What is chemical weathering?

Question

i am an 11th grader in a 9th grade earth science class to pass my regents and i dont have my notes on weathering!!!!!!!!!!!! SOMEBODY PLEASE HELP ME!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Answer 1

breaking down of rock, soil, minerals due to heat, water, ice etc.

Answer 2

Chemical weathering is the breaking down of rock by chemical action, as opposed to mechanical weathering. In mechanical weathering, rock is broken off little bit by little bit, but the little bits are chemically the same as the rock they came from. In chemical weathering, the rock is altered chemically. Some rocks, when altered by the normal amount of acidity in rain, turn to clay.

Answer 3

Chemical weathering attacks the surfaces of rocks (and cars, buildings) and is accelerated by increasing the available surface area.

Answer 4

chemical weathering is the breaking down of something without changing it's composition.

Answer 5

Chemical weathering involves the change in the composition of rock, often leading to a 'break down' in its form.


Solution

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 oxides 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 4.0. Sulfur dioxide, SO2, comes from volcanic eruptions or from fossil fuels, can become sulfuric acid within rainwater, which can cause solution weathering to the rocks on which it falls.

One of the most well-known solution weathering processes is carbonation, the process in which atmospheric carbon dioxide leads to solution weathering. Carbonation occurs on rocks which contain calcium carbonate such as limestone 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 reactions as follows:

CO2 + H2O ⇌ H2CO3
carbon dioxide + water ⇌ carbonic acid
H2CO3 + CaCO3 ⇌ Ca(HCO3)2
carbonic acid + calcium carbonate ⇌ calcium bicarbonate


Hydration

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, it increases in volume, thus setting up physical stresses within the rock.Iron oxides are converted to Iron hydroxides Evidence: Surface flaking [exfoliation] E.g. the hydration of anhydrite forms gypsum

A freshly broken rock shows differential chemical weathering (probably mostly oxidation) progressing inward. This piece of sandstone was found in glacial drift near Angelica, New York
[edit] Hydrolysis
Hydrolysis is a chemical weathering process affecting Silicate minerals. In such reactions, pure water ionizes slightly and reacts with silicate minerals. An example reaction:

Mg2SiO4 + 4H+ + 4OH- ⇌ 2Mg2+ + 4OH- + H4SiO4
olivine (forsterite) + four ionized water molecules ⇌ ions in solution + silicic acid in solution
This reaction results in complete dissolution of the original mineral, assuming enough water is available to drive the reaction. However, the above reaction is to a degree deceptive because pure water rarely acts as a H+ donor. Carbon dioxide, though, dissolves readily in water forming a weak acid and H+ donor.

Mg2SiO4 + 4CO2 + 4H2O ⇌ 2Mg2+ + 4HCO3- + 4H4SiO4
olivine (forsterite) + carbon dioxide + water ⇌ Magnesium and bicarbonate ions in solution + silicic acid in solution
This hydrolosis reaction is much more common. Carbonic acid is consumed by silicate weathering, resulting in more alkaline solutions because of the bicarbonate. This is an important reaction in controlling the amount of CO2 in the atmosphere and can affect climate.

Aluminosilicates when subjected to the hydrolosis reaction produce a secondary mineral rather than simply releasing cations.

2KAlSi3O8 + 2H2CO3 + 9H2O ⇌ Al2Si2O5(OH)4 + 4H4SiO4 + 2K+ + 2HCO3-
Orthoclase - aluminosilicate feldspar + carbonic acid + water ⇌ Kaolinite - a clay mineral + silicic acid in solution + potassium and bicarbonate ions in solution

Oxidation

Within the weathering environment chemical oxidation of a variety of metals occurs. The most commonly observed is the oxidation of Fe2+ (iron) and combination with oxygen and water to form Fe3+ hydroxides and oxides such as goethite, limonite, and hematite. 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'.


Sulfation

Sulfur dioxide can react directly with limestone producing gypsum (calcium sulfate) which is more soluble than calcium carbonate and which is easily dissolved and washed away by subsequent rain. On areas of a building which are sheltered from rain, a gypsum crust may accumulate and trap soot particles derived from fossil fuel combustion.


Biological

A number of plants and animals may create chemical weathering through release of acidic compounds.

The most common form of biological weathering is the release of 'chelating compounds', i.e. acids, by trees so as to break down elements such as Aluminium and Iron in the soils beneath them. Once broken down, such elements are more easily washed away by rainwater. This process exists as metals such as iron can be toxic and hinder the a tree's growth. Extreme release of chelating compounds can easily affect surrounding rocks and soils, and may lead to Podsolisation of soils.