When you leave a saturated NaCl saline solution in a glass cylinder, what happens when water dries up? Likely you thought cubic salt crystals would form at the bottom of the glass, and then grow bigger as water evaporates. You have this mental picture perhaps from reading a textbook, or maybe watching an animated video. Eventually only salt crystals are left in the cup. Well, that’s not wrong. But that is also not the full story.
What actually happens as salt water dries up in a cup is more complicated. You start with a glass cylinder about 2/5 filled with maximally-saturated salt water at 26 wt%, with about 1g of undissolved salt remaining, shown on the left. As water evaporates, at first those undissolved crystals continue to grow. But more crystals form on the surface of the solution. Before you realize, these crystals turn into lumps and form a seemingly live colonies of wet salt creeping up the glass wall. They will cover the entire inner wall, then march over and past the rim of the cylinder to the outside wall. By the time all water dries up, the glass cylinder is covered completely in a rind of salt, as shown on the right.

I couldn’t stop thinking, “what a creepy way for salt water to dry up!”
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