My textbook defines melting point of a solid as the point when the solid vapour pressure is equal to the liquid vapour pressure.
However when I search for solid vapour pressure, I get information on sublimation. This doesn't satisfy me because I know that not all solids sublime easily, but they can melt easily.
So my question is, what is solid vapour pressure? Is it really same as sublimation or is a different phenomenon at work here?
For a solid to have a vapour pressure we require the solid to be in equilibrium with the vapour i.e. there is a pressure and temperature at which the chemical potentials of the solid and vapour are the same.
If we look at a typical pressure-temperature phase diagram it looks something like this:
and it is only along the red line that the solid and vapour are in equilibrium, so it is only for this range of pressures and temperatures that the solid has a vapour pressure. But this is exactly the region in which the solid sublimes directly to a vapour without passing through a liquid phase. That's why the solid can have a vapour pressure only if it sublimes.
I have some different approaches to understand it If we know how to find vapour pressure of solid, we get the answer of this question. We take a sealed container in which a piece of ice is taken. We give as much pressure of water vapour that it cannot liquefied. We note the vapour pressure at different temperature. Now,we plot a graph vapour pressure vs temperature and compare the graph with vapour pressure of liquid vs temperature. We get the freezing point. I think u get the answer how we find the vapour pressure in solid phase
