An example of chemical weathering is the decomposition of limestone (a type of sedimentary rock) by carbonation. Limestone is primarily composed of the mineral calcite, which is a carbonate mineral. When carbon dioxide (CO2) from the atmosphere dissolves in rainwater or in the soil, it forms carbonic acid (H2CO3) through the following chemical reaction:
CO2 (g) + H2O (l) ⇌ H2CO3 (aq)
Carbonic acid is a weak acid, but it is sufficient to react with the calcium carbonate (CaCO3) in limestone, leading to the chemical weathering of the rock. The chemical reaction between carbonic acid and calcium carbonate can be represented as follows:
CaCO3 (s) + H2CO3 (aq) → Ca(HCO3)2 (aq)
In this reaction, the calcium carbonate in limestone reacts with carbonic acid to form calcium bicarbonate, which is soluble in water. As a result, the limestone gradually dissolves, and the calcium and carbonate ions are carried away in solution.
Over time, this chemical weathering process can lead to the formation of various landforms, such as caves, sinkholes, and limestone pavements. The dissolution of limestone by carbonation is a common and important example of chemical weathering, and it plays a significant role in shaping karst topography in regions where limestone is prevalent.
It is worth noting that chemical weathering can also occur through other processes, such as oxidation, hydrolysis, hydration, and solution, and it involves the breakdown and alteration of various minerals in rocks over geological time scales.