I recently froze some tap water in plastic bottles and I later made the ice melt completely (the idea was keeping ice bottles nearby me to fight the heat). When the water returned to liquid a lot of tiny white dust/crystal flakes were visible. These were heavier than water and tended to sink.
At the beginning I thought they were little chunks of plastic that got detached from the walls of the bottle due to the severe changes in temperature, but then by comparing the state of the bottles (seemingly unaltered wrt the beginning) with the quantity of flakes I concluded it had to be something else.
The second hypothesis was that the flakes were actually crystals from the minerals that are in the tap water (I hear that in my region tap water is full of calcium for instance). However I do not understand how would these crystals form:
- how could it be possible? and why don't they return/remain diluted in the water?
- do they form during the freezing or the melting phase?
I would say that during the freezing phase the fact that the solution freezes at lower temperatures than pure water could isolate portions of liquid with higher and higher concentration of minerals as the freezing goes on. These will remain mobile till the very end of the freezing process. In the final stage the space for the liquid part to move around will be very limited, giving chance to form crystals. But maybe I am wrong and there is something I am missing.
So any hypothesis on how these crystals form? Maybe they are not even crystals like I am trying to say?