Calcium carbonate dissolves in water

In our atmosphere there is always a small amount of the gas carbon dioxide. This likes to dissolve in water, which is why rainwater or groundwater always contains dissolved carbon dioxide or carbonic acid:

1st step of the cycle: carbon dioxide + water -> carbonic acid

If the carbonated water flows over limestone, the lime is broken down by the acid into water-soluble calcium hydrogen carbonate:

2nd step: carbonic acid + calcium carbonate -> calcium hydrogen carbonate

The water containing calcium hydrogen carbonate (hard water) is washed away via the groundwater. Wherever water evaporates or is heated, the calcium hydrogen carbonate is converted back into lime and carbon dioxide. The first two steps are reversed:

3rd step: calcium hydrogen carbonate -------> calcium carbonate + carbon dioxide + water

In nature, this process takes place primarily in stalactite caves. The water flowing down the stalactite evaporates slowly and forms lime (stalactite). If hard water evaporates in water pipes, for example in coffee machines, limescale also forms and the pipe becomes calcified.

However, the greatest amount of lime is produced by marine animals such as corals, snails and mussels. With the help of the hard water, they build their limestone casings, releasing carbon dioxide and water into the environment. This closes the lime cycle. The limestone mountains were created this way!

A clear distinction is often not made between burnt lime (calcium oxide CaO), slaked lime / hydrated lime (calcium hydroxide Ca (OH) 2 and limestone (calcium carbonate CaCO)3).

Manufacture and application

Environmental impact and health

Wastewater treatment / drinking water treatment

In the case of wastewater neutralization, hydrated lime is predominantly used, but quicklime is also used. The latter is only converted into calcium hydroxide by extinguishing with water. Lime is also used in wastewater technology to condition sludge or as a precipitant (usually in combination with neutralization).

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