The crystal structure could not possibly be disrupted, as the compounds had never mixed in liquid form. So my question is: how does that work?
Really?
I take it from you question that my assumption is wrong
Is it just the case that a small amount of one or both of the pure substances is melting at the edges of the crystals, and that when this happens, this tiny amount of impure liquid can dissolve more of the two pure solids, and that this is what causes the melting point depression?
Ah, I see you figured it out.
Try not to see it as simply a disruption of crystal structure. Try also to visualize it as driven by entropy. Can you make some comparisons between a pure solid, a melted pure solid, and a mixture, in terms of entropy,or disorder?
In order of increasing entropy:
Solid < Pure Liquid < Mixture (generally; it is possible that the intermolecular forces in the mixture could be more 'organizing' than the intermolecular forces of the liquid; but if we assume an ideal mixture, then this progression is true).
So my model makes sense then? In a simple—almost-ideal—mixture, the small amount of liquid forming at the edges of the finely ground crystals are dissolving the two pure crystalline solids, forming a rapidly spreading entropically favored liquid?
Is there anything else going on with the vapor pressure of the solids or anything else?
Also, would it be fair to assume that if we have two pure compounds that are miscible, A and B, and we melt both and then freeze the mixture, that mixed solid will have a lower melting point than the melting point of the two mixed pure solids, which will in turn be lower than either of the single pure solids?
i.e.: Melting point of mixed solid < melting point of two pure solids mixed together < melting point of either solid
Or would the melting point of the mixed solid and the melting point of the two pure solids mixed together be the same? Assuming that both are ground equally.