Water’s behavior when transitioning from a liquid to a solid state is distinctive. Freezing is a phase change where a liquid transforms into a solid as its temperature decreases. While many materials solidify predictably, water exhibits peculiar characteristics during this process. These unique properties contribute to phenomena observed in nature and everyday life.
Molecular Changes During Freezing
Water (H2O) molecules are loosely associated through hydrogen bonds in their liquid form, continuously forming and breaking as they slide past one another. As liquid water cools, these molecules lose kinetic energy and slow down.
Upon reaching its freezing point, typically 0°C (32°F) under standard pressure, water molecules begin to arrange into a more structured pattern. Hydrogen bonds become stable, locking molecules into fixed positions. This molecular rearrangement results in an open, hexagonal crystal lattice structure, which is the solid form known as ice. This ordered, hexagonal arrangement is key to ice’s unusual physical properties.
The Unique Expansion of Water
Most substances contract and become denser when they solidify. Water, however, expands as it freezes. This expansion is directly linked to the hexagonal crystal lattice formed during solidification.
In this lattice, hydrogen bonds hold water molecules farther apart than in the liquid state. This creates empty spaces within the ice structure, leading to a lower density. Because ice is approximately 9% less dense than liquid water, it floats. This is a notable anomaly, as most materials are denser in their solid form.
Influences on the Freezing Process
The freezing point of water can be influenced by external factors. Impurities, such as salt, are a significant factor. When salt dissolves in water, its ions interfere with the formation of the ice crystal lattice. This interference requires a lower temperature for water to freeze, a phenomenon known as freezing-point depression. Saltwater, for instance, freezes below 0°C.
Supercooling is another phenomenon, where water remains liquid below its normal freezing point. This occurs when there are no impurities or nucleation sites—tiny particles or surfaces—for ice crystals to form. Supercooled water will eventually freeze if disturbed, such as by shaking or introducing a small ice crystal, which provides the necessary nucleation site for molecules to align and solidify.