Why does ice float?

Particles that make up solids have attractions to each other that hold the particles very close in fixed positions. Particles in liquids have more kinetic energy, so they are able to move around more, but they are still held very close together. In most substances, the particles in a liquid state are a bit farther apart then they are in a solid state. Because liquid particles are not packed as tightly together, the liquid state of a substance takes up more space than the same mass of the solid state. This difference in density means that a piece of solid will generally sink to the bottom of a liquid of the same substance.

Water does not follow the predicted pattern of density that is found in most other substances. Remember, the ice cube floats in water, while the oil cube sinks in oil. If the solid ice were denser than liquid water, it would sink. But it does not, which means that ice is less dense than liquid water.

As with other substances, the water particles vibrate in place in ice and slip past one another in liquid water. Ice is similar to many solids because water particles are arranged in a specific pattern that repeats throughout ice. However, this patterns differs from the pattern of particles in most solids because the spacing between water particles in ice is greater than the spacing between water particles in liquid water. As a result, fewer water particles in ice are packed into the same amount of space compared to liquid water. So, ice is less dense than liquid water.

The lower density of ice compared to liquid water does not only mean that ice cubes float in your water cup. This property of water is important for life on Earth because both liquid water and ice are part of Earth’s hydrosphere. The hydrosphere, which contains all the water on Earth’s surface, is an essential part of the Earth system. It interacts with the atmosphere and biosphere in many ways. For example, large pieces of floating ice provide penguins, polar bears, and walruses with a resting place as they look for food in the ocean.