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Is the bottle empty? Sure, it is empty of crumpled paper balls, but the bottle is not completely empty.
When the Epsom salt is mixed in the hot water, it doesn’t just disappear. It’s still there, it just dissolves into the water. As the sun heats the liquid mixture, the water evaporates, leaving the salt crystals behind in the plastic lid. The forming of crystals is called crystallization. Some forms of salt are obtained through this very same process. Sea salt is often produced in dry climates such as the Mediterranean.