Pressure and melting of ice.

[Chocolatebear89]

Which of the following will increase the rate at which ice melts in a closed container if all other parameters are kept constant?
A. Adding water with a temperature of 0°C
B. Lowering the temperature below 0° C
C. Loweringthepressure
D. Raising the pressure

I was wondering which one is correct, and why?

---

Members don't see this ad.

 

[fizzgig]

my guess would be D, because they are checking that you know water is weird and has a solid phase with LESS density than its liquid phase, which is not the case for most substances.

 

[wanderer]

Yup, the slope of the solid-liquid equilibrium in the phase diagram of water is the opposite of that expected, since higher pressures melt ice. There is also a popular myth that when you ice skate the pressure under the skates is great enough to melt the ice, and that provides for a slick surface. The pressure isn't nearly high enough for that to be true, but that explanation might serve as a good memory aid.

 

[Pulsar]

You can prove it to yourself mathematically with the Clapeyron equation:

dP/dT = dH / (TdV)

dH is somewhere around 40kJ/mol depending on pressure, and T is obviously positive. dV is negative for water (it expands on freezing), but positive for nearly every other substance. This is the key. When you plug these values in you get:

dP/dT = +/(+*-) < 0

Therefore, the slope of the pressure vs. temperature line is < 0 and the slope of the temperature vs. pressure line must be < 0. This indicates that as pressure increases, freezing temperature decreases. I believe the physical reason is because the excess pressure will hinder the molecule's attempts to expand against the environment as it freezes.

 

[Chocolatebear89]

So if it wasn't water, it would be lowering the pressure to increase the melting rate?

 

[Pulsar]

So if it wasn't water, it would be lowering the pressure to increase the melting rate?

Yes -- plug in a positive dV to the Clapeyron equation, and you get dT/dP > 0. So lowering the pressure lowers the freezing point, which means it will melt at a higher temperature.