Freezing is a physical process that marks the transition of a substance from a liquid state to a solid state. This transformation occurs when the temperature of the liquid is lowered to its freezing point, which for pure water at standard pressure is 0 degrees Celsius (32 degrees Fahrenheit). While most liquids contract and become denser when they solidify, water exhibits an unusual exception. The way water molecules behave during this phase change is unique, resulting in a solid form with distinct properties.
The Molecular Dance: Slowing Down
Liquid water molecules are constantly in rapid, random motion throughout the volume of the liquid. This motion is a manifestation of their kinetic energy, which keeps them from locking into a fixed position. As the liquid’s temperature drops, heat energy is removed, causing the average speed of the molecules to decrease significantly.
This reduction in kinetic energy allows the attractive forces between the water molecules to exert a greater influence. Even in the liquid state, temporary attractions known as hydrogen bonds are continually forming and breaking. Cooling the water slows the molecular movement enough that these attractions begin to persist for longer periods, setting the stage for the rigid structure of ice to form.
Creating the Crystal Lattice
The formation of solid ice is governed by the specific way water molecules interact due to the polarity of their structure. Each water molecule consists of one oxygen atom bonded to two hydrogen atoms, creating a bent shape where the oxygen side carries a slight negative charge and the hydrogen sides carry a slight positive charge. These partial charges allow the hydrogen atom of one molecule to be attracted to the oxygen atom of a neighboring molecule, forming a structured hydrogen bond.
When the temperature reaches the freezing point, the slowed molecules have insufficient kinetic energy to break these hydrogen bonds as quickly as they form. The molecules lock into an orderly, fixed, three-dimensional arrangement that defines the ice crystal lattice. This structure is an open, repeating hexagonal pattern where each molecule is connected to four neighbors.
Why Ice Floats
The open structure of the ice crystal lattice is responsible for water’s most famous anomaly: its solid form is less dense than its liquid form. In the liquid state, water molecules are closely packed, even with the transient hydrogen bonds. When the water freezes, the permanent hydrogen bonds push the molecules into the fixed, open hexagonal arrangement, spacing them farther apart.
This fixed, open geometry means that a given mass of ice occupies a larger volume than the same mass of liquid water. The increased volume results in a lower density, causing ice to float on the surface of liquid water. The expansion during freezing can generate substantial pressure, which is why water freezing inside confined spaces, like plumbing pipes or glass containers, can cause them to burst.
Beyond Zero: Supercooling and Nucleation
The freezing process does not always initiate exactly at 0 degrees Celsius, especially in the absence of impurities. Water can remain in its liquid state even when cooled below its standard freezing point, a phenomenon known as supercooling. This occurs because the formation of the first tiny ice crystal requires a starting point, a process called nucleation.
In typical, impure water, this starting point is provided by a foreign particle, such as dust or a microscopic air bubble, which acts as a nucleus for the crystal structure to begin. If water is highly purified and kept perfectly still, it lacks these nucleation sites and can be supercooled to temperatures as low as approximately -48 degrees Celsius. Once a nucleus is introduced or the supercooled water is disturbed, the molecules rapidly align into the crystal lattice, causing the liquid to solidify almost instantly.