The question of whether running water freezes at a different temperature than still water is rooted in physics and thermodynamics. While the temperature at which water must freeze is a constant, the temperature at which it does freeze in a moving state is far more complex. The difference lies not in changing the fundamental freezing point, but in delaying the process through kinetic and heat transfer effects. This phenomenon explains why rivers and faucets can remain liquid even when the air temperature is well below freezing.
The Thermodynamic Freezing Point of Water
The thermodynamic freezing point is the defined temperature at which liquid water and solid ice can coexist in equilibrium under standard atmospheric pressure. For pure water, this constant temperature is precisely 0 degrees Celsius (0°C) or 32 degrees Fahrenheit (32°F). At this point, the phase change from liquid to solid occurs as water molecules lose kinetic energy, allowing them to arrange into a rigid, crystalline lattice structure. For this transition to occur, the liquid must reach a point where the solid state is thermodynamically favored over the liquid state.
The Role of Motion in Delaying Freezing
Water motion does not change the defined thermodynamic freezing point of 0°C, but it significantly alters the kinetics of the freezing process. Flowing water appears resistant to freezing primarily because movement is a highly effective way to manage heat loss. The continuous churning and mixing of the water body prevents the formation of a stable, cold surface layer.
Heat Transfer (Convection)
This mixing process, called convection, constantly brings slightly warmer water from the lower depths up to the surface. As the surface water loses heat to the cold air, it is immediately replaced by the warmer water below, effectively resetting the cooling process. This prevents the entire water mass from reaching 0°C uniformly and stops the sustained heat loss needed for ice formation.
Crystal Disruption
Furthermore, the kinetic energy of the moving molecules actively disrupts the initial formation of the ice crystal lattice. For water to solidify, its molecules must slow down enough to align themselves into the fixed hexagonal structure of ice. The physical force of flowing water breaks apart any nascent microscopic ice crystals that begin to form. This requires a greater energy loss or a longer time exposed to sub-freezing temperatures before freezing can finally occur.
Supercooling and the Initiation of Ice Crystals
The resistance of running water to freezing is closely linked to the phenomenon of supercooling, where water remains liquid below its freezing point of 0°C. This supercooled state requires a physical trigger, known as a nucleation site, to initiate the formation of ice crystals. Nucleation sites are typically microscopic impurities, dust particles, air bubbles, or tiny imperfections on a surface. In moving water, the constant flow and turbulence dramatically reduce the chance for a stable nucleation site to exist long enough for a crystal to grow. The water is continuously mixed and agitated, preventing the necessary molecular stability needed to transition from the liquid to the solid phase. When supercooled water is finally disturbed or introduced to an ice seed, the liquid rapidly and almost instantly freezes.
Factors That Truly Change Water’s Freezing Point
In contrast to the kinetic delays caused by motion, certain physical factors genuinely alter the thermodynamic freezing point of water. The most common factor is the presence of dissolved impurities, such as salt or minerals, which introduces a colligative property known as freezing point depression. This property interferes with the water molecules’ ability to form an orderly ice structure. The result is that the solution must be cooled to a lower temperature to overcome the interference and initiate freezing. For example, the salt content in seawater typically lowers its freezing point to about -1.8°C.