# Boiling Point lower than Melting Point?

Under the Wikipedia entry for Metaldehyde is listed a Melting Point of $\pu{246^\circ C}$ and a Boiling point of $\pu{110^\circ C}$ (actually, it says it sublimes).

How can a Boiling point be lower than the Melting Point? Even if $\pu{110^\circ C}$ is actually a sublimation temperature how can it then subsequently melt at a higher temperature of $\pu{246^\circ C}$?

Sublimation is not boiling but there is a reason for listing it instead. Sublimation is direct vaporisation from the solid. The most familiar is solid dry ice producing $\ce{CO2}$ gas. So if metaldehyde melts at $\pu{246^\circ C}$ then one might suppose that the boiling point is higher. However, look at the generic $P$ vs. $T$ diagram below. If we assume that at $P = 1000$ is normal atmospheric pressure then a compound with this $P$ vs $T$ sublimes. As we define boiling to occur only at normal pressure and be the liquid to vapour (gas) transition then this compound can never boil at normal pressure. To become a liquid high temperature and pressure is needed. So you can see why the table lists sublimation at normal pressure rather than boiling.