A carbonate is a salt or ester of carbonic acid, H2CO3, containing the carbonate ion [CO3] 2-. A solution of carbon dioxide in water reacts with a base to form a carbonate.


Carbonates are common rock-forming minerals, occurring as calcite and limestone (both calcium carbonate, CaCO3), siderite (FeCaCO3), magnesite (MgCO3), dolomite (CaMg(CO3)2), and ankerite (Ca(Fe,Mg,Mn)(CO3)2).


The carbonates of the alkali metals and alkaline earth metals are stable (the latter insoluble). Most other metal carbonates are precipitated as basic carbonates, e.g. 2PbCO3·Pb(OH)2. All carbonates except the alkali metal carbonates lose carbon dioxide on strong heating.


Carbonate esters are important solvents.


The bicarbonates (hydrogen carbonates) contain the [HCO3] - ion and important salts include NaHCO3 and Ca(HCO3)2. Bicarbonates are formed by the action of carbon dioxide on carbonates in aqueous solution; this reaction is reversed on heating. Dissolved calcium and magnesium bicarbonates give rise to hard water.