This has bothered me for a long time, and no one can explain it to me. It involves some pretty familiar and primitive P-chem that every chemist has experience with: melting-point depression (or elevation) in impure solid samples. What’s going on? Why does the mechanical contact of two crystalline solids affect their melting points?

 

Let’s take the most common example: mp depression due to impurities.

 

Pure A has a sharp mp of 150 deg C. Pure B has a mp of 170 dC (sharp).

 

When we mix A and B together though, we find the mechanical mixture begins to melt at some lower temperature than either, say 120 d C.

The explanation I was always given (with considerable hand-waving) is that either A or B begins to melt below its expected (pure) value and begins to act as a solvent for the other.

 

But this is a mechanical mixture, crystals of A and B just pushed around together on a watch glass, touching each other but not (as far as I know) interpenetrating or contaminating each other’s crystal structure in a way that might affect its melting point. Neither one has any reason to suddenly start melting at a lower temperature and then “dissolve” the other.

 

I'd be interested in hearing anyone's ideas on the mechanism of mp depression solids.

You need to study Eutectic mixtures.

 

https://en.wikipedia.org/wiki/Eutectic_system

When each solid remains pure, the case is easy to understand. Molecules can separate still as easily from the solid to float away in the liquid (ptovided both liquids mix easily), while there are fewer molecules of the proper kind in the liquid that can deposit on each solid. This favours melting.