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Q & A: floating ice

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Most recent answer: 10/22/2007
We all know that water expands when frozen... If you have a glass of water with ice cubes floating, does the water level rise, fall, or remain the same as the cubes melt?
- Bob Diamond
Hauppauge, NY, USA
Hi Bob- This is a nice puzzle question. The Greek physicist/mathematician Archimedes figured out the idea behind the answer. A floating object displaces an amount of liquid with the same weight as the object. That means that if you have a boat floating in water, a volume of water equal to the volume of the boat below the water line would weigh the same as the boat. If you melt an ice cube, you don't change its weight. So the water from it has just the right volume to fill the space that the ice cube had displaced. The water level doesn't go up or down.

Before you conclude that the ocean levels won't change if the polar ice melts, remember that the Antarctic ice sheet isn't floating but sitting on land. If it melts, sea levels will rise. (and global warming warms the oceans, too, making the water expand, making the levels rise further).

(published on 10/22/2007)

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