Water expands by about nine percent when it freezes into ordinary ice.

Ordinary ice takes up about nine percent more volume than the same mass of liquid water.

Hydrogen bonding arranges water molecules into an open crystal structure.

Its lower density is why ordinary ice floats on liquid water.

Most familiar substances become denser when they solidify because their particles settle into a compact arrangement. Water's ordinary freezing behavior is an important exception. Each water molecule has a bent shape and an uneven electrical charge distribution. Attractions called hydrogen bonds connect neighboring molecules. In ordinary hexagonal ice, those connections favor an open framework with more empty space than the continually rearranging structure of liquid water near its freezing point.

The expansion concerns the same mass before and after freezing. It does not mean that water gains material or that each molecule swells by nine percent. Using approximate densities of one gram per cubic centimeter for liquid water and 0.917 for ice gives a volume ratio near 1.09. Exact values depend on temperature and pressure, so nine percent is a useful rounded description rather than a universal conversion factor applying to every form of ice under every condition.

Liquid water also behaves unusually before it freezes. At ordinary pressure, it becomes denser as it cools toward approximately four degrees Celsius. Below that temperature, further cooling increasingly favors more open molecular arrangements, and its density decreases again. This maximum matters in freshwater lakes: water near four degrees can sink beneath colder surface water. Once surface water freezes, the buoyant ice stays above the liquid instead of settling to the bottom and allowing successive exposed layers to freeze there.

Floating ice helps insulate the water underneath, especially when snow accumulates above it. That does not make every lake incapable of freezing solid; shallow water and severe conditions can still do so. Salinity, wind-driven mixing, and local climate also alter the simple freshwater picture. The central consequence is that an ice cover can leave a liquid habitat below, allowing aquatic organisms to survive winters in many environments where an imagined sinking solid would produce a very different seasonal structure.

The same expansion can damage plumbing and containers. Water trapped in a confined space cannot always accommodate the larger solid volume, and stresses can rise enough to rupture the surrounding material. Real pipe failures involve the movement and confinement of remaining liquid as ice develops, not merely a uniform block pushing equally everywhere. Freezing also contributes to rock weathering where water enters cracks, although actual frost processes depend on water supply, temperature history, and the mechanical properties of the rock.

“Ordinary ice” is a necessary qualification. Water has multiple crystalline solid phases at different pressures and temperatures. Some high-pressure forms are denser than liquid water, so the floating-ice rule is not a statement about every possible arrangement of H₂O. The familiar ice cube, a frozen pond surface, and a snow crystal normally involve the low-pressure structures relevant to everyday conditions. Linking the macroscopic expansion to a particular molecular arrangement explains both the usefulness and the limits of the nine-percent fact.

Freezing expansion is a statement about equal masses in different phases. It does not mean water gains matter when it freezes. Keeping mass and volume separate explains both why ordinary ice floats and why a container can be damaged even though none of its water has been added or removed.