What happens when acid reacts with limestone?

15 June 2008

Question

What happens in a reaction between acid rain and limestone?

Answer

Limestone is mostly made up of the mineral calcium carbonate (CaCO3). This is not very soluble, so rocks don't dissolve very quickly. But if you add an acid, you add hydrogen ions (H+), which will react with the carbonate to form hydrogen carbonate HCO3- ions, which are very soluble in water, and the limestone will dissolve. Or, if there is more acid, two hydrogen ions will react with a carbonate to form carbonic acid - H2CO3 - which will decompose to form carbon dioxide - CO2 - which eventually bubbles off into the atmosphere, and water H2O.

The acid can come from a variety of sources: sulphur and nitrogen oxides released by burning fossil fuels will form sulphuric, sulphurous, nitrous and nitric acids, and carbon-dioxide itself can dissolve in water to form carbonic acid.

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