thumb|right|200px|Ball-and-stick model of the carbonate ion, CO₃²⁻ In chemistry, a carbonate is a salt or ester of carbonic acid.

Applications

To test for the presence of the carbonate anion in a salt, the addition of dilute mineral acid (e.g. hydrochloric acid) will yield carbon dioxide gas.

Carbonate-containing salts are industrially and mineralogically ubiquitous. The term "carbonate" is also commonly used to refer to one of these salts or carbonate minerals. Most common is calcite, or calcium carbonate, CaCO₃, the chief constituent of limestone. The process of removing carbon dioxide from these salts by heating is called calcination.

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

Structure and bonding

The carbonate ion is a polyatomic anion with the empirical formula CO₃²⁻ and a molecular mass of 60.01 daltons; it consists of one central carbon atom surrounded by three identical oxygen atoms in a trigonal planar arrangement, and has D_3h molecular symmetry. The carbonate ion carries a negative two formal charge and is the conjugate base of the hydrogen carbonate ion, HCO₃⁻, which is the conjugate base of H₂CO₃, carbonic acid.

The structure and bonding of the carbonate ion cannot be properly represented by its Lewis structure, which depicts CO₃²⁻ with two long single bonds and one short double bond:

Simple, localised Lewis structure of the carbonate ion

Like the isoelectronic nitrate ion, resonance structures can be used to depict the carbonate ion:

Resonance structures of the carbonate ion

In reality, CO₃²⁻ has three equally long C-O bonds:

Delocalisation and partial charges on the carbonate ion Space-filling model of the carbonate ion

Chemical properties

A carbonate salt forms when a positively charged ion attaches to the negatively charged oxygen atoms of the ion, forming an ionic compound:

2 M⁺ + CO₃²⁻ → M₂CO₃

M²⁺ + CO₃²⁻ → MCO₃

2 M³⁺ + 3 CO₃²⁻ → M₂(CO₃)₃

Most carbonate salts are insoluble in water at standard temperature and pressure, with solubility constants of less than 1×10⁻⁸. Exceptions include sodium, potassium and ammonium carbonates.

In aqueous solution, carbonate, bicarbonate, carbon dioxide, and carbonic acid exist together in a dynamic equilibrium. In strongly basic conditions, the carbonate ion predominates, while in weakly basic conditions, the bicarbonate ion is prevalent. In more acid conditions, aqueous carbon dioxide, CO₂(aq), is the main form, which, with water, H₂O, is in equilibrium with carbonic acid - the equilibrium lies strongly towards carbon dioxide. Thus sodium carbonate is basic, sodium bicarbonate is weakly basic, while carbon dioxide itself is a weak acid.