Ice water is water in a state of thermal equilibrium with ice, meaning both solid and liquid phases coexist simultaneously. This coexistence occurs at a specific, fixed temperature defined by the freezing point of water. For pure water at standard atmospheric pressure, this temperature is precisely 0° Celsius (32° Fahrenheit).
The Thermodynamic Baseline
The scientific standard for the phase transition between water and ice is set at 0°C (273.15 Kelvin). This baseline applies only to chemically pure water maintained at standard atmospheric pressure. The presence of both liquid water and solid ice guarantees this exact temperature because the system is balanced between freezing and melting. Any heat added causes ice to melt, and any heat removed causes water to freeze, but the mixture’s temperature remains constant. This reliability means a mixture of pure ice and water is historically used to calibrate precision thermometers.
The Role of Latent Heat
The mechanism forcing the ice-water mixture to hold steady at 0°C is the latent heat of fusion. When ice melts, it must absorb a significant amount of energy to change the state of the water molecules, not to raise the temperature. This phase change energy, known as the enthalpy of fusion, is approximately 334 Joules per gram of ice melted. This energy is absorbed from the surrounding liquid water, preventing the water’s temperature from rising.
The absorbed heat is consumed in breaking the hydrogen bonds holding the ice’s crystalline structure together. Until these bonds are broken, water molecules cannot gain enough thermal energy to increase kinetic movement, which would register as a temperature rise. This energy absorption acts as a thermostat, ensuring the mixture remains at 0°C as long as ice is present, even if heat is added. Only once all the ice has melted will the water begin to absorb the added heat and raise its temperature above the freezing point.
Real-World Variables
In practical scenarios, the temperature of ice water can deviate slightly from the ideal 0°C due to real-world factors. Most tap water contains dissolved gases and minerals, which act as impurities and cause minor freezing-point depression. These solutes interfere with ice crystal formation, slightly lowering the equilibrium temperature where ice and water coexist. A more significant factor is the lack of thermal equilibrium, where water far from the ice cubes may be marginally warmer than the water directly adjacent to the melting ice.
The most dramatic variable is the deliberate addition of a solute, such as salt, which greatly magnifies freezing-point depression. When salt dissolves in ice water, the resulting ions disrupt the water’s ability to freeze, requiring a much lower stabilization temperature. For instance, a concentrated salt solution can lower the equilibrium temperature to as low as -21°C, a principle used in de-icing roads and making homemade ice cream. This effect explains why standard, impure ice water may register a temperature fractionally above 0°C, while a salt-ice mixture can intentionally achieve temperatures far below the standard freezing point.