Editing Weathering (section)

===Dissolution===

[[File:Weathered limestone cores.jpg|thumb|[[Limestone]] [[core samples]] at different stages of chemical weathering, from very high at shallow depths (bottom) to very low at greater depths (top). Slightly weathered limestone shows brownish stains, while highly weathered limestone loses much of its carbonate mineral content, leaving behind clay. Limestone drill core taken from the carbonate West Congolian deposit in [[Kimpese]], [[Democratic Republic of Congo]].]]

Dissolution (also called ''simple solution'' or ''congruent dissolution'') is the process in which a mineral dissolves completely without producing any new solid substance.<ref>{{cite book |last1=Birkeland |first1=Peter W. |title=Soils and geomorphology |date=1999 |publisher=Oxford University Press |location=New York |isbn=978-0195078862 |page=59 |edition=3rd}}</ref> Rainwater easily dissolves soluble minerals, such as [[halite]] or [[gypsum]], but can also dissolve highly resistant minerals such as [[quartz]], given sufficient time.<ref>{{cite book |last1=Boggs |first1=Sam |title=Principles of sedimentology and stratigraphy |date=2006 |publisher=Pearson Prentice Hall |location=Upper Saddle River, N.J. |isbn=0131547283 |edition=4th |page=7}}</ref> Water breaks the bonds between atoms in the crystal:<ref name="Environmental Studies in Sedimentar">{{cite journal|jstor=43418626|title=Environmental Studies in Sedimentary Geochemistry|last1=Nicholls|first1=G. D.|journal=Science Progress (1933- )|year=1963|volume=51|issue=201|pages=12–31}}</ref>

[[File:SiO H2O.jpg|Hydrolysis of a silica mineral]]

The overall reaction for dissolution of quartz is
:{{chem2|SiO2 + 2 H2O -> H4SiO4}}

The dissolved quartz takes the form of [[silicic acid]].

A particularly important form of dissolution is carbonate dissolution, in which atmospheric [[carbon dioxide]] enhances solution weathering. Carbonate dissolution affects rocks containing [[calcium carbonate]], such as [[limestone]] and [[chalk]]. It takes place when rainwater combines with carbon dioxide to form [[carbonic acid]], a [[weak acid]], which dissolves calcium carbonate (limestone) and forms soluble [[calcium bicarbonate]]. Despite a slower [[reaction kinetics]], this process is thermodynamically favored at low temperature, because colder water holds more dissolved carbon dioxide gas (due to the retrograde [[solubility]] of gases). Carbonate dissolution is therefore an important feature of glacial weathering.<ref>{{cite journal |last1=Plan |first1=Lukas |title=Factors controlling carbonate dissolution rates quantified in a field test in the Austrian alps |journal=Geomorphology |date=June 2005 |volume=68 |issue=3–4 |pages=201–212 |doi=10.1016/j.geomorph.2004.11.014|bibcode=2005Geomo..68..201P }}</ref>

Carbonate dissolution involves the following steps:

:CO<sub>2</sub> + H<sub>2</sub>O → H<sub>2</sub>CO<sub>3</sub>
:carbon dioxide + water → carbonic acid

:H<sub>2</sub>CO<sub>3</sub> + CaCO<sub>3</sub> → Ca(HCO<sub>3</sub>)<sub>2</sub>
:carbonic acid + calcium carbonate → calcium bicarbonate

Carbonate dissolution on the surface of well-jointed limestone produces a dissected [[limestone pavement]]. This process is most effective along the joints, widening and deepening them.<ref name="Geology and geomorphology">{{cite web|url=http://www.limestone-pavements.org.uk/geology.html|title=Geology and geomorphology|last=Anon|work=Limestone Pavement Conservation|publisher=UK and Ireland Biodiversity Action Plan Steering Group|accessdate=30 May 2011|archive-url=https://web.archive.org/web/20110807234809/http://www.limestone-pavements.org.uk/geology.html|archive-date=7 August 2011|url-status=dead}}</ref>

In unpolluted environments, the [[pH]] of rainwater due to dissolved carbon dioxide is around 5.6.  [[Acid rain]] occurs when gases such as sulfur dioxide and nitrogen oxides are present in the atmosphere.  These oxides react in the rain water to produce stronger acids and can lower the pH to 4.5 or even 3.0. [[Sulfur dioxide]], SO<sub>2</sub>, comes from volcanic eruptions or from fossil fuels, and can become [[sulfuric acid]] within rainwater, which can cause solution weathering to the rocks on which it falls.<ref>{{cite journal |last1=Charlson |first1=R. J. |last2=Rodhe |first2=H. |title=Factors controlling the acidity of natural rainwater |journal=Nature |date=February 1982 |volume=295 |issue=5851 |pages=683–685 |doi=10.1038/295683a0|bibcode=1982Natur.295..683C |s2cid=4368102 }}</ref>