Many people have observed that ice floats in water, which seems to defy common sense, as most people expect solid forms of liquids to be heavier than their liquid forms. Like everything else that floats, ice floats because it is less dense than water, demonstrating the property of buoyancy. In addition, ice is also an illustration of the fascinating properties of water, a liquid which behaves in some very unusual ways.
When you see ice float, you are looking at a demonstration of the scientific principle of buoyancy. Objects are said to be buoyant when they are able to displace their own weight in water before sinking. As the object settles in the water, an equal force pushes up against the bottom of the object, causing it to float, rather than sink. When the surface area and density of a substance are right, that substance floats rather than sinking to the bottom.
In the case of most liquids, the cooler the liquid is, the more dense it becomes. Water, however, reaches its maximum density above the freezing point. As water freezes, its molecules arrange themselves into a matrix, creating spaces between them which didn't exist before. As a result, ice is around 9% less dense than water at its densest point, which makes ice float in water, rather than sinking.
However, you may have noticed that when you look at ice and water together, the ice doesn't just float on the surface; part of the ice is typically submerged. Sometimes, an entire piece of ice will be submerged, as is the case with ice cubes in a glass. In the case of something like an ice sheet in the Arctic, the huge surface area of the ice ensures that it will be buoyant, because it will displace its own weight before it sinks. A small piece of ice like an ice cube, on the other hand, may sink before it can displace its own weight.
The fact that ice floats is a fortunate thing for the natural environment. If ice sank to the bottom, the world's oceans, lakes, and rivers would slowly freeze from the bottom up, eventually turning into solid ice. Instead, ice floats along the surface, gradually melting down in response to temperature changes, except for the Arctic and Antarctic regions, where permanent sheets of ice exist year round because the temperatures are so cool.