The carbonate ion is the simplest oxocarbon anion, consisting of a carbon atom bonded to three oxygen atoms for a net charge of 2-.
The carbonate ion is one of the most commonly encountered ions. It is present in both inorganic and organic compounds. Most inorganic carbonates are insoluble, with the exception of the alkali carbonates and ammonium carbonate.
The singly protonated form of the carbonate ion, the bicarbonate ion, also exists, but is generally only confined to coexistence with alkali metal ions in the solid state. It coexists with carbonate and carbonic acid in solution.
The doubly protonated conjugate acid of the carbonate ion, carbonