Why ice floats on water

Iceberg floating on water or ice cubes floating in lemonade are no surprise. In nature, however, it is quite an atypical phenomenon. But why is it that ice floats on water?

Ice is a clear hexagonal mineral that is abundant on Earth and in space. People use it for various purposes, such as cooling, sports or art.^[1]

Historically, water has density of 1 g/cm³ at 4 °C. As the temperature decreases or increases, the density of the water decreases. When the temperature reaches 0 °C at normal atmospheric pressure, it changes to a solid state. Density will decrease by approximately 8.3% and volume will increase by 9%. Ice then floats on water simply because it has a lower density than water.^[2]

However, to explain the lower density, we need to look at what happens to water at the molecular level. The individual water molecules are bound to each other by a weak hydrogen bond. A higher temperature of a substance means that the substance has a higher kinetic energy of individual molecules. It is strong enough in liquid water to break the hydrogen bond. Thus, the individual molecules are constantly bound to each other and subsequently torn off. When the temperature drops to 0 °C, the kinetic energy becomes so weak that it can no longer break the hydrogen bond and the water molecules form a hexagonal lattice. This spatial arrangement causes the individual molecules to be on average farther apart than the individual molecules of liquid water, which reduces the density 8.3 times.^[3]