High School Earth Science: Weathering and Formation of Soil - Weathering — Flashcards

Chemical weathering occurs when rocks undergo chemical reactions that change their mineral composition. Most minerals form under conditions of high pressure and temperature deep within the crust or even in the mantle. If the minerals reach Earth’s surface where pressure and temperature are much lower, they become unstable. As a result, the minerals change chemically to other minerals, such as clay, that are more stable under conditions on the surface.    
Water is the most important agent of chemical weathering. Water is a polar molecule, meaning that one side of the molecule is slightly positive and the other side is slightly negative. The positive side of each water molecule attracts negative ions and the negative side attracts positive ions. In this way, water molecules separate ions from their compounds and surround them. Water can completely dissolve some minerals, such as salt.    
Two other important agents of chemical weathering are carbon dioxide and oxygen, both of which are gases in Earth’s atmosphere.    
- Carbon dioxide combines with water as raindrops fall through the atmosphere. This forms a weak acid, called carbonic acid, which can dissolve some types of rock.    
- Oxygen is very reactive chemically. A common type of chemical reaction involving oxygen is oxidation. The most familiar example of oxidation is the reaction of iron with oxygen to create iron oxide, or rust. Minerals rich in iron break down as the iron rusts. This is why some soils are red in color.