Describe the two processes in which sediments containing calcium carbonate can be formed.

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akindtexan eNotes educator| Certified Educator

One way is when acidic water is passed through limestone. The chemical process is the result of the reduced ph of rain when it is exposed to carbon dioxide in the atmosphere. At a PH of around 5.6, rainwater is slightly acidic. When water reacts with the carbon dioxide in the air, it forms carbonic acid, H2CO3. Carbonic acid dissociates in water, leaving two H ions and a CO3 ion. These two H ions can each react with CaCO3, forming an aqueous Ca ion and an HCO3 ion. These are both washed away, where they will be left to evaporate somewhere else. Upon evaporation, the calcium recombines with CO3, and calcium carbonate is left as sedimentation.

A second common reaction is with nitric oxide, a gas formed by lightning strikes. This NO combines with O2 to form NO2, and 3NO2 combine with H2O to form 2HNO3 and NO. Nitric acid, or HNO3, which dissociates in water to form H ions and NO3 ions. Like before, the H ions bind with the CaCO3 to make calcium ions and HCO3 ions; both of which are soluble and are washed away to be deposited elsewhere.

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