[MITaylor]

     Everything I've seen says that intermolecular forces are removed both for melting and boiling a substance.  Is it that just some are melted and all are boiled? Because in that case, I don't see why some sublimation couldn't occasionally occur as a solid melts.
     I'm not even really sure what I'm saying anymore - I'm just really confused!  But basically, what's the difference between melting and boiling?
     Thanks!

[Corribus]

Well, I'm not sure what you're asking, either, to be honest. What's the difference between melting and boiling? Melting is a conversion between a liquid and solid; boiling - more or less - is a conversion between a liquid and a gas. That may sound trite, but for most substances there's a well-defined distinction between the various phases. Actually, boiling is a fair bit more complicated. Strictly speaking, evaporation is the conversion between liquid and gas. Boiling is a specific type of evaporation that occurs when specific conditions are met.

Given that one way that one could define the phases of matter is by the strength of intermolecular forces that hold molecules together, then it would be OK to say that both melting and boiling involve a reduction in the strength of intermolecular forces. Actually, I think it would be better to say that reducing the strength of intermolecular forces - or, rather, providing enough energy such that intermolecular forces are no longer strong enough to hold molecules in place - are what cause melting and boiling (evaporation).

Hope that helps.

[Arkcon]

     I'm not even really sure what I'm saying anymore - I'm just really confused!  But basically, what's the difference between melting and boiling?

That's OK, try to figure out what confusing you.  Why do you think that they should be the same, and not merely similar, using what Corribus: said.

[Enthalpy]

About sublimation: it does happen, and always. It's just that the resulting pressure can be small.

In other words, solids do have a vapour pressure, even without first melting. Some solids have a big vapour pressure, for instance >1atm or a good fraction of it, before melting - then we say they sublimate. Others have a small vapour pressure until they're really hot, and these melt before evaporating, so we don't usually care about sublimation.

If you check vapour pressure curves, they show very little around the melting point. I attach the example of methane (thankfully pinched from Air Liquide) which melts at 91K.