Learn more physics!
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
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
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.
(republished on 07/25/06)
Follow-Up #1: density and volume
are density and volume the same.
- shae (age 14)
tyler, tx us
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.
(published on 03/03/07)
Follow-up on this answer.