'Pure' should dissolve in water - there should be nothing else left over.
The problem with this is a lot of 'pure' has already been cut or contains non soluble ingredients so having some residue after dissolving in water isnt really a good way to test purity.
unless the cut dissolves in water too...its kinda handy that you can filter out the cut that way
Make sure to put a proportionate amount of ethanol (~10% ) to act as a preservative if you plan to store it for a long time. Otherwise it might get a little bit moldyDoes anyone know the stability of MDMA in water? How long does it stay active before it begins to break down? I wonder if freezing it in this form would extend it's efficacy (I bet it would).