If I float an ice cube in a glass of water filled to the brim, will the water overflow when the ice melts?

(Lansing State Journal, March 17, 1993)

Question submitted by A. Schneider.

Strangely enough, the answer is no. This question requires you to remember "Ask Science Theatre" columns of long ago about why things float. Remembering back to those articles we know that things float if their densities (how much stuff is in a certain amount of space) are less than the density of water. The density of the ice cube is less than the density of water and the ice floats. This is important because if ice did not float then aquatic creatures would not be able to survive the long winter when rivers and lakes freeze on top.

The other important requirement of floating is that when something floats the weight of water it moves exactly equals the weight of the object. If you took the ice cube out of the glass, the water level would lower by an amount equal to the exact weight of the ice cube. Similarly, when the ice cube melts it will turn back into water and fill up only the volume that it used to displace. So, the water will not overflow.

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