Like anything that floats on water, ice floats because it has a lower density than water. This seems strange to most people (myself included at times) for a couple reasons. In most of our experiences, solids are supposed to sink. And then, ice IS water, isn’t it? Let us indulge both ideas, and see where they lead.
We think of solids as denser than water in general, because most of the solids we work with are. But there are exceptions, and most people are familiar with several. The wood of many trees, for example, will float. So will Styrofoam, and a number of plastics. Obviously then, a floating solid is nothing out of the ordinary.
Ice is indeed the solid form of water. Steam is the gaseous form of water, as is “water vapor”. Both the solid and gas forms of water get special names. Liquid water though, we just call that water, and that causes a little bit of confusion right off the bat. So, yes, despite the naming conventions, ice is a form of water.
Not only is ice solid water, it is a crystalline form of water. You’ve probably seen ice crystals on windowpanes in winter (if you live somewhere cold, anyhow). Similarly, ice cubes, ice rinks, and icicles all are composed of water molecules, arranged in a crystal structure. This crystal structure is rigid, and leaves relatively large empty spaces between the water molecules. Compare that to liquid water, where the water molecules are free to move, and they pull one another close using “hydrogen bonding”. (I’ll spare you the details here – go read my article about “intermolecular interactions” if you really want to know.) As a result, there is a lot less space between the molecules of liquid water, and it is more dense than ice as a result. Since ice is less dense than water, it must float.
Interesting asides:
Air bubbles (and other crud) are the cause of the white bubbly appearance of the center of ice cubes and other ice objects. Everything that was dissolved in the water before freezing had to drop out as the water froze, but since it froze from the outside edges inwards, they were all pushed to the middle before doing so. Those air bubbles can add a little buoyancy to the ice cube, but are not necessary for the ice to float.
Ice actually has more than one crystalline form. As temperatures get colder (or pressures get higher), the ice crystals undergo changes in arrangement to accommodate the new environment better. We only observe one or two of these ice “phases” in nature. Other forms can be created in the lab, or in more extreme environments, such as planets with massive gravity (high pressure) and/or far from the sun or other star (low temperature). There are over a dozen different crystal structures of ice. (There are a few amorphous – non-crystalline – ones as well.)
