The vast majority of liquids contract as they transition from a liquid to a solid state, a natural consequence of molecules slowing down and packing more tightly together. The phenomenon of a substance increasing its volume upon freezing is a highly unusual physical behavior. This counterintuitive expansion means the solid form is less dense than the liquid form, causing the solid to float on its own liquid. This unique characteristic has profound consequences for life and geology on Earth.
Why Water Is Different
The reason water behaves differently lies in its molecular structure and the forces holding it together. A water molecule, composed of two hydrogen atoms and one oxygen atom, possesses a bent shape that makes it highly polar. This polarity causes an attraction between the positive charge on the hydrogen atom of one molecule and the negative charge on the oxygen atom of a neighboring molecule, forming hydrogen bonds.
In liquid water, these hydrogen bonds are constantly breaking and reforming, allowing the molecules to remain in a relatively close, disorganized arrangement. This liquid state allows for maximum density, which water reaches at approximately four degrees Celsius. As the temperature drops further toward the freezing point, the molecules slow down, and the hydrogen bonds become stable and fixed.
At the moment of freezing, these fixed hydrogen bonds force the water molecules to arrange themselves into a highly ordered, open crystalline structure, specifically a hexagonal lattice known as Ice I\(_\text{h}\). This precise, geometric arrangement creates significant empty space, similar to a microscopic honeycomb. Because the molecules are held farther apart in this solid lattice than they were in the dense liquid, the overall volume increases by about nine percent upon freezing.
Practical Impacts of Ice Expansion
The approximately nine percent volume increase of water as it turns to ice generates immense pressure that profoundly impacts both natural and engineered systems. In geology, this force is the engine behind frost wedging, a form of mechanical weathering. When water seeps into tiny cracks and fissures in rocks or pavement, the subsequent expansion upon freezing can exert pressures upwards of 2,100 pounds per square inch.
Repeated cycles of freezing and thawing cause these cracks to widen progressively, leading to the fragmentation of rock formations and the breakdown of roads and sidewalks. This relentless force is responsible for shaping landscapes, contributing to the formation of scree slopes, and creating potholes in asphalt. In engineered structures, this expansion can be destructive, most notably causing water pipes to burst in cold weather.
The expansion is also a fundamental condition for the survival of aquatic life in cold climates. Because ice is less dense, it floats, forming an insulating layer on the surface of lakes and ponds. This floating ice shield prevents the water beneath from freezing solid, maintaining a liquid environment at the bottom where temperatures remain at about four degrees Celsius. If water became denser when frozen, ice would sink, leading to the progressive freezing of bodies of water from the bottom up, which would devastate entire aquatic ecosystems.
Substances That Contract When Freezing
Water’s behavior is a rare exception to the standard physical rule governing phase change for the vast majority of materials. For most substances, a decrease in kinetic energy as a liquid cools allows the molecules to settle into a more compact, orderly arrangement when they solidify. The atoms or molecules in the resulting solid crystal are packed more tightly than in the liquid state, causing contraction in volume and an increase in density.
Liquids like oils, alcohols, and liquid metals such as mercury all contract and become denser upon freezing. If a solid form of these materials were placed into their respective liquids, the solid would immediately sink.
While water is the only common substance to expand upon freezing at standard atmospheric pressure, it is not entirely alone. A small number of other elements and compounds also form open crystalline structures that are less dense than their liquid counterparts. These rare exceptions include elements like silicon, gallium, germanium, antimony, and bismuth, a property sometimes exploited in specialized casting to ensure sharp, detailed molds.