Why does ice float?

What is happening?

Water molecules form hydrogen bonds. In ordinary ice, these interactions organise molecules into an open crystalline structure that leaves more space between them than in liquid water.

Because the same mass occupies a larger volume, ice has a lower density and floats. Liquid water reaches its maximum density near 4 °C under ordinary pressure.

What changes it?

Floating ice matters far beyond a glass of water. A surface layer of ice can insulate the liquid below, helping lakes avoid freezing solid from the bottom upward.