I recently started a small project of making home-made chromium alum crystals, as can be found on many sites, videos and recipes all around the internet.

I tried calculating the amount of salt I needed from wikipedia's page for the chromium alum, and then went nowhere, since the solubility data stated there does not say if it is for the dodecahydrate or anhydrous form. I tried it for the dodecahydrate and failed since no seed crystals formed after two days.

Then I added about 100% more alum to the solution while heated to about 80°C (not measured) and it all dissolved, and then after some hours some nice seed crystals formed.

After the initial seed crystal creation, I was left with a small amount of saturated solution and some extra seeds that I wouldn't use.

Just for curiosity sake, I then went to try to heat this solution to boil of the water and try to make some anhydrous chromium alum, or at least the solid, crystalline hydrated form.

Then, first, the solution took a goopy, molten plastic-like appearance, and then, after several hours, it went from a dark purple (hydrated?) clumpy mass to a greyish-green colored powder that didn't change at all even at the maximum temperature of my (bad quality) hot plate that hovers around 250°C.

I believed to have made anhydrous chromium alum (wikipedia stated that it is a greyish-brown powder), and then proceeded to take a small sample of this powder and re-hydrate it.

To my surprise, it didn't dissolve in distilled water at all! Having read something somewhere about some salts being quite soluble in water but dissolving very slowly, I then waited several days, stirring it occasionally, and nothing changed.

What happened to my chromium alum?

Wikipedia states a melting point of 89°C that could be related to the plastic-like melt observed, but also states a boiling point of 400°C that while I was never able to observe since my hot plate never reaches anything close to this temperature, should have left me with molten chrome alum. I know some compounds increase in viscosity with temperature increase, so I wouldn't be surprised to not get a flowing liquid.

But instead I got a greyish-green powder at around 250°C that is not water soluble.

Wikipedia also does not state decomposition temperature for this compound.



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