Carbonic acid

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6 atoms, 5 bonds.

IUPAC name: carbonic acid
Molar mass: 62.0240(54) g/mol
Atoms: 6
SMILES: C(=O)(O)O
InChIKey: BVKZGUZCCUSVTD-UHFFFAOYSA-N
PubChem CID: 767

Made of

Structural formula of carbonic acid. Each unlabelled corner is a carbon atom. It shows which atoms are joined to which, not the shape. The model above shows the shape.

Rain, soda water and human blood each hold a little carbonic acid, made when dissolved carbon dioxide reacts with the water around it. Its molecule joins one carbon, two hydrogen and three oxygen atoms, 62.024 g/mol in all. The US National Oceanic and Atmospheric Administration (NOAA) calls it a weak acid that comes apart in water into a hydrogen ion and a bicarbonate ion. ChEBI, a database of small biological molecules, names it the conjugate acid of hydrogencarbonate, another name for bicarbonate: the acid that ion becomes when it gains a proton.

Only a sliver of the dissolved gas ever becomes the acid. Such a solution is mostly hydrated CO₂, the gas loosely bound to the water around it, according to Stephen Lower’s Chem1 textbook. The acid itself cannot be isolated pure under ordinary conditions.

Structure and bonding

One carbon, three oxygens and two hydrogens make up carbonic acid. The carbon sits in the middle, bonded to all three oxygens, one by a double bond. Each hydrogen hangs from one of the other two oxygens, never from the carbon. Turned edge-on, all six atoms fall into one plane. In this computed model the angles around the carbon are uneven: about 125° between the double bond and each single C–O, about 110° between the two single ones.

Ball-and-stick shows the double bond as the shortest of the three, 1.22 Å in the model against 1.35 Å for each single C–O. That matches Chem1’s rule that multiply bonded atoms lie closer together than singly bonded ones. Stick drops the balls and keeps the flat frame. Space-filling packs the spheres tightly: three oxygens crowd round the carbon, and the hydrogens show only as small caps on two of them.

The model draws the neutral molecule, which is not what dissolved carbon dioxide mostly becomes. The small share that becomes carbonic acid splits into hydrogen ions and bicarbonate ions, HCO₃⁻, and the viewer draws neither.

Where it turns up

Rain turns acidic on its way down. Falling drops take in carbon dioxide from the air, and part of it reacts to give carbonic acid. On Chem1’s account the same happens to all pure water left in contact with air. Freshly distilled water shows the same effect, its pH creeping downward as the gas dissolves from the air.

The tingle of soda water on your tongue is carbonic acid too. It forms where bubbles of carbon dioxide touch the mucous membranes, the moist lining of the mouth, as Chem1 describes it.

Chem1 traces the ocean’s large capacity for carbon dioxide to slightly alkaline water carrying it on to bicarbonate and carbonate ions. How far surface pH has fallen is set out on the carbon dioxide page, and what that means for shells on the calcium carbonate page.

Why it matters

ChEBI lists carbonic acid as a mouse metabolite, a substance the animal’s body chemistry produces, and human cells make it too. Every cell releases carbon dioxide as a by-product of its metabolism, and in water part of the gas reacts to form the acid. Converted to bicarbonate, it travels safely in the blood to the lungs. There carbonic acid loses water, and the carbon dioxide it gives up leaves in the breath. Carbonic acid that the body makes from food takes the same exit, most of it breathed out almost at once, Chem1 notes.

Blood is buffered, kept at a steady pH, while carbon dioxide from metabolism is added to it and then removed by the lungs. In Chem1’s account the carbonate system is the body’s main buffer, and at normal physiological pH it exists mostly as bicarbonate, not as the acid drawn in the model.

Properties

Properties of carbonic acid Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 767.
Property	Value	Unit	Source
Molecular formula (Hill order)	CH₂O₃
Molar mass	62.0240(54) g/mol	g/mol	IUPAC/CIAAW 2024
Hydrogen atoms	2
Carbon atoms	1
Oxygen atoms	3
Bonds	5
Single bonds	4
Double bonds	1
PubChem molecular weight	62.025 g/mol	g/mol	PubChem

Safety

Carbonic acid is a mild acid, formed from carbon dioxide and water in a nonhazardous reaction, on the description in PubChem’s carbon dioxide record. A single company’s notification to the European Chemicals Agency (ECHA) gives it the signal word Warning, calling it harmful if swallowed, inhaled or in contact with skin. The same notification lists skin irritation, serious eye irritation and possible respiratory irritation, all gathered in the PubChem Laboratory Chemical Safety Summary for carbonic acid.

Questions

Can you buy pure carbonic acid?

Not as a pure chemical. Under ordinary conditions the acid cannot be isolated on its own, and it gives up water to become carbon dioxide again. New Zealand's Environmental Protection Authority has not approved it as a chemical to be used by itself. Its salts are another matter: carbonates and hydrogen carbonates are the forms in which, according to MeSH, the acid exists.

