View source for Carbonate

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[[Image:Carbonate-3D-balls.png|thumb|right|200px|[[Ball-and-stick model]] of the carbonate ion, CO<sub>3</sub><sup>2−</sup>]]
In [[chemistry]], a '''carbonate''' is a [[salt]] or [[ester]] of [[carbonic acid]].

==Applications==
[[Soda water]]                                     (also known as Seltzer water) is water with CO<sub>2</sub> dissolved under pressure.  The taste of soda water was discovered by the 18th century chemist [[Joseph Priestley]].

To test for the presence of the carbonate anion in a salt, the addition of dilute mineral acid (e.g. [[hydrochloric acid]]) will yield carbon dioxide gas.

Carbonate-containing [[salt]]s are industrially and [[mineral]]ogically ubiquitous.  The term "carbonate" is also commonly used to refer to one of these salts or [[carbonate minerals]]. Most common is [[calcite]], or calcium carbonate, the chief constituent of [[limestone]].  The process of removing carbon dioxide from these salts by heating is called [[calcination]].

The term is also used as a verb, to describe the process of raising carbonate and [[bicarbonate]] concentrations in water, see also [[carbonated water]], either by the introduction under pressure of [[carbon dioxide]] gas into the water, or by dissolving carbonate or bicarbonate salts into the water.

==Chemical properties==
The '''carbonate ion''' is a [[polyatomic ion|polyatomic]] [[anion]] with the [[empirical formula]] [[carbon|C]][[oxygen|O]]<sub>3</sub><sup>2−</sup> and a molecular mass of 60.01 [[dalton (unit)|daltons]]; it consists of one central carbon [[atom]] surrounded by three identical oxygen atoms in a [[trigonal planar]] arrangement. The carbonate ion carries a negative two [[formal charge]] and is the [[conjugate acid|conjugate base]] of the [[bicarbonate|hydrogen carbonate]] ion, HCO<sub>3</sub><sup>−</sup>, which is the conjugate base of H<sub>2</sub>CO<sub>3</sub>, carbonic acid.

A carbonate salt forms when a positively charged ion attaches to the negatively charged oxygen atoms of the ion, forming an ionic [[chemical compound|compound]]. Most carbonate salts are [[solubility|insoluble]] in [[water]] at [[standard temperature and pressure]], with [[solubility constant]]s of less than 1×10<sup>−8</sup>. Exceptions include [[sodium carbonate|sodium]], [[potassium carbonate|potassium]] and [[ammonium carbonate|ammonium]] carbonates.

In [[aqueous solution]], carbonate, bicarbonate, carbon dioxide, and carbonic acid exist together in a [[chemical equilibrium|dynamic equilibrium]]. In strongly basic conditions, the carbonate ion predominates, while in weakly basic conditions, the [[bicarbonate]] ion is prevalent. In more acid conditions, aqueous [[carbon dioxide]], CO<sub>2</sub>(aq), is the main form, which, with water, H<sub>2</sub>O, is in equilibrium with carbonic acid - the equilibrium lies strongly towards carbon dioxide. Thus [[sodium carbonate]] is basic, [[sodium bicarbonate]] is weakly basic, while carbon dioxide itself is a weak acid.  

[[Carbonated water]] is formed by dissolving CO<sub>2</sub> in water under pressure.  When the partial pressure of CO<sub>2</sub> is reduced, for example when a can of soda is opened, the equilibrium for each of the forms of carbonate (carbonate, bicarbonate, carbon dioxide, and carbonic acid) shifts until the concentration of CO<sub>2</sub> in the solution is equal to the solubility of CO<sub>2</sub> at that temperature and pressure.  In living systems an enzyme, [[carbonic anhydrase]], speeds the interconversion of CO<sub>2</sub> and carbonic acid.

In organic chemistry a carbonate can also refer to a [[functional group]] within a larger molecule that contains a carbon atom bound to three oxygen atoms, one which is double bonded.  The [[VSEPR_theory|VSEPR]] shape of the carbonate ion is trigonal planar or triplanar

== Acid-base chemistry ==
The carbonate ion (CO<sub>3</sub><sup>2−</sup>) is a strong base. It is a conjugate base of the weakly acidic [[bicarbonate]] ([[IUPAC]] name [[bicarbonate|hydrogen carbonate]] HCO<sub>3</sub><sup>−</sup>), itself a strong conjugate base of the still weakly acidic [[carbonic acid]]. As such in aqueous solution, the carbonate ion seeks to reclaim hydrogen atoms.

It works as a buffer in the blood as follows:
when pH is too low, the concentration of hydrogen ions is too high, so you exhale CO<sub>2</sub>. This cause the equation to shift left, essentially decreasing the concentration of H<sup>+</sup> ions, causing a more basic pH.

When pH is too high, the concentration of hydrogen ions in the blood is too low, so the kidneys excrete bicarbonate (HCO<sub>3</sub><sup>−</sup>).  This causes the equation to shift right, essentially increasing the concentration of hydrogen ions, causing a more acidic pH.

== Carbonate salts ==
{{seealso|Category:Carbonates}}

* Carbonate overview:
{{Template:Carbonates}}

==History==
It is generally thought that the presence of carbonates in [[Rock (geology)|rock]] is unequivocal evidence for the presence of liquid water.  Recent observations of the [[Planetary nebula]] [[NGC 6302]] shows evidence for carbonates in space <ref>Kemper, F., Molster, F.J., Jager, C. and Waters, L.B.F.M. (2002) The mineral composition and spatial distribution of the dust ejecta of NGC 6302.  ''Astronomy & Astrophysics'' '''394''', 679-690.</ref>, where aqueous alteration similar to that on Earth is unlikely. Other minerals have been proposed which would fit the observations.

Significant [[Carbonates on Mars|carbonate deposits]] have not been found on Mars via remote sensing or in situ missions, even though Martian meteorites contain small amounts and groundwater may have existed at both Gusev<ref>Squyres et al., (2007) [http://dx.doi.org/10.1126/science.1139045 doi 10.1126/science.1139045]</ref> and Meridiani Planum<ref>Squyres et al., (2006) [http://dx.doi.org/10.1029/2006JE002771 doi 10.1029/2006JE002771]</ref>.

==References==
{{reflist|2}}

{{Oxides of carbon}}

==See also==

[[Bicarbonate]]

[[Category:Carbonates| ]]
[[Category:Oxoanions]]

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