Difference Between Freezing Point and Melting Point
For the most part, the freezing point and melting point of water are the same temperature. But, sometimes the freezing point is much lower than the melting point. Water experiences supercooling. Supercooling is when very pure water, free of dissolves gases or impurities, lacks nucleation sites that allow ice formation. Supercooling potentially lowers the freezing point of water as low as −48.3 °C or −55 °F!
Effect of Pressure on the Melting Point of Water
Pressure affects the boiling point, freezing point, and melting point of water. Two ways to estimate the effect of pressure on melting point are consulting a phase diagram and using the Clausius-Clapeyron equation, which relates pressure and temperature between two phases of matter. Increases pressure lowers the melting point of water. For example, at 800 bars (11603 psi) of pressure, the melting point of water is -6.9 °C. As you decrease pressure, eventually you reach a point where solid ice vaporizes rather than melts into a liquid.
Other Factors That Affect the Melting Point of Water
In addition to pressure, other factors affect melting point, including impurities, the structure of the ice, and the starting size of the solid.
Impurities disrupt the bonds between molecules, making it easier to overcome intermolecular forces between them. In water and most other compounds, impurities increase the the melting point. So, dirty ice melting at a higher temperature than pure ice.
The familiar form of solid water is hexagonal ice (ice Ih), but water molecules organize into other crystal forms that have different melting points.
In nanoscale ice, the phenomenon of melting point depression comes into play. Melting point depression is the lowering of the melting point with reduction in sample size. In the everyday world, ice consists of many water molecules, so melting point depression is not an issue. But, if you only have a few water molecules, the melting point lowers because the ice has a larger surface to volume ratio than normal. The cohesion between a few molecules increases, making it harder to separate them and change phase. Basically, the water molecules bind more strongly to each other because they don’t have as many neighbor molecules influencing them with intermolecular forces.
Melting point depression is a very different process from freezing point depression, in which impurities lower the freezing point of a substance. As stated, impurities raise rather than lower the melting point of ice.
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- Haynes, William M., ed. (2011). CRC Handbook of Chemistry and Physics (92nd ed.). CRC Press. ISBN 978-1439855119.