Is carbonic acid a strong or weak acid?

Weak. In water it releases H⁺ and becomes the bicarbonate ion, HCO₃⁻, in NOAA's description. As it is usually met, in a solution of carbon dioxide, Chem1 lists it with boric acid among the very weak acids. The figure usually quoted for its strength is a blend, measured on a solution that is mostly dissolved carbon dioxide, so the pure acid is considerably stronger than that figure suggests.

Is carbonated water the same as carbonic acid?

Not quite. Carbon dioxide dissolved in a drink stays mostly as the gas, loosely bound to the water around it in what Chem1 calls a hydrate. Only a very small fraction of that hydrate combines into H₂CO₃, so the drink holds far more dissolved gas than acid. The acid it does hold is a mild one, as PubChem's carbon dioxide record describes it.

Made of the same elements

HCOOH Formic acid 5 atoms, 4 bonds.
CH₂O Formaldehyde 4 atoms, 3 bonds.
H₂C₂O₄ Oxalic acid 8 atoms, 7 bonds.
C₂H₄O₃ Glycolic acid 9 atoms, 8 bonds.
CH₃COOH Acetic acid 8 atoms, 7 bonds.
CH₃OH Methanol 6 atoms, 5 single bonds.
CH₃CHO Acetaldehyde 7 atoms, 6 bonds.
C₂H₄O Ethylene oxide 7 atoms, 7 single bonds.

All 51 compounds made of hydrogen, carbon and oxygen

C₂H₆O₂ Ethylene glycol 10 atoms, 9 single bonds.
C₃H₆O₃ Lactic acid 12 atoms, 11 bonds.
CH₃OCH₃ Dimethyl ether 9 atoms, 8 single bonds.
C₂H₅OH Ethanol 9 atoms, 8 single bonds.
C₂H₅COOH Propionic acid 11 atoms, 10 bonds.
C₃H₆O Acetone 10 atoms, 9 bonds.
C₃H₈O₃ Glycerol 14 atoms, 13 single bonds.
C₄H₆O₅ Malic acid 15 atoms, 14 bonds.
C₃H₈O₂ Propylene glycol 13 atoms, 12 single bonds.
C₃H₇OH 1-Propanol 12 atoms, 11 single bonds.
C₃H₇COOH Butyric acid 14 atoms, 13 bonds.
CH₃COOC₂H₅ Ethyl acetate 14 atoms, 13 bonds.
C₃H₈O Isopropyl alcohol 12 atoms, 11 single bonds.
C₇H₆O₃ Salicylic acid 16 atoms, 16 bonds.
C₄H₆O₆ Tartaric acid 16 atoms, 15 bonds.
C₆H₅COOH Benzoic acid 15 atoms, 15 bonds.
C₄H₈O Butanone 13 atoms, 12 bonds.
C₆H₅OH Phenol 13 atoms, 13 bonds.
C₆H₅CHO Benzaldehyde 14 atoms, 14 bonds.
C₄H₉OH 1-Butanol 15 atoms, 14 single bonds.
C₂H₅OC₂H₅ Diethyl ether 15 atoms, 14 single bonds.
C₈H₈O₃ Methyl salicylate 19 atoms, 19 bonds.
C₈H₈O₃ Vanillin 19 atoms, 19 bonds.
C₆H₈O₆ Ascorbic acid 20 atoms, 20 bonds.
C₉H₈O₄ Aspirin 21 atoms, 21 bonds.
C₆H₈O₇ Citric acid 21 atoms, 20 bonds.
C₆H₁₂O₆ Fructose 24 atoms, 24 single bonds.
C₆H₁₂O₆ Glucose 24 atoms, 24 single bonds.
C₁₄H₁₄O₃ Naproxen 31 atoms, 32 bonds.
C₁₅H₁₆O₂ Bisphenol A 33 atoms, 34 bonds.
C₁₃H₁₈O₂ Ibuprofen 33 atoms, 33 bonds.
C₁₀H₂₀O Menthol 31 atoms, 31 single bonds.
C₂₀H₁₄O₄ Phenolphthalein 38 atoms, 41 bonds.
C₁₂H₂₂O₁₁ Lactose 45 atoms, 46 single bonds.
C₁₂H₂₂O₁₁ Sucrose 45 atoms, 46 single bonds.
C₁₈H₂₄O₂ Estradiol 44 atoms, 47 bonds.
C₁₉H₂₈O₂ Testosterone 49 atoms, 52 bonds.
C₁₆H₃₂O₂ Palmitic acid 50 atoms, 49 bonds.
C₂₁H₃₀O₂ Progesterone 53 atoms, 56 bonds.
C₂₁H₃₀O₅ Cortisol 56 atoms, 59 bonds.
C₁₈H₃₄O₂ Oleic acid 54 atoms, 53 bonds.
C₂₇H₄₄O Cholecalciferol 72 atoms, 74 bonds.
C₂₇H₄₆O Cholesterol 74 atoms, 77 bonds.