Why Does Ice Float?
In short, ice floats because it is less dense than water. The only factor that determines if something floats or not is its density. Air is less dense than water, which is why beach balls float. Rocks are denser, that is why they sink, regardless of their weight.
Most of us have probably never put much thought into what ice floating really means. It is something we have just accepted. But when we think about it, ice is unique in this sense. There is not a single other solid that does not float, so why does ice float?
Ice is Unique!
Generally, the solid version of a compound is denser that the liquid version. As the temperature rises, molecules move less and get closer together. Eventually, the molecules form strong, tight bonds and almost stop moving. That is not the case with water though. Ice is one of the few solids that is less dense than the its liquid counterpart. The reason is because of the hydrogen bonds.
Water and ice are held together by covalent bonds and hydrogen bonds. As water freezes, the hydrogen bonds create a hexagonal structure, which pushes them away from each other. Since the bonds are far away from each other, there is more empty space, so they are less dense.
Cold water is still denser than warm water though. Before water completely freezes, it follows the same properties most other liquids do. The particles move less and get closer to each other. It is not until water freezes that the hydrogen bonds move the particles away from each other. Since cold water is denser, the warmest water will always be at the top of any body of water.
Real Life Examples
You have probably heard of pipes bursting in the winter, this is because the water freezes. When a pipe is filled with water, and it gets too cold, the water will freeze. Since ice takes up more space, it needs to go somewhere. The water cannot move forward or backwards, so it has to move out and burst the pipe.
If you ever put a full bottle in the freezer, you likely returned to it popped or at least stretched out. This is once again the result of ice needing more space for the same amount of molecules.
Does this Matter?
Yes! If ice did not float, we would not be able to live on this planet. Water is actually at its densest at about 40°F/4°C, so cold water sinks, but ice floats. If ice did not float, the bottoms of rivers and lakes would freeze every winter. All the animals living in them would die. The bottom of the ocean would also freeze, so no deep-sea animals there either. There would be no animals in either pole since there would be no ice in the poles. The ice in the poles would be at the bottom of the ocean.
Most ecosystems in the world would cease to exist if ice was denser than water.
Cold water going to the bottom is also very important for the water cycle. The water cycle depends on this water travelling to different points.
Why Can Humans Float?
Humans can only float in certain situations. Human lungs are filled with air and help us float. Our lungs act as big balloons inside our bodies. This is why it is easier to float when we breathe in. Fatter people also float easier. Since fat is less dense than muscle, people with more fat float easier than others.
The original theory behind objects floating is known as Archimedes’ principle after the Greek philosopher and mathematician Archimedes.