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Q & A: How can pressure melt ice?

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Most recent answer: 10/22/2007
How can pressure melt ice?
- Brian Kenney (age 11)
St. Matthew, Seattle, WA USA
Dear Brian,

That's an excellent question! The answer to this is the same as the answer to another question: Why does ice float?

To answer both of these questions we must first realize that something very interesting happens to water when it freezes: It takes up more space! To see that this it true, fill a glass half full of water and make a pencil mark on the glass exactly at the top of the water (better ask your mom or dad to help with this experiment). Now put the glass in the freezer overnight. When you look at the glass in the morning it will have ice in it, and the top of the ice will be slightly above the pencil mark. This is because the water expanded as it froze.

Since the ice still weighs exactly the same as the water did the night before, but is now taking up more space, it will float. You know this is true...just drop an ice cube in a glass of water and you will see it stay at the surface.

OK, so what does this have to do with pressure and melting. Well, the answer is exactly the same as the above, only in reverse. If we take some ice and push on it hard by putting lots of pressure on it, we are really just trying to make it take up less space. One way for ice to take up less space is for it to turn back into water, which is the answer to your question.

To verify that water takes up less space than ice, just leave your glass of ice (from the last experiment) on the counter until it melts and you will see that the water is back below the line you drew on the glass.

Hope this helps. MS

(published on 10/22/2007)

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