Pure water, under standard atmospheric pressure at sea level, changes from a liquid to a solid at 0 degrees Celsius (°C), which is equivalent to 32 degrees Fahrenheit (°F). This temperature marks the point where liquid water and solid ice can coexist. The freezing point is a fundamental scientific constant, but it can be altered by changes in pressure or the presence of dissolved substances.
Defining the Standard Freezing Point
The standard freezing point (0°C or 32°F) is established under standard atmospheric pressure, defined as one atmosphere (atm) at sea level. The Celsius scale was historically defined by setting the freezing point of water at 0°C and the boiling point at 100°C.
The Fahrenheit scale places the freezing point at 32°F, a system still widely used for everyday temperature reporting in some countries. In scientific contexts, the Kelvin scale is often used, where the standard freezing point is 273.15 Kelvin (K). This value represents the temperature where molecular motion theoretically stops at absolute zero, making it the scientific standard for precision.
The Molecular Process of Solidification
The transition from liquid water to solid ice is a physical process driven by the removal of energy. For water to solidify, a quantity of energy known as the latent heat of fusion must be removed from the liquid. The temperature remains constant at the freezing point until the phase change is complete.
Liquid water molecules constantly form and break temporary connections called hydrogen bonds. As the temperature drops, the molecules lose kinetic energy, and these bonds become more permanent. Once the liquid reaches the freezing point, the water molecules arrange themselves into a highly organized, repeating crystalline lattice structure. This ordered arrangement is less dense than liquid water, a unique property allowing ice to float.
External Variables That Alter the Freezing Point
The presence of dissolved substances, known as solutes, is the most common way the freezing point of water is lowered, a phenomenon called freezing-point depression. Solutes like salt or alcohol interfere with the ability of water molecules to form the organized crystalline structure required for ice. This disruption means the temperature must drop further to initiate solidification.
This principle is applied by using salt on icy roads, which can depress the freezing point to approximately -21°C (-6°F), depending on the concentration. Antifreeze in car engines uses compounds like ethylene glycol to keep the coolant liquid far below 0°C. The extent of the freezing point lowering depends only on the concentration of the dissolved particles, not their chemical identity.
Pressure also slightly affects the freezing point. Unlike most substances, increasing the pressure on water actually lowers its freezing point, though the effect is minimal under normal circumstances. This unusual behavior occurs because solid ice is less dense than liquid water, meaning increased pressure favors the more compact liquid state.
Supercooling
Supercooling occurs when pure water remains liquid even when cooled below 0°C. This state happens when there are no impurities or rough surfaces, called nucleation sites, for the first ice crystals to form. If supercooled water is disturbed or a nucleation site is introduced, the water can freeze almost instantaneously. Extremely pure water has been observed to remain liquid down to about -48°C before freezing spontaneously.