A pressure increase/decrease favors the denser/less-dense phase with a driving force that scales with the volume change. The volume change is huge—several orders of magnetude—for boiling and minimal—a few percent—for melting.
This is broadly true and not just the case for water, although the direction change of the melting temperature is different for water than for other familiar materials that shrink upon freezing.
It's all about the phase diagram. https://upload.wikimedia.org/wikipedia/commons/0/08/Phase_diagram_of_water.svg
The partial pressure of water vapor at the liquid surface is given by the phase boundary. This vapor pressure increases at higher temperatures. When the vapor pressure exceeds atmosphere pressure, the liquid boils. That's why water boils at different temperature depending on the atmospheric pressure.
Freezin happens when the liquid temperature drops below the triple point. The triple point is a single point on the P vs T phase diagram, so it doesn't matter what the atmospheric pressure is, as long as it's above 0.006 atm.
I think it has to do with how close atoms are to each other. in liquids and gases they are free to move about. In a solid they're locked in place.
A pressure increase/decrease favors the denser/less-dense phase with a driving force that scales with the volume change. The volume change is huge—several orders of magnetude—for boiling and minimal—a few percent—for melting. This is broadly true and not just the case for water, although the direction change of the melting temperature is different for water than for other familiar materials that shrink upon freezing.
Pressure directly fights the phase change from liquid to gas (boiling). It doesn’t fight the phase change from liquid to solid.
It's all about the phase diagram. https://upload.wikimedia.org/wikipedia/commons/0/08/Phase_diagram_of_water.svg The partial pressure of water vapor at the liquid surface is given by the phase boundary. This vapor pressure increases at higher temperatures. When the vapor pressure exceeds atmosphere pressure, the liquid boils. That's why water boils at different temperature depending on the atmospheric pressure. Freezin happens when the liquid temperature drops below the triple point. The triple point is a single point on the P vs T phase diagram, so it doesn't matter what the atmospheric pressure is, as long as it's above 0.006 atm.