Which one of the following is not an example of chemical weathering ?

The correct answer is C, Salt Crystal Growth. It is a form of physical, or mechanical, weathering: the rock breaks into pieces but its chemical make-up does not change. In dry regions, salty water seeps into cracks and pores of rocks. When the water evaporates, salt crystals grow and press on the walls of the crack, slowly widening it until grains and flakes break off. This is common in deserts and on coasts, and it works like frost wedging, where freezing water expands in cracks. Chemical weathering, in contrast, changes the minerals themselves through reactions with water, oxygen and acids, and it is fastest in hot, wet climates. Option A is wrong because acid action, such as carbonic acid dissolving limestone, is chemical weathering. Option B is wrong because hydrolysis, the reaction of water with minerals like feldspar to form clay, is chemical. Option D is wrong because oxidation, such as iron in rocks turning to rust-coloured iron oxide, is chemical. Exam tip: salt crystal growth, frost wedging and exfoliation are physical; solution, carbonation, hydrolysis and oxidation are chemical.