When CO2 Is Dissolved in Water What Is Formed? Carbonic Acid

When carbon dioxide (CO₂) dissolves in water, it forms carbonic acid (H₂CO₃). The chemical equation is CO₂ + H₂O ⇌ H₂CO₃. This is a reversible reaction, and at any given time only a small fraction of the dissolved CO₂ exists as carbonic acid — roughly 1% or less at typical temperatures and pressures. Most remains as dissolved CO₂ molecules.

How Carbonic Acid Forms and What It Does

Once formed, carbonic acid quickly dissociates into bicarbonate (HCO₃⁻) and hydrogen ions (H⁺), which makes the solution slightly acidic. The reaction is slow without an enzyme called carbonic anhydrase, which speeds it up in biological tissues Royal Society of Chemistry. In the ocean, this process drives the carbonate-bicarbonate buffer system that regulates pH. For example, when you open a soda can, the dissolved CO₂ escapes as bubbles, and the drink’s acidity drops because less carbonic acid remains.

A concrete example: if you bubble CO₂ through pure water at room temperature, the pH drops from about 7 to around 5–6, depending on the amount of gas dissolved PubChem. This same reaction happens in your bloodstream, where CO₂ from metabolism is converted to carbonic acid and then to bicarbonate for transport to the lungs.

The equilibrium shifts with temperature. Cold water holds more dissolved CO₂ and therefore produces more carbonic acid, which is why a chilled soda tastes sharper than a warm one.

In the human body, red blood cells contain carbonic anhydrase, which converts CO₂ into carbonic acid thousands of times faster than the uncatalyzed rate Royal Society of Chemistry. This allows the blood to carry CO₂ from tissues to the lungs efficiently. In the environment, more atmospheric CO₂ dissolving into seawater increases carbonic acid concentration and lowers pH — a process called ocean acidification. This reduces the availability of carbonate ions that shellfish and corals need to build their calcium carbonate shells.

Key Takeaways

• When CO₂ dissolves in water, it forms carbonic acid (H₂CO₃) through a reversible chemical reaction.
• Carbonic acid is a weak acid that partially dissociates into bicarbonate and hydrogen ions, lowering the pH of the solution.
• This process is essential for pH buffering in oceans and blood, and it explains why carbonated drinks are acidic.
• The reaction is accelerated by the enzyme carbonic anhydrase in living organisms, allowing rapid CO₂ transport and acid-base regulation.