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Acid rain is rain that has an excess of protons (H+) present, usually as a result of pollutants in the air. Limestone is a rock that is composed of calcium carbonate (CaCO3). Acidic water can dissolve the CaCO3 because protons react with the carbonate ion to form carbonic acid. The equation is shown below:
2H+ + CO3(2-) --> H2CO3
As the protons react with the carbonate ions in the limestone, the rock is gradually etched away as it dissolves. Over millions of years, underground cave systems can form in the limestone.
Sandstone is a similar situation. It is a sedimentary rock formed by the cementing together of tiny grains of rock crystals. The cement is usually composed of calcium carbonate (CaCO3), so the acidic water dissolves this cement over time just like limestone and the individual grains can be washed away with water or wind.
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