|MadSci Network: Chemistry|
You may be experiencing opalescence. Opalescence is due to light scattering by undissolved suspended particles in the liquid (solution). If this liquid is solidified to an amorphous mass it may do so resulting in a solid solution (a glass). This glass may contain trapped particles that may scatter light in such a way that opalescence results.
I have done the same experiment you have done with our local tap water in Canada and have not seen this "milky" condition, but a nice golden glassy material. You said that using potassium bitartrate does not cause this opalescence caused by your tap water. Without knowing the composition of your tap water it is difficult to tell. Your experiment with NaCl in distilled water indicates that your tap water may contain lots of Cl. What seems to be happening is that NaCl is getting excluded from the glass matrix resulting in NaCl particles that end up trapped in between the glass framework of the sucrose mass. The cooling down process is critical here, with both speed and stirring having en effect. You do not provide details in regard to these two factors, but I am sure they are involved in all of this. Stirring and slow cooling would ensure uniform dispersion with little or no opalescence.
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