What is the equation for the reaction of hydrochloric acid and calcium carbonate, both before and after balancing? 

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When calcium carbonate reacts with hydrochloric acid the products are calcium chloride, carbon dioxide gas and water. Here is the unbalanced equation:

`CaCO_3 + HCl -> CaCl_2 + CO_2 + H_2O`

Here is the balanced equation:

`CaCO_3 + 2 HCl -> CaCl_2 + CO_2 + H_2O`

Carbonates tend to react with acid to produce CO2. This particular reaction is an example of a double replacement reaction combined with decomposition. Carbonic acid, H2CO3, is a product that decomposes into CO2 and H2O.

The reverse of this decompostion also takes place. The amount of carbon dioxide that will react with water depends on the temperature, pressure and amount of CO2 present. Acidification of the ocean is caused by the uptake of atmospheric CO2 by ocean water. 

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