Hardness of Water.
Water is said to be hard if it doesn't form a lather (bubbles) with soap.
Calcium and magnesium ions cause water to be hard.
Hardness is described as "permanent" or "temporary".
Rain water is naturally acidic due to dissolved carbon dioxide,
(see the Carbon Cycle).
The acid in rain water is carbonic acid, H2CO3(aq).
Rocks containing magnesium carbonate or calcium carbonate (limestone),
will react with dilute carbonic acid forming hydrogencarbonates.
carbonic acid + calcium carbonate calcium hydrogencarbonate.
H2CO3(aq) + CaCO3(s) Ca(HCO3)2(aq)
carbonic acid + magnesium carbonate magnesium hydrogencarbonate.
H2CO3(aq) + MgCO3(s) Mg(HCO3)2(aq)
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