Ice, the solid form of water, is often associated strictly with a temperature of 0 degrees Celsius (32 degrees Fahrenheit), the standard freezing and melting point at sea level atmospheric pressure. However, the idea that all ice must exist only at this single temperature is a misunderstanding of phase transitions. Ice can exist at different temperatures because the freezing point is merely the temperature at which water transitions between liquid and solid states. Once formed, ice is simply a cold solid that can be cooled far below the transition point, and its melting point can even be shifted by external forces.
Why Ice Stays at 0°C During Melting
The reason an ice and water mixture maintains a steady 0°C while melting is due to a thermodynamic principle called the latent heat of fusion. When heat energy is continuously added to ice at its melting point, that energy is not immediately used to raise the temperature of the water molecules. Instead, the incoming energy is completely absorbed to break the rigid hydrogen bonds holding the water molecules in their crystalline, solid lattice structure.
This energy, known as latent heat, is required to change the state of matter from solid to liquid without changing the temperature. Only after all the ice has been converted into liquid water can any additional heat begin to increase the temperature of the resulting liquid. Therefore, 0°C is the temperature of equilibrium, where both freezing and melting can happen simultaneously, effectively locking the temperature during the transition.
The Range of Cold: How Freezing Temperature Differs from Ice Temperature
While 0°C is the maximum stable temperature for the common form of ice (Ice I\(_{\text{h}}\)) under normal atmospheric conditions, ice can be significantly colder. Once the water has completely solidified, it acts like any other solid material, and its temperature will drop to match its surroundings. Ice deep inside a household freezer can easily reach temperatures of -18°C or lower.
In naturally occurring environments, such as the vast ice sheets of Antarctica, ice temperatures drop much further. Deep ice cores have recorded stable temperatures approaching -60°C, and the record cold on Earth’s surface has been measured at nearly -90°C. Theoretically, solid ice could be cooled down to absolute zero, approximately -273.15°C, though this is practically impossible to achieve. The standard hexagonal ice structure remains stable down to temperatures as low as -268°C.
Supercooled Water: Liquid Below Zero
The phenomenon of supercooling allows water to remain liquid well below its standard freezing point. This state is metastable, meaning it is temporarily stable but prone to sudden change, and it occurs when water is cooled slowly without impurities or disturbances. Water molecules do not automatically form a solid structure at 0°C; they require a nucleation point—a tiny seed or surface—around which the first ice crystal can form.
In the absence of these starting points, such as dust particles or container scratches, pure water can be supercooled. Scientists have achieved supercooled water temperatures down to about -48.3°C before the water spontaneously forms ice crystals through homogeneous nucleation. A slight shock or the introduction of a tiny ice crystal will instantly trigger rapid crystallization, often freezing the entire volume in seconds.
High Pressure and Warm Ice
The most unusual temperature variation for ice occurs under extreme pressure, which alters the water molecule’s crystal structure and shifts the melting point. Water’s solid-liquid phase boundary is unique because increasing pressure initially causes the melting point to decrease slightly, dropping to a minimum of -21.9°C at 209.9 megapascals (MPa). This explains why the pressure from an ice skate blade can cause a thin layer of ice to melt, allowing the skater to glide.
However, at far greater pressures, water molecules are forced into more compact crystal arrangements, creating exotic forms known as high-pressure ice polymorphs. At pressures above 632.4 MPa, the melting point rapidly increases, meaning these ice structures can exist at temperatures above 0°C. Ice V, a polymorph that forms at extremely high pressures, is stable up to a temperature of about 6.8°C, showing that ice is not strictly limited to sub-zero temperatures. These high-pressure ices, like Ice V and Ice VI, are theorized to exist inside the cores of large icy moons and distant planets.