Now that winter has finally arrived here in Happy Valley and the ice outside is enough to keep even you in bed, have you ever wondered why it is that ice floats on top of water? Why is it that your solid ice cubes float to the top of your glass of water? Nearly every solid, if placed in its liquid form, will sink to the bottom. Luckily for us, the properties of water are different.
The meaning behind this mystery lies in the different properties of solid and liquid water. Unlike most other substances on Earth, the solid form of water floats on the liquid form. This is caused by the change in density, the amount of mass in a volume. With the exception of water, most substances on Earth become denser as they become colder. The solid ice will float because its density is lower than that of water. It is about 9% less dense than water. The denser water sinks to the bottom forcing the less dense ice to the surface.
What makes water molecules different from other molecules is that they attract each other in an organized fashion. As the water cools, the molecules begin to bind to each other, forming a hexagonal pattern. Water is at its densest point at 4C. After that point, the water molecules move very slowly and attract to each other. In most substances, the molecules are more tightly packed together in solid form. But in ice, the hexagonal pattern of the attracting water molecules leaves empty spaces. This is why your water expands when making ice cubes. The empty space between the hexagonal shapes makes the solid form less dense than the liquid form so that it floats to the top.
Thanks to this oddity of physics, the water in our oceans and seas remain in liquid state. If the solid form of ice happened to be denser than water, the ice would sink to the bottom. If this happened, the ice on the bottom would begin to freeze up toward the surface. Eventually, nearly all the water on Earth would become solid ice and never melt. Luckily, ice floats and remains on the surface so that the water underneath remains in liquid form.
This trait of physics also applies to other forms of water. "Heavy water" is used to cool nuclear reactors. An ice cube of heavy water will sink in ordinary water. Ordinary water contains two hydrogen atoms, but heavy water has two deuterium atoms. This causes a difference in weight. Deuterium atoms weigh about twice as much as hydrogen atoms and the extra mass of the deuterium adds enough weight so that an ice cube of heavy water sinks in ordinary water. But like ordinary water, an ice cube of heavy water will float to the top of a glass of heavy water.
So on your next icy Monday morning or tall glass of ice water, remember that density causes solid ice to float on liquid water.