The melting point (MP) of a pure solid is defined as the temperature at which the solid and liquid phases exist together in thermodynamic equilibrium. Unlike the boiling point (BP), which is routinely reported alongside a pressure value, the melting point is almost always specified by temperature alone. This omission reflects the fundamental physical differences between the solid-to-liquid and liquid-to-gas phase transitions. The negligible effect pressure has on melting is governed by principles of thermodynamics, making the pressure value largely redundant in standard reporting.
Why Pressure Heavily Influences Boiling
The strong dependence of a substance’s boiling point on external pressure is due to the dramatic change in volume that occurs during the liquid-to-gas transition. When a liquid vaporizes, the molecules gain enough kinetic energy to overcome the intermolecular forces holding them together. This results in an enormous expansion of volume, typically increasing by a factor of 1,000 or more for a given amount of substance.
Boiling is defined as the temperature at which the substance’s vapor pressure equals the surrounding external pressure. If the external pressure is high, the molecules require significantly more energy, and thus a higher temperature, to transition into the gaseous state. Conversely, lowering the external pressure allows the liquid to boil at a much lower temperature. This direct and significant relationship necessitates reporting the pressure alongside the boiling point for accuracy.
The Insensitivity of Solid-Liquid Transitions
The primary reason pressure has such a minimal effect on the melting point lies in the small change in molar volume (\(\Delta V\)) when a solid melts. The Clapeyron equation, a thermodynamic relationship, demonstrates that the change in melting temperature with pressure is directly proportional to the volume change of the phase transition. In the transition from a solid to a liquid, the particles remain tightly packed, and the volume typically changes by less than 10%.
For most substances, the liquid phase occupies a slightly greater volume than the solid phase, meaning that increasing pressure slightly raises the melting point. Pressure resists the expansion that accompanies melting, requiring a slightly higher temperature to force the transition. Because the change in volume is so tiny, even a very large change in pressure, such as a hundred atmospheres, only causes a fractional degree change in the melting temperature.
Water is a well-known exception where the solid (ice) is less dense than the liquid, meaning the volume decreases upon melting. In this unique case, increasing pressure actually slightly lowers the melting point because the pressure favors the denser, smaller-volume liquid phase. Even with this reverse effect, the magnitude of the change remains extremely small. For instance, a pressure increase of 150 times the standard atmospheric pressure only lowers the melting point of ice by about one degree Celsius. This negligible scale of temperature adjustment is the physical reason pressure is not considered a variable of consequence for the melting point.
Implicit Assumptions of Standard Conditions
When a melting point is reported without an accompanying pressure value, it operates under the implicit assumption of standard atmospheric pressure. This value is approximately 1 atmosphere (atm) or 101.3 kilopascals (kPa). Since the physical effect of pressure on the solid-liquid transition is so small, minor fluctuations in atmospheric pressure do not produce a measurable or practically significant shift in the melting point.
A change in altitude or daily weather patterns might cause an atmospheric pressure fluctuation of a few percent. This small pressure change would alter the boiling point noticeably, but it would have an insignificant effect on the melting point of a pure substance. Including the pressure value in melting point reports would provide little additional scientific utility. The lack of a reported pressure value signals that the measurement was taken under normal laboratory conditions, where the small volume change makes the pressure variable functionally redundant.