Science Puzzle
Ice in a Full Glass: Will It Overflow?
Fill a glass to the very top with water, then gently float an ice cube in it. The water bulges right at the brim but does not spill. The ice sticks up above the surface, so surely when it melts the extra water must run over the edge.
When the ice cube melts completely, what happens?
The Answer
The level does not change at all. Not a drop spills. A floating object pushes aside exactly its own weight of water. The ice cube floats because it is a little less dense than water, so it settles with most of its body submerged and a small cap above.
Here is the key: the underwater part of the cube is already holding a cube-shaped hole in the water. When the ice melts, it turns into water, and that meltwater has exactly the volume of the hole the submerged part was making. It slots neatly into the space it was already occupying. The bit that stuck up above the surface was the extra bulk of being frozen, and it shrinks away as the ice becomes denser water.
This is why melting sea ice, which is already floating, does not by itself raise sea level. Ice sitting on land, like glaciers, is a different story, because that water is not in the sea yet.
The principle: Floating and displacement. A floating object displaces its own weight of fluid, and melting ice fills exactly the space its submerged part was already occupying.