|MLA Citation:||Bloomfield, Louis A. "Question 507"|
How Everything Works 17 Jan 2018. 17 Jan 2018 <http://howeverythingworks.org/print1.php?QNum=507>.
The minerals that you see in the thawed bottle of water were originally dissolved or suspended in the water. But as the water froze, the ice crystals excluded those impurities and they remained in the liquid portion of the water. Eventually the liquid portion of the water dwindled away and the minerals were forced to come out of solution as solid particles. When the water thawed, those minerals failed to redissolve (they're often only weakly soluble in water and have great difficulty redissolving).
This phenomenon whereby crystallizing a liquid separates out its impurities is very useful in chemistry—many important chemicals, notably medicines, are purified in this manner. Similarly, freezing water is an important way of purifying it in some locations—native people in cold countries have used sea ice (the pure ice that forms when seawater freezes) as a source of fresh water for centuries. And you may have noticed that when you eat frozen juice, you can suck away the sweet flavored portion and leave behind only the pure ice portion—because the sugar and flavors have been excluded from the pure ice crystals during freezing.