A: Thanks for the question James! Ice floats because when water freezes (that is, when the temperature drops below 32 degrees Fahrenheit or 0 degrees Celsius) it becomes less dense than when it is in its liquid form. That is, if you had a cup of water and a cup of ice, there would be less “stuff”—fewer water molecules—in the cup of ice than the cup of water. The reason this happens is because of water’s hydrogen bonds.
Each molecule of water is made of one oxygen atom and two hydrogen atoms which look like this. This is why we can also write ‘water’ as ‘H2O’.
The oxygen atom of one molecule will be attracted to a hydrogen atom on another molecule. This attraction is called a hydrogen bond, because once it is formed it is not too easy to pull the molecules far away from each other (and, of course, because it involves hydrogen!).
When the water is a liquid, these hydrogen bonds are fairly flexible, allowing
the water molecules to slide past each other. However, when the water freezes
the hydrogen bonds become less flexible, and they hold the water molecules
farther away from each other than they would if they were liquid. Because the
molecules are held farther away from each other, there are fewer of them in a
cup than there would be in a cup of liquid water. This is what makes the ice
less dense, and is the reason why the ice floats on top of liquid water!
the water molecules to slide past each other. However, when the water freezes
the hydrogen bonds become less flexible, and they hold the water molecules
farther away from each other than they would if they were liquid. Because the
molecules are held farther away from each other, there are fewer of them in a
cup than there would be in a cup of liquid water. This is what makes the ice
less dense, and is the reason why the ice floats on top of liquid water!