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Q & A: ice cube floating in a glass of water

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Most recent answer: 10/22/2007
Q:
When you put an ice cube in a glass of water, filled up to the top what happens and why does it happen?
- Katie (age 15)
South WIndsor High School, connecticut
A:
We assign this question all the time to our students. Since the ice, when it floats, displaces exactly its weight in water, when it melts, the water it melts into takes the same volume that the ice cube displaced in the water. So the water level should remain the same.

A small correction to this reasoning comes from the fact that the water cools down as the ice cube melts. Cool water is more dense than warm water, and the water level will go down just a bit as the water shrinks.

A small correction to the small correction comes from the fact that water expands a bit before it freezes. It has a maximum density at around 4 degrees Celsius. If the water has a temperature between 0 and 4 C, and the ice cube melts, the water level should go up because water expands as it cools down in this narrow temperature range.

Tom

(published on 10/22/2007)

Follow-Up #1: density and volume

Q:
are density and volume the same.
- shae (age 14)
tyler, tx us
A:
It's almost the opposite. "Density" tells us how much is there per unit volume. For example mass density is the amount of mass per unit volume. If you take one gram of water, when it has high density that means it fits in a small volume. When it has low density, it's spread over a big volume.  There’s a formula,  density = mass / volume. 

Mike W.

Lee H

(published on 10/22/2007)

Follow-up on this answer.