Carbonic acid

Carbonic acid

Names
Preferred IUPAC name Carbonic acid
Other names Carbon dioxide solution Dihydrogen carbonate Acid of air Aerial acid Hydroxymethanoic acid
Identifiers
CAS Number	463-79-6 Y
3D model (Jmol)	Interactive image
ChEBI	CHEBI:28976 Y
ChEMBL	ChEMBL1161632 Y
ChemSpider	747 Y
ECHA InfoCard	100.133.015
KEGG	C01353 Y
InChI InChI=1S/CH2O3/c2-1(3)4/h(H2,2,3,4) Y Key: BVKZGUZCCUSVTD-UHFFFAOYSA-N Y InChI=1/CH2O3/c2-1(3)4/h(H2,2,3,4) Key: BVKZGUZCCUSVTD-UHFFFAOYAU
SMILES O=C(O)O
Properties
Chemical formula	H2CO3
Molar mass	62.03 g/mol
Density	1.668 g/cm3
Solubility in water	Exists only in solution
Acidity (pKa)	3.6 (pKa1 for H2CO3 only), 6.3 (pKa1 including CO2(aq)), 10.32 (pKa2)
Related compounds
Related compounds	Acetone Urea Carbonyl fluoride
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Y  (what is YN ?)
Infobox references

Carbonic acid is a chemical compound with the chemical formula H₂CO₃ (equivalently OC(OH)₂). It is also a name sometimes given to solutions of carbon dioxide in water (carbonated water), because such solutions contain small amounts of H₂CO₃. In physiology, carbonic acid is described as volatile acid or respiratory acid, because it is the only acid excreted as a gas by the lungs. It plays an important role in the bicarbonate buffer system to maintain acid–base homeostasis.

Carbonic acid, which is a weak acid, forms two kinds of salts, the carbonates and the bicarbonates. In geology, carbonic acid causes limestone to dissolve producing calcium bicarbonate which leads to many limestone features such as stalactites and stalagmites.

It was long believed that carbonic acid could not exist as a pure compound. However, in 1991 it was reported that NASA scientists had succeeded in making solid H₂CO₃ samples.

When carbon dioxide dissolves in water it exists in chemical equilibrium producing carbonic acid:

The hydration equilibrium constant at 25 °C is called K_h, which in the case of carbonic acid is [H₂CO₃]/[CO₂] ≈ 1.7×10⁻³ in pure water and ≈ 1.2×10⁻³ in seawater. Hence, the majority of the carbon dioxide is not converted into carbonic acid, remaining as CO₂ molecules. In the absence of a catalyst, the equilibrium is reached quite slowly. The rate constants are 0.039 s⁻¹ for the forward reaction (CO₂ + H₂O → H₂CO₃) and 23 s⁻¹ for the reverse reaction (H₂CO₃ → CO₂ + H₂O). The addition of two molecules of water to CO₂ would give orthocarbonic acid, C(OH)₄, which exists only in minute amounts in aqueous solution.