Water in its pure form, is soft water, and is an incredible solvent. When the rain falls, and perculates through the ground, and in particular rock formations, the universal solvent picks up minerals. Over a certain parts per million, the water becomes hard.
Hard water comes with many problems. The fantastic solvent, becomes less affective, and reacts with soaps making it scummy and poor at cleaning. When heated the water leaves a crusty deposit called limescale, which looks unsightly, but also damages the material it crusts upon.
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