The freezing point of a substance is the precise temperature at which its liquid and solid phases exist in equilibrium. Although often treated as a constant, this temperature is dependent on the surrounding pressure. Changes in pressure shift the balance between the two states, requiring a corresponding temperature adjustment to maintain equilibrium. The outcome is not universal; the freezing point of some materials increases with pressure, while for others, it decreases, a difference determined by the material’s atomic structure.
How Pressure Affects Most Substances
For the vast majority of known materials, including metals, organic compounds, and minerals, an increase in external pressure causes the freezing point to rise. This means a higher temperature is needed to keep the substance liquid when subjected to pressure. The solid state of these materials is typically denser and occupies less volume than the liquid state.
When a material solidifies, its atoms pack into a tight, ordered crystalline lattice, resulting in a more compact structure than the liquid’s disordered arrangement. Applying external pressure favors the phase that takes up the least amount of space, which is the solid phase. To melt this denser solid under increased pressure, extra thermal energy must be supplied to force the molecules apart into the less dense liquid form, thus increasing the freezing temperature.
The Unique Behavior of Water and Ice
Water stands out as a major exception to this general rule, exhibiting anomalous behavior. When liquid water freezes into common hexagonal ice, it expands, meaning the solid phase is less dense and occupies about nine percent more volume than the liquid. Because of this volume increase upon freezing, applying pressure to water has the opposite effect compared to most substances: it lowers the freezing point.
The unique structure of ice is responsible for this effect, as water molecules form an open, hydrogen-bonded tetrahedral lattice upon freezing. This ordered arrangement creates empty space within the crystal structure, making it less dense than the liquid where molecules are packed more closely. When pressure is applied to ice, it shifts to the state that occupies less volume—the liquid water state. This shift causes the ice to melt at a temperature below the standard 0°C point to alleviate the pressure.
Why Density Determines the Outcome
The fundamental physics governing how pressure affects the freezing point is based on the principle that a system at equilibrium shifts to counteract an applied stress. When pressure is increased on a substance at its freezing point, the equilibrium between the liquid and solid phases must shift to reduce the overall volume.
If the solid is denser than the liquid, the system reduces volume by favoring the solid phase, which pushes the freezing temperature higher. Conversely, if the solid is less dense, as in the case of water, the system reduces volume by favoring the liquid phase, which pushes the freezing temperature lower. The difference in volume between the solid and liquid states dictates the slope of the phase boundary on a pressure-temperature diagram. A positive volume change upon freezing results in a negative slope, meaning higher pressure leads to a lower freezing temperature.
Real World Examples of Pressure Induced Freezing Changes
The effect of pressure on the freezing point manifests in several natural and industrial scenarios. One common example is the pressure melting under an ice skate blade, where high pressure exerted on the narrow edge was historically thought to lower the melting point, creating a thin layer of lubricating water. While direct pressure only accounts for a fraction of the melting, it illustrates the principle for water.
High-Pressure Ice Forms
Under extreme conditions, such as those found in the interiors of icy moons or large exoplanets, immense pressure forces water to abandon its open structure. This results in exotic, high-density forms of ice, such as Ice VII and Ice X. These crystalline phases are more compact than liquid water, causing the freezing point to increase drastically at gigapascal pressures. On Earth, these high-pressure forms are only observed in laboratory experiments using diamond anvil cells.
Industrial Applications
In metallurgy, the rule that pressure increases the freezing point is exploited in processes like squeeze casting or high-pressure die casting. Applying high pressure to a molten metal during solidification forces the material into its denser solid state. This helps eliminate internal defects like porosity and refines the microstructure of the finished component. This industrial technique ensures the production of stronger, higher-quality metal parts by using pressure to control the phase transition.