A mysterious crystal that melts at two different temperatures
The actual publication that resolves this riddle is here: <a href="https://pubs.acs.org/doi/10.1021/acs.cgd.8b01459" rel="nofollow">https://pubs.acs.org/doi/10.1021/acs.cgd.8b01459</a><p>The gist is that a chemical called acetaldehyde phenylhydrazone (APH) produces crystals in the solid state. Very minute contamination of the crystal can cause the melting point to go from 96 degrees C (uncontaminated) to 56 degrees C (contaminated), even if the contaminating agent (an acid) is virtually undetectable. A contaminated (low melting point crystal) and uncontaminated crystal appear identical under crystal structure and spectroscopy.<p>The cause turns out to be isomerization. The solid and liquid phases of APH have different stable isomer ratios (solid: 100% “Z” form, liquid: 37% Z, 62% “E” form); the contaminant appears to catalyze the isomeric transformation. With the catalyst, the Z form can transition quickly into the E form, making the liquid phase more accessible and lowering the melting point; without, the solid Z has to melt into the unstable liquid Z which then slowly (spontaneously) changes into the stable Z/E mix.<p>The paper’s worth a read, if only to see how thoroughly the authors tried to rule out any other possible explanation for this very weird behavior - doing many, many tests of the different solid samples and checking and rechecking equipment.