General Trends:
* Most substances: For most substances, increased pressure leads to an increased melting point. This is because pressure compresses the molecules, making it harder for them to transition from a solid to a liquid state. The increased pressure effectively "holds" the molecules together in a solid structure.
* Water: Water is a notable exception to this general trend. Increased pressure lowers water's melting point, allowing it to melt into liquid at lower temperatures. This is because the liquid form of water is denser than its solid form (ice), so increased pressure favors the liquid state.
Factors Influencing the Relationship:
* Density change upon melting: The key factor is the difference in density between the solid and liquid phases. If the solid is denser, increasing pressure will favor the solid phase and thus raise the melting point. If the liquid is denser, increasing pressure will favor the liquid phase and thus lower the melting point.
* Molecular structure: The structure and bonding within the molecules also influence the melting point and its response to pressure.
* Specific substance: Each substance has unique properties, and the relationship between pressure and melting point needs to be determined experimentally.
Examples:
* Carbon dioxide: Solid CO₂ (dry ice) sublimes (goes directly from solid to gas) at atmospheric pressure. However, under high pressure, it can be liquefied, and then eventually solidified into a denser form of solid CO₂.
* Ice: At atmospheric pressure, ice melts at 0°C. But at high pressure, the melting point drops, and ice melts at lower temperatures. This explains how ice skates work, as the pressure applied by the blade melts a thin layer of ice, reducing friction.
In summary:
While there are general trends, the relationship between pressure and melting point is complex and specific to each substance. It depends primarily on the density difference between the solid and liquid phases, the molecular structure, and the specific substance's properties.
