The precise temperature of ice water is a fixed point in science, used to calibrate thermometers and define temperature scales. When ice and pure liquid water are mixed together and allowed to reach a thermal balance, the temperature settles exactly at \(0^\circ\) Celsius (\(32^\circ\) Fahrenheit). This represents the standard condition for the coexistence of water’s solid and liquid states.
The Scientific Baseline Temperature
The temperature of \(0^\circ\text{C}\) is the point at which water freezes and ice melts, provided the water is pure and under standard atmospheric pressure. This specific temperature is known as the freezing point and the melting point. At this point, the liquid water and the solid ice are in thermodynamic equilibrium, meaning the rate of freezing equals the rate of melting.
This equilibrium is the reason a glass of ice water remains cold for a long time. If the temperature were to drop slightly below \(0^\circ\text{C}\), the liquid water would begin to freeze until all of it turned to ice. Conversely, if the temperature were to rise even fractionally above \(0^\circ\text{C}\), the ice would begin to melt until only liquid water remained.
The Physics of Latent Heat
The remarkable stability of ice water’s temperature is explained by the concept of Latent Heat of Fusion. This refers to the large amount of energy required to change water from a solid (ice) to a liquid state without causing a temperature change in the mixture. For water, this value is approximately \(334\) Joules of energy for every gram of ice melted.
When heat energy enters a glass of ice water from the warmer surrounding air, that energy is not used to increase the kinetic energy of the water molecules. Instead, the absorbed heat is immediately used to break the strong intermolecular hydrogen bonds holding the water molecules in the rigid crystalline structure of ice. This effectively acts as a thermal buffer.
Only once all the ice has melted is the added energy available to increase the kinetic energy of the water molecules, which is recorded as a temperature rise. This mechanism makes ice water an effective and stable coolant; the melting ice continuously draws heat from the liquid water, ensuring the entire mixture remains fixed at \(0^\circ\text{C}\) until the last piece of ice disappears.
How Impurities Affect Ice Water Temperature
The precise \(0^\circ\text{C}\) baseline applies only to pure water at standard sea-level pressure. The presence of dissolved substances, or impurities, significantly alters this temperature through a process known as freezing point depression. When a solute like salt is added to water, the dissolved particles interfere with the ability of water molecules to arrange themselves into the orderly crystal structure of ice.
This interference requires the mixture to be cooled to a lower temperature before the water can successfully freeze. For example, typical seawater contains about \(3.5\%\) salt and has a freezing point of approximately \(-2^\circ\text{C}\). Adding salt to a maximum concentration can lower the freezing point even further, sometimes down to about \(-21^\circ\text{C}\).
Extreme pressure can also have a minor effect, as seen at the base of thick glaciers where the weight can cause ice to melt at temperatures slightly below \(0^\circ\text{C}\). The presence of dissolved impurities is the primary factor that causes ice water to be slightly colder than the \(0^\circ\text{C}\) standard.