A mysterious crystal that melts at two different temperatures
The actual publication that resolves this riddle is here: https://pubs.acs.org/doi/10.1021/acs.cgd.8b01459The 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.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.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.