carbonic acid A weak acid, H₂CO₃, formed in solution when carbon dioxide is dissolved in water: CO₂ + H₂O ↔ H₂CO₃

The acid is in equilibrium with dissolved carbon dioxide, and also dissociates into hydrogencarbonate and hydrogen ions. These reactions, catalysed by carbonic anhydrase, take place in the red blood cells when carbon dioxide diffuses into them from the surrounding tissue cells. See also hydrogencarbonate.

car·bon·ic ac·id • n. a very weak acid, H₂CO₃, formed in solution when carbon dioxide dissolves in water.