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Water - Solubility - Ions

Temporary and Permanent Hardness.

Both calcium hydrogencarbonate and magnesium hydrogencarbonate
decompose (split up) when heated.
The original insoluble carbonate is reformed.
This happens when water is boiled.

calcium hydrogencarbonate carbon dioxide +  water + calcium carbonate.
Ca(HCO₃)_2(aq)                       CO_2(g)     +    H₂O_(l)   +       CaCO_3(s)

magnesium hydrogencarbonatecarbon dioxide+water+magnesiumcarbonate
Mg(HCO₃)_2(aq)                       CO_2(g)     +    H₂O_(l)   +       MgCO_3(s)

Boiling the water causes the precipitation of solid calcium carbonate
or solid magnesium carbonate.
This removes the calcium ions or magnesium ions from the water,
and so removes the hardness.
Therefore, hardness due to hydrogencarbonates is said to be temporary.

Other types of calcium ion or magnesium ion in water
such as calcium chloride - CaCl₂, calcium sulfate - CaSO₄,
magnesium chloride - MgCl₂ or magnesium sulfate - MgSO₄
are not removed by boiling the water.
These ions in water are said to cause permanent hardness.
Calcium sulfate is called gypsum.

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