Here's why:

* Chemical Composition: Limestone is primarily composed of calcium carbonate (CaCO3). Calcium carbonate is a basic compound, meaning it has a high pH and can neutralize acidic substances.

* Reaction with Acid: When acid rain (containing sulfuric and nitric acids) comes into contact with limestone, a chemical reaction occurs:

* CaCO3 (limestone) + H2SO4 (sulfuric acid) → CaSO4 (calcium sulfate) + H2O (water) + CO2 (carbon dioxide)

This reaction neutralizes the acid, preventing it from further acidifying the environment.

* Other Contributing Factors: Other factors that contribute to limestone's effectiveness include:

* High Porosity: Limestone is porous, allowing rainwater to infiltrate and react with the rock.

* Presence of Other Minerals: Limestone often contains other minerals, like dolomite (CaMg(CO3)2), which also contribute to buffering capacity.

While other rock types like dolomite, marble, and some igneous rocks with calcium-rich minerals can also provide some buffering, limestone is generally considered the most effective due to its high calcium carbonate content and favorable chemical reactions.