Water expands when it freezes due to the way hydrogen bonding organizes water molecules into a crystalline structure. In liquid water, molecules move rapidly and hydrogen bonds constantly break and reform, allowing them to pack relatively close together. As temperature drops toward freezing, molecular motion slows and hydrogen bonds stabilize. The molecules then arrange into a rigid hexagonal lattice structure that is more open and spacious than the liquid configuration. This geometric arrangement forces molecules further apart than they were in liquid form, causing the same number of molecules to occupy a larger volume. The result is that ice is approximately 9% less dense than liquid water.

This expansion is unusual because most substances become denser when they freeze. Water's behavior is distinctive because of both the specific shape of the water molecule and the particular way hydrogen bonds function between water molecules. Before complete freezing, water exhibits another unusual property: it reaches maximum density at approximately 4°C, then expands slightly as it continues cooling to 0°C before the more pronounced expansion occurs during the phase transition to ice.

This property has profound real-world consequences. Because ice is less dense than liquid water, it floats rather than sinking. This means bodies of water freeze from the top down rather than solidifying completely from the bottom up, allowing aquatic life to survive beneath the ice layer. The floating ice also provides insulation that slows further freezing. On the practical side, the expansion of freezing water exerts significant pressure that can crack water pipes, bottles, and rocks, making winter pipe bursting a common household problem.