Why does ice float on water?
Ice floats because it is about 9 percent less dense than liquid water. When water freezes, its molecules lock into an open crystal that takes up more space.
Most substances get denser when they turn solid, so their solid form sinks. Water is a rare exception. In liquid water, molecules move around and pack fairly closely. As water cools below about 4°C (39°F), hydrogen bonds start holding the molecules in a fixed, six-sided pattern with lots of empty space between them.
That open structure gives ice a density of about 0.92 grams per cubic centimeter, compared with about 1.00 for liquid water. Anything less dense than the liquid around it floats, which is why ice cubes bob at the top of a glass and roughly 90 percent of an iceberg sits below the surface.
Why it matters
Because ice forms on top, lakes and ponds freeze from the surface down. The ice layer insulates the water beneath it, so fish and other life can survive the winter in the liquid water below. If ice sank, many lakes would freeze solid from the bottom up.
Did You Know?
Water is at its densest at about 4°C (39°F), not at freezing. That is why the bottom of a deep lake stays at around 4°C all winter, even when the surface is frozen solid.