Ice floats on liquid water because solid ice is less dense than liquid water under ordinary conditions. When an object is placed in a fluid, buoyancy acts upward and depends on the weight of fluid displaced by the submerged volume. An object floats when it can displace enough fluid for the buoyant force to balance its weight while remaining partially above the surface.
Because the density of ordinary ice is about 0.917 grams per cubic centimetre near its melting point, while liquid freshwater has a density close to 1.0 grams per cubic centimetre near its maximum density, a given mass of ice occupies more volume than the same mass of liquid water. Consequently, a piece of ice can displace a volume of water whose mass equals the mass of the ice before the ice becomes fully submerged. This is the physical reason ice floats rather than sinking.
The unusual density relationship between water and ice originates in the molecular structure of water. A water molecule consists of two hydrogen atoms bonded to an oxygen atom and has a bent geometry with an uneven distribution of electrical charge. This allows neighbouring water molecules to form hydrogen bonds. In liquid water, hydrogen bonds are continually breaking and reforming as molecules move. The molecules can therefore pack relatively closely, although the arrangement remains dynamic and disordered.
When water freezes under ordinary pressure, the molecules become organised into a crystalline structure in which hydrogen bonding favours an open arrangement. The crystal lattice contains more empty space than the typical local structure of liquid water. This increases the volume occupied by a given mass and lowers the density. The molecular geometry and hydrogen bonding are therefore central to the macroscopic floating behaviour of ice.
The crystal structure of ordinary ice at Earth's surface conditions is commonly called ice Ih. Its hydrogen bonded arrangement produces a relatively open hexagonal structure. The exact molecular arrangement is more complicated than a simple drawing of widely separated hexagonal rings, because real ice contains disorder in the orientations of water molecules and can exhibit defects. Nevertheless, the key structural feature is that the solid lattice is relatively open compared with the average molecular arrangement in liquid water.
This is unusual because many substances become denser when they solidify. In many materials, cooling reduces molecular motion and allows particles to pack more efficiently, increasing density. Water behaves differently over part of its temperature range because the formation of hydrogen bonded structures introduces additional open space. Ice therefore occupies more volume than an equal mass of liquid water under ordinary pressure.
Water itself has another unusual density property. Liquid freshwater reaches its maximum density at approximately 4 degrees Celsius under atmospheric pressure, rather than becoming steadily denser all the way down to its freezing point. As liquid water cools from higher temperatures towards 4 degrees Celsius, it contracts and becomes denser. Below approximately 4 degrees Celsius, the hydrogen bonded structure becomes increasingly important and the water begins to expand as it approaches freezing.
At 0 degrees Celsius, ordinary ice is less dense than the corresponding liquid water. This density maximum is extremely important for aquatic environments. In a typical cold freshwater lake, the densest water can sink towards the bottom until the deeper water approaches 4 degrees Celsius, while colder water remains nearer the surface. Eventually surface water can freeze, forming an ice layer that floats.
Floating ice has major environmental consequences. Because ice forms a layer at the surface rather than sinking to the bottom, it can act as a thermal barrier between relatively warmer water beneath and very cold air above. Ice and snow also have relatively low thermal conductivity compared with liquid water, particularly when snow contains trapped air. This can reduce the rate at which heat escapes from a body of water during winter.
As a result, many freshwater lakes do not simply freeze solid from the bottom upward under ordinary winter conditions. Aquatic organisms can survive beneath surface ice because liquid water remains below the freezing layer. The density anomaly of water therefore influences freshwater ecosystems, seasonal mixing and climate processes. Without this unusual behaviour, the thermal structure and biological conditions of many lakes and ponds would be very different.
The buoyancy of ice can be explained quantitatively using Archimedes principle. A floating object displaces a volume of fluid whose weight equals the weight of the object. Since ice is less dense than water, only a fraction of the ice needs to be below the water surface to displace a sufficient amount of water. The submerged fraction is approximately equal to the ratio of ice density to liquid water density under the relevant conditions.
For ice with density around 0.917 grams per cubic centimetre and water near 1.0 grams per cubic centimetre, roughly 92 percent of the ice volume is submerged and roughly 8 percent remains above the waterline, ignoring complications involving salinity, temperature and shape. This is why icebergs can have most of their volume below the surface. The familiar statement that only a small part of an iceberg is visible is therefore a direct consequence of density and buoyancy.
Not all forms of ice have exactly the same density, and the density of water depends on pressure, temperature and dissolved substances. Ice has numerous crystalline phases that form under different pressure and temperature conditions, and some high pressure ice phases have structures and densities different from ordinary ice Ih. Seawater also behaves differently from pure freshwater because dissolved salts lower its freezing point and alter its density.
Ice formed from seawater can have a different composition and density from ideal pure ice, although the solid ice lattice excludes much of the salt. These details matter in oceanography and glaciology. Ice floating in seawater affects sea level, ocean circulation and climate processes differently from land based ice melting into the ocean. Floating sea ice does not displace exactly the same amount of seawater as freshwater ice would displace freshwater because the fluid densities differ.
The statement that ice floats should not be interpreted as meaning that all frozen water or every solid form of water must float under every condition. Buoyancy depends on the density of the solid relative to the surrounding fluid. The ordinary surface ice familiar on Earth is less dense than liquid water at comparable conditions, which causes it to float. This unusual property ultimately arises from the molecular structure of water and hydrogen bonding.
The effect links molecular physics with large scale phenomena including lake ecology, sea ice, glaciers, iceberg behaviour and climate. It is also an important example of why macroscopic properties cannot always be predicted from the simplest intuition about cooling. Water's density maximum near 4 degrees Celsius and the open crystalline structure of ice are consequences of molecular interactions that give water several anomalous thermal and structural properties.
