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Water - Solubility - Ions

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, H₂CO_3(aq).

Rocks containing magnesium carbonate or calcium carbonate (limestone),
will react with dilute carbonic acid forming hydrogencarbonates.

carbonic acid + calcium carbonate calcium hydrogencarbonate.
H₂CO_3(aq)    +       CaCO_3(s)                  Ca(HCO₃)_2(aq)

carbonic acid + magnesium carbonate magnesium hydrogencarbonate.
H₂CO_3(aq)    +       MgCO_3(s)                            Mg(HCO₃)_2(aq)

These hydrogencarbonates are slightly soluble in water, causing hardness.

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