FactSentinel

Ice is less dense than liquid water, and that difference is the reason it floats. At 0°C, liquid water has a density of about 1.000 grams per cubic centimeter, while ice has a density of roughly 0.917 grams per cubic centimeter. Because ice is about 9 percent less dense, it displaces a volume of water heavier than itself, so buoyant force keeps it afloat with roughly 90 percent of its volume submerged. This is the principle behind the familiar sight of icebergs, where most of the mass sits below the surface.

The cause lies in molecular structure. Water molecules form hydrogen bonds with one another. When water freezes, these bonds lock the molecules into an open hexagonal lattice that holds them slightly farther apart than they are in the liquid state, where molecules move around and pack more closely. The result is a larger volume for the same mass, which means lower density. This is why water expands when it freezes, and why sealed bottles or pipes can burst in freezing temperatures.

A common misconception is that most substances behave this way. In fact, the solid form of nearly all other materials is denser than its liquid form and sinks in it. Water is an unusual exception. Another misunderstanding is that cold water is always denser than warm water. Pure water reaches its maximum density at about 4°C, and it becomes less dense as it cools further toward freezing. This property allows ice to form at the top of lakes, insulating the water below and helping aquatic life survive winter.