Carbonate is a compound ion with a charge of -2 and an elemental formula of CO32-. An aqueous solution of carbon dioxide contains a minute amount of H2CO3, called carbonic acid, which dissociates to form hydrogen ions and carbonate ions. It would be a fairly strong acid if carbonic acid existed in pure form, but the equilibrium favors carbon dioxide and so such solutions are fairly weak. In biological systems the enzyme carbonic anhydrase catalyzes this interconversion between carbon dioxide and carbonate ions.

Carbonate-containing salts are industrially and minerologically ubiquitous. The term "carbonate" is also commonly used to refer to one of these salts. Most common is limestone, or calcium carbonate.

The term is also used as a verb, to describe the process of raising carbonate and bicarbonate concentrations in water, see also carbonated water, either by the introduction under pressure of carbon dioxide gas into the water, or by dissoving carbonate or bicarbonate salts into the water.

At one time, it was thought that the presence of carbonates in rock was unequivocal evidence for the presence of liquid water. Recent observations of two planetary nebulae reported in the January 17, 2002 issue of the scientific journal Nature indicate that carbonates can form in interplanetary space.

Carbonates were detected in the Gusev Crater on Mars by the Mars Exploration Rover Spirit on January 9, 2004. [1]