Grades K–2 · Materials & Matter
Why can ice float in liquid water?
An ice cube bobs at the top of a drink, while a small stone drops to the bottom. The ice is solid, but being solid does not make everything sink. What matters is how much material is packed into the space it takes up.
When water freezes into ordinary ice, its tiny parts settle into a pattern with more space between them. The same amount of water now takes up more room. Freezing has not added any water. It has changed how the water is arranged.
A piece of ice weighs less than the same-sized space filled with liquid water. Water pushes upward on the ice. Before the whole piece goes under, that push can hold up its weight. Some ice stays above the surface, while most stays below it.
The two containers compare equal amounts before and after freezing. The lower picture shows how the surrounding water supports floating ice.
The floating ice on a pond is made of the same water as the liquid underneath. In cold weather, ice can form a lid on top while water remains below. A change too small for our eyes to see helps explain a change large enough to cover a pond.
About this explanation
The drawings explain ordinary freshwater ice in liquid water. The size change is enlarged for visibility, and the floating picture is not a measurement.
Starting knowledge: Recognize liquid water and ice and compare how much room two shapes occupy.
Sources
- American Chemical Society — Changing State—Freezing
- U.S. Geological Survey — Water Density