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Recrystalization of aluminum sulphate on surface of liquid


I am trying to grow a single mono crystal of aluminum sulphate. The problem that i am encountering is that a solid polychrystal layer forms on the top surface of the beaker instead of crystal growth happening at the bottom.
I tried placing a saturated solution into food dehydrator in a wide dish and also a 250ml beaker under my AC intake, so that there is a draft over the beaker as well as just leaving beaker sitting in house.  I also have access to vacuum chamber, so can pull vacuum on beaker if need be.
What i do not understand is what can i do to prevent crystals from forming on the surface and have them form on the bottom instead. My concern with crystals growing on the surface is that they will block evaporation and they are poly crystals, so mono-crystal growth is kind of hard...

Try suspending a needle in the solution, pointing downwards so the point is halfway down. Use a taller container to slow down evaporation. The crystal may start to grow on the needle point first. its better to have lower supersaturation for this. If the nucleation rate is too high you'll get multiple crystals.

Why not take a crystal of Al2(SO4) and put it in the solution. It will become a point of nucleation.


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