[Ben Cohen]

Hi all,

How do I use phase diagrams to tell me information about density?

For example, http://www.cem.msu.edu/~mantica/cem152/prep1/sulfurphasediagram.html shows the phase diagram of sulfur. How do I tell anything about the density of different phases from the diagram?

Thanks in advance.

{MOD Edit: replace ridiculous link}

[XGen]

Check the slope of the line that separates the two phases. If it's positive, like in this one, the left-hand phase is more dense. If it's negative, then right-hand side is more dense.

[Ben Cohen]

Check the slope of the line that separates the two phases. If it's positive, like in this one, the left-hand phase is more dense. If it's negative, then right-hand side is more dense.

So in this case, the rhombic is more dense than the monoclinic, and the monoclinic is more dense than the liquid?
And the vapor is just irrelevant because it's almost always less dense than anything else?

[fledarmus]

Think about what the phase diagram is actually telling you. What happens physically when you increase pressure on something? The molecules get closer together and the substance becomes more dense. So if you are following a phase diagram along increasing pressure and your substance changes form, the form at the higher pressure will be denser than the form at the lower pressure. Otherwise, it would not be favored by an increase in pressure.

[curiouscat]

Check the slope of the line that separates the two phases. If it's positive, like in this one, the left-hand phase is more dense. If it's negative, then right-hand side is more dense.

And the mathematical basis for that is this equation: