Chemical weathering involves the decomposition of rocks by the alteration of rocks-forming minerals. Chemical weathering involves the change in the composition of rocks, often leading to a break-down in its form. This type of weathering happens over a period of time.
Modes of Chemical Weathering:
Rainfall is naturally slightly acidic because atmospheric carbon dioxide dissolves in the rainwater producing weak Carbonic Acid. In unpolluted environments, the rainfall pH is around 5.6. Acid rain occurs when gases such as Sulphur dioxide and nitrogen oxide are present in the atmosphere. These oxides react in the rainwater to produce stronger acids and can lower the pH to 4.5 or even lesser. Sulphur Dioxide, which comes from volcanic eruption or from fossil fuels, can become sulphuric acid within rainwater, which can cause solution weathering to the rocks on which it falls. One of the most well-known solution weathering process is carbonation ; the process in which atmospheric carbon dioxide leads to solution weathering. Carbonation occurs on rocks which contain Calcium Carbonate such as lime stone and chalk. This takes place when rain combines with carbon dioxide or an organic acid to form a weak carbonic acid which reacts with calcium carbonate (the limestone) and forms calcium bicarbonate. This process speeds up with a decrease in temperature and therefore is a large feature of glacial weathering.
The reaction is as follows:
Carbondioxide +Water———-Carbonic Acid
(Calcite+Carbonic Acid ———-Calcium+Bicarbonate
Carbonation on the surface of well-joined limestone produces a dissected limestone pavement which is most effective along the joints, widening and deepening them.
Within the weathering environment, chemical oxidation of a variety of metals occurs. The most commonly observed is the oxidation of Iron and its combination with oxygen and water to form Iron Hydroxides. This gives the affected rocks a reddish brown colouration on the surface which crumbles easily and weakens the rock. This process is better known as rusting.
Hydration is a form of chemical weathering that involves the rigid attachment of H+ and OH- ions to the atoms and molecules of a mineral. When rock minerals take up water, the increased volume creates physical stresses within the rock. Hydration also allows for the acceleration of other decompositional reactions by expanding the crystal lattice offering more surface area for reaction.
Hydrolysis is the weathering reaction that occurs when the two surface of water and compound meet. It involves the reaction between mineral ions and the ions of water (OH- and OH+), and results in the decomposition of the rock surface by forming new compounds, and by increasing the pH of the solution involved through the release of the hydroxide ions. Hydrolysis is especially effective in the weathering of common silicate and alumino-silicate minerals because of their electrically charged crystal surfaces.