Editing Carbon dioxide (section)

=== In aqueous solution ===
{{See also|Carbonic acid}}
Carbon dioxide is [[soluble]] in water, in which it reversibly forms {{chem2|H2CO3}} (carbonic acid), which is a [[Acid strength|weak acid]], because its ionization in water is incomplete.
:{{chem2|CO2 + H2O <-> H2CO3}}

The [[Henry's law|hydration equilibrium constant]] of carbonic acid is, at 25&nbsp;°C:
:<math chem>K_\mathrm{h} = \frac{\ce{[H2CO3]}}{\ce{[CO2_{(aq)}]}} = 1.70 \times 10^{-3}</math>
Hence, the majority of the carbon dioxide is not converted into carbonic acid, but remains as {{CO2}} molecules, not affecting the pH.

The relative concentrations of {{CO2}}, {{chem2|H2CO3}}, and the [[deprotonation|deprotonated]] forms {{chem2|HCO3-}} ([[bicarbonate]]) and {{chem2|CO3(2-)}}([[carbonate]]) depend on the [[pH]]. As shown in a [[Bjerrum plot]], in neutral or slightly alkaline water (pH > 6.5), the bicarbonate form predominates (>50%) becoming the most prevalent (>95%) at the pH of seawater. In very alkaline water (pH > 10.4), the predominant (>50%) form is carbonate. The oceans, being mildly alkaline with typical pH = 8.2–8.5, contain about 120&nbsp;mg of bicarbonate per liter.

Being [[diprotic acid|diprotic]], carbonic acid has two [[acid dissociation constant]]s, the first one for the dissociation into the bicarbonate (also called hydrogen carbonate) ion ({{chem2|HCO3-}}):

:{{chem2|H2CO3 <-> HCO3- + H+}}
:''K''<sub>a1</sub> = 2.5 × 10<sup>−4</sup> mol/L; p''K''<sub>a1</sub> = 3.6 at 25&nbsp;°C.<ref name=Green>{{Greenwood&Earnshaw2nd|pages=305–314|name-list-style=vanc}}</ref>
This is the ''true'' first acid dissociation constant, defined as
:<math chem>K_\mathrm{a1} = \frac{\ce{[HCO3- ][H+]}}{\ce{[H2CO3]}}</math>
where the denominator includes only covalently bound {{chem2|H2CO3}} and does not include hydrated {{CO2}}(aq). The much smaller and often-quoted value near 4.16 × 10<sup>−7</sup> (or pK<sub>a1</sub> = 6.38) is an ''apparent'' value calculated on the (incorrect) assumption that all dissolved {{CO2}} is present as carbonic acid, so that 
:<math chem>K_\mathrm{a1}{\rm{(apparent)}}=\frac{\ce{[HCO3- ][H+]}}{\ce{[H2CO3] + [CO2_{(aq)}]}}</math>
Since most of the dissolved {{CO2}} remains as {{CO2}} molecules, ''K''<sub>a1</sub>(apparent) has a much larger denominator and a much smaller value than the true ''K''<sub>a1</sub>.<ref>{{cite book | vauthors = Jolly WL | title = Modern Inorganic Chemistry | publisher = McGraw-Hill | date = 1984 | pages = 196 | isbn = 978-0-07-032760-3}}</ref>

The bicarbonate ion is an [[amphoteric]] species that can act as an acid or as a base, depending on pH of the solution. At high pH, it dissociates significantly into the [[carbonate]] ion ({{chem2|CO3(2-)}}):
:{{chem2|HCO3- <-> CO3(2-) + H+}}
:''K''<sub>a2</sub> = 4.69 × 10<sup>−11</sup> mol/L; p''K''<sub>a2</sub> = 10.329

In organisms, carbonic acid production is catalysed by the [[enzyme]] known as [[carbonic anhydrase]].

In addition to altering its acidity, the presence of carbon dioxide in water also affects its electrical properties. [[File:Millipore co2.svg|thumb|400px|Electrical conductivity of carbondioxide saturated desalinated water when heated from 20 to 98&nbsp;°C. The shadowed regions indicate the error bars associated with the measurements. A comparison with the temperature dependence of vented desalinated water can be found [https://commons.wikimedia.org/wiki/File:Electric_conduction_of_vented_and_CO2_saturated_desalinated_water_as_function_of_temperature.svg here] .]]  When carbon dioxide dissolves in desalinated water, the electrical conductivity increases significantly from below 1 μS/cm to nearly 30 μS/cm. When heated, the water begins to gradually lose the conductivity induced by the presence of <math> \mathrm{CO_{2}} </math> , especially noticeable as temperatures exceed 30&nbsp;°C.

The [[Conductivity (electrolytic)#Conductivity of purified water in electrochemical experiments|temperature dependence]] of the electrical conductivity of fully deionized water without {{CO2}} saturation is comparably low in relation to these data.