The carbonate system

The chemistry linking atmospheric carbon dioxide, rock weathering, water pH and limescale. It is the single most important equilibrium in natural waters.

Why hard water scales: Heating drives off carbon dioxide, shifting the equilibrium back toward solid calcium carbonate. The scale in a kettle is the limestone the water dissolved on its way to you.
Temporary versus permanent hardness: Hardness paired with bicarbonate precipitates on heating and is temporary. Hardness paired with sulphate or chloride does not, and is permanent.
Alkalinity: Bicarbonate is the buffer that resists pH change. Waters with little of it — soft upland waters — are the ones acid deposition damaged.
Ocean acidification: The same equilibrium at global scale. More atmospheric CO₂ means more carbonic acid, more hydrogen ions and lower ocean pH.

Text description

Carbon dioxide dissolves in water to form carbonic acid, which dissociates into bicarbonate and hydrogen ions, and bicarbonate further into carbonate. Calcium carbonate rock dissolves in the presence of carbonic acid, releasing calcium and bicarbonate into the water. Heating reverses this: bicarbonate decomposes, carbon dioxide is driven off, and calcium carbonate precipitates as limescale.