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(Video) Why Does Ice Float in Water? - George Zaidan and Charles Morton

by TED-Ed

Have you ever wondered why solid water (ice) floats on top of liquid water? What makes water so special in the first place? Check out this animated video to learn how water's ability to form hydrogen bonds affects its density. You'll also see what would happen if ice did sink. It's a scary thought to have!

Density of Water in Different Phases

Solids, liquid, and gasses have their own sets of properties and behaviors. The main changes that can occur during these phase changes include the breaking or formation of intermolecular interactions – like hydrogen bonds – and the breaking or formation of covalent and ionic bonds.

Due to water’s bent geometry and the atoms that comprise it, water is a polar) molecule. It has a permanent dipole. The electronegativity difference between hydrogen and oxygen is so large that water forms hydrogen bonds. Hydrogen bonds are intermolecular bonds or “attractions” between a very electronegative atom and hydrogen. The only elements that participate in hydrogen bonding are nitrogen, oxygen, and fluorine.

Within liquid water, there is a dynamic breaking and reformation of hydrogen bonds. The water molecules continually move past each other. What allows for the movement of the $H_{2}O$ molecules? The kinetic energy. Remember, the temperature measures the average kinetic energy. As the temperature starts to drop, the water's translation motion slows. As the temperature continues to drop, the hydrogen bonds will eventually completely stop breaking and reforming. This halting of movement and breaking of intermolecular attraction causes the rigid structure of solid water.

Within solid water, there is quite a bit of space between molecules compared to liquid water. The ice's increase in empty space is caused by the rigidity of the “frozen” intermolecular attractions. If we were to look at the same sized area in ice and water, there would be more $H_{2}O$ molecules in liquid water than in ice. That's why water expands upon freezing. This is also due to the decrease in density caused by the halting of dynamically broken and reformed intermolecular bonds!

This difference in empty space affects the density of the molecule. The fewer molecules of $H_{2}O$ there are in a volume of water, the lighter the mass will be therefore decreasing the density.

$$Density=mass\div{volume}$$

As a result, it makes sense that ice has a density of .92 g/mL while liquid water has a density of 1.0 g/mL. This also explains why ice floats in liquid water.

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