# Interpreting from phase diagram whether O2 will float or sink

The normal melting and boiling points of $\ce{O2}$ are $\pu{-218 ^\circ C}$ and $\pu{-183 ^\circ C}$, respectively. Its triple point is at $\pu{-219 ^\circ C}$ and $\pu{1.14 Torr}$, and it's critical point is $\pu{-119 ^\circ C}$ and $\pu{49.8 atm}$. Will $\ce{O2 (s)}$ float or sink on $\ce{O2 (l)}$?

I quickly sketched the phase diagram of oxygen:

Pressure of the molecules will affect how compact the molecules are in solid and liquid, so I guess we can explain whether it will float or sink using the slope?

How does the slope tell us about the densities of $\ce{O2}$ solid and liquid ?

And in addition, how do I use intermolecular forces to explain this?

• Floating or sinking is a matter of comparing the densities, or molar volumes of two phases; even though one can easily find the density of an ideal gas ($V_m = RT/p$), I'm unaware of how a phase diagram can be used for it. from what I remember, solid oxygen is heavier and will sink. – andselisk Dec 12 '17 at 6:35

As to the intermolecular forces, I think the usual hand-waving along the lines of "less freedom $\to$ less motion $\to$ lower volume" will do, and even that is redundant. Having solid denser than liquid is normal. If anything, it is the opposite situation that would require an explanation.