Science File / An exploration of issues and trends affecting science, medicine and the environment

Q: Why does ice float?

A: Water is a rare exception to the rule that materials get smaller as they get colder. Liquid water does become more dense as it cools--until it reaches its greatest density at about 39 degrees Fahrenheit. Further cooling then causes it to expand, becoming less dense. By the time the water freezes into ice, it is actually less dense than at any other temperature, and it floats in liquid water. This expansion caused by the change in density is why water in a sealed bottle shatters the bottle when it freezes. If ice did not float, ice formed on ponds during winter would sink to the bottom and the ponds would become completely frozen, killing everything in them. Many ponds would remain largely frozen solid throughout most of the year.