Which of the following is not a process of chemical weathering?

Frost action is not a process of chemical weathering. It is a process of physical (mechanical) weathering. Frost action (also called freeze-thaw weathering) occurs when water seeps into cracks in rocks and freezes. As water freezes, it expands by about 9%, exerting pressure on the surrounding rock. Repeated cycles of freezing and thawing gradually widen the cracks until pieces of rock break off. No chemical change occurs in this process.

The other three options are all genuine processes of chemical weathering:

• Hydrolysis is a chemical reaction between minerals in rock and water. Feldspar in granite, for example, reacts with water to form clay minerals (kaolinite), weakening the rock.
• Oxidation is the reaction between minerals (especially iron-bearing minerals) and oxygen in the presence of moisture. It produces iron oxides (rust), giving rocks a reddish-brown colour and making them crumble.
• Carbonation is the reaction between carbon dioxide dissolved in rainwater (forming weak carbonic acid) and calcium carbonate in limestone. The calcium carbonate dissolves, creating features such as caves, sinkholes, and karst landscapes.

Exam tip: Physical weathering breaks rock without changing its chemical composition. Chemical weathering alters the mineral composition of the rock through reactions with water, oxygen, or acids.