ice freezing and life
- Dale Young (age 48)
It's hard to figure out whether slightly different conditions would have been just as favorable for life adjusted to those conditions as our conditions are for our types of life. You may be right that that having ice float is overall more favorable than it would be for ice to sink. It's really hard to speculate accurately about such things, in part because it's hard to imagine what different types of life might have formed. Before anyone explored the deep-sea thermal vents, who would have imagined the crazy types of life that thrive there?
(published on 01/15/2015)