dr_mabeuse

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.

I'm sure there's an easy answer to this, but it's bothered me for a while. It's about the Twin Paradox. So Allen stays on earth while twin brother Bob roars off in a space ship to some distant star at a significant fraction of the speed of light. Bob reached his destination and completes his business, then roars back to earth again at again, some near-light speed. When the brothers finally meet back up on earth again, Bob is significantly younger than Allen. Bob's near-c velocity has slowed time for him compared to what Allen experienced. That's how I understand the Twin Paradox. But-- All things being relative, couldn't we just as easily look at this as Bob's space ship sitting stationary while the earth, Allen, and the whole universe rushes past Bob at near-light speeds? In that case, isn't it Allen who's traveling at near-c speeds, and Allen who would come back younger than Bob? Obviously, Bob can't be both older and younger than Allen. Where did I go wrong?