Why does ice float on water?

Drop an ice cube into a glass of water and it floats. It seems so ordinary that most people never question it. But it is actually quite unusual. For almost every other substance in the world, the solid form is denser and heavier than the liquid form, so it sinks. Water breaks this rule, and that strange behavior is not just a fun fact. It is something that shapes life on our entire planet.

Whether something floats or sinks depends on its density, which means how much mass is packed into a given amount of space. If an object is less dense than the liquid it sits in, it floats. If it is denser, it sinks. So the real question is not simply why ice floats, but why frozen water ends up being less dense than liquid water. For most materials, freezing packs the molecules tighter and makes the solid denser. Water does the opposite.

The secret lies in how water molecules arrange themselves. In liquid water, the molecules are close together but constantly moving and jostling in a fairly random way. When water cools and freezes into ice, the molecules lock into a fixed, orderly pattern called a crystal lattice. Because of the shape of water molecules and the way they bond to one another, this orderly ice structure is actually more spread out than the liquid. The molecules are held slightly farther apart than they were as a liquid. More space between the same molecules means lower density, so ice is less dense than water and floats.

This quirk of water has enormous consequences. Because ice floats, when a lake or pond freezes in winter, the ice forms a layer on the top rather than sinking to the bottom. That floating ice acts like a blanket, insulating the water below and keeping it from freezing solid. Fish, plants, and other creatures can survive the winter in the liquid water beneath. If ice sank instead, bodies of water could freeze from the bottom up, which would be devastating for aquatic life. A simple floating ice cube is a small sign of one of nature's most important rules.

This article was written using information from Wikipedia.