Why Does Ice Float?

In general, things float on water or other liquids because they don’t have as much mass as the water. Simply put, objects that float are less dense. Because stones, bricks, non-porous metals etc. have more mass they displace the water and sink.

Water freezes at 32 degrees Fahrenheit and when it does it becomes less dense than liquid just a few degrees warmer. This is primarily due to a process called bonding. Water is composed of two oxygen molecules and one hydrogen molecule. With water this bonding involves hydrogen molecules that have been found to separate the oxygen molecules and keep them apart. Scientific research describes this as a lattice formation of crystals. We call it ice.

If we were to get a magnified view of water in its liquid state we would see that the two oxygen molecules are much closer than they are in the solid state of ice. Water in its liquid state is denser than ice in most situations – there are more molecules in each square inch of liquid water than in the ice.

We may well be confused by the fact that the solid form of water (ice) is less dense than the liquid form. This is the opposite of what generally happens with other materials in their liquid and solid states. Most other liquids become denser when they solidify. But the special properties of water make this amazing liquid react differently.

Another term that might help with our understanding of why ice floats on water is “buoyancy.” This can be described as the ability of an object to displace its weight before sinking. Blocks of ice might be large but they remain buoyant because of the lattice effect described above. Their weight is spread over a larger area because of the spacing created by hydrogen bonding.

So, buoyancy involves the density of the water and the relative density of the ice. This helps explain why objects float more readily in salt water as opposed to fresh water. Salt water is denser than fresh water and that helps large objects such as ships and icebergs stay afloat. It’s also easier to swim in the ocean because of this effect.

Another key to envisioning this process is the surface area of the object in relation to its weight and density. This helps explain why ice cubes might sink in a liquid while a huge iceberg floats on the ocean surface.

Most scientific measurements show that ice is approximately 9 or 10 percent less dense than water in its liquid state. We also know that bodies of water freeze from the top down, a phenomenon that is also due to the lower density of ice. Nature provides a way for fish to continue to live in lakes and streams because the water remains in liquid state below the surface.

Finally, we may be able to get a better idea of how a specific volume of liquid water will “grow” or take up more space. Water placed in a small, sealed container can change to ice and expand enough due to the bonding process that it will break out of the container. This is an interesting “power” ice has because of the internal changes it goes through.