Question: A solid substance has a vapor pressure of 100 mmHg at the melting point (100C). Assuming an atmospheric pressure of 760 mmHg, describe the behavior of this solid as the temperature is raised from room temperature to its melting point.

My answer: The solid will remain a solid as the temperature is raised to its melting point because the atmospheric pressure is too great to allow it to melt.

Is this okay?

Thanks from Sheryl

No. The part "..because the atmospheric pressure is too great to allow it to melt" is wrong. It does vaporize at a faster rate as temp is raised, vaporization is proportional to vapor pressure. This is sublimation.

Okay, since the melting point is relatively insensitive to pressure, and considering that the melting point is also usually the freezing point of a substance, the substance will be in an equilibrium state of solid and liquid.

Is this correct?

Thanks from Sheryl

No. See my post. This solid is vaporizing as it is heated, then when it gets to mp, it melts (and will still vaporize).

See this post of yours yesterday, we missed it.

one last thought to remember: When something melts, it is endothermic, it is absorbing energy, so the reaction is cooling. Same thing on vaporization: it is endothermic, and is a cooling reaction.
So when one heats something that is subliming, the energy goes not into all heating the substance, a lot of the energy goes into vaporization. The balance of those energies depends on "vapor Pressure" as we measure it. High vapor pressure most energy goes into vaporization.

I don't understand how it can vaporize at 760 mmHg when its vapor pressure is 100 mmHg. It seems that the much higher pressure of 760 would prevent it from vaporizing. This seems to be the point I don't get.


Okay, vaporization is proportional to vapor pressure which must mean that it can vaporize at much higher v.p. than its vapor pressure.

A much higher pressure, not vapor pressure.

Hope this is not too confusing.

The vaporization RATE is affected by the atmospheric pressure, the higher the A.P. the lower the v.r.

Atmosphere is gaseous, it cannot prevent melting or vaporization.

Isn't vaporization rate is affected by the partial pressure of the vapor in the atmosphere, but not total atmospheric pressure?

Two samples of paradichlorobenzene:
1) one at three atmospheres of air, but the partial pressure of PDB in one is zero.
2) the second PDB sample at .6 atm air pressure, but the partial pressure of PDB vapor in the "air" is .5 atm. Assume the vapor pressure of PDB is less than .5 atm at given temp.

The first will vaporize at a much greater rate than the second. In fact, the second will gain mass (condense) due to the vp of the vapor is greater than the vp of the solid. Vaporization rate is an equilibrium process, the forward reaction (solid or liquid>gas) is dependent on temperature, the reverse (condensation) is dependent on the difference in the partial pressure of the gas vs vapor pressure of substance.

I finally get it. I had atmospheric v.p. and the solid's v.p. somehow reversed in my mind.

Thank you


How dose gas become a solid?

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