The salts of carbonic acid are called carbonates and hydrogen carbonates. When water containing dissolved CO2 is pressurized, a weakly acidic solution (formally known as H2CO3) is formed. Carbonic acid is an unstable compound that decomposes into water and carbon dioxide. Many types of carbonates - also known as spar - are found in nature, including mountains such as the Dolomites and coral reefs.

Carbonates are the inorganic salts and organic esters of carbonic acid, which is an inorganic compound.

There are two series of salts derived from the diphotonic (dibasic) acid:

• Hydrogen carbonates, also known as primary carbonates, with the general formula MIHCO3
• Secondary carbonates, with the general formula MI2CO3. The secondary carbonates are based on the double negatively charged carbonate ion CO32-.

The esters of carbonic acid with the general structural formula R1-O-C(=O)-O-R2, where R1 and R2 are carbon-containing alkyl or aryl radicals, are also referred to as carbonates and are described under carbonic acid esters. Polymeric carbonates have the structural formula (O-R-O-C(=O)-)n

Carbonates are ionic salts and therefore generally crystalline solids at room temperature. They are formed by the reaction of a base and an acid, in this case sodium hydroxide and carbonic acid. The carbonate anion does not give the compounds their own color, so their color can be determined by the cation involved. Carbonates are odorless. With the exception of alkali carbonates, they are only slightly soluble in water, so that most metal ions precipitate when they react with alkali carbonates in aqueous solution.

The carbonate anion has a completely planar structure, with 120° bond angles between the individual oxygen atoms. The distances between all three O atoms and the central carbon atom are the same and, at around 130 pm, lie between the length of the C-O single bonds (143 pm) and the C=O double bonds (123 pm).

Carbonates are ubiquitous in nature and can occur in many different forms.

They are common minerals, but do not form a common mineral class with the nitrates, as Strunz (9th edition) suggests. Borates are also classified in the class of carbonates according to Dana's classification system, which is commonly used in the English-speaking world. An important representative of the carbonates is witherite, a barium carbonate mineral. Calcite, aragonite and vaterite are all calcium carbonate minerals, and siderite (iron spar) is an iron carbonate mineral. Azurite and malachite are basic copper carbonates; magnesite is a magnesium carbonate; rhodochrosite (manganese spar) is a manganese carbonate; gaylussite, natrite and pirssonite are sodium carbonates; soda and trona are sodium compounds; smithsonite (zinc spar) is a zinc carbonate; bastnäsite is used to extract rare earth metals from ore deposits. Dolomite (calcium magnesium carbonate) and "potash" (potassium carbonate), which only occurs in nature as a mixture, are further examples of these minerals.

In the hydrosphere, there are large quantities of carbonates dissolved in the oceans, lakes and rivers. These carbonates are used by almost all living organisms as skeletal substances. From a geological point of view, sedimentites (limestone) are more common than metamorphic or igneous rocks (carbonatite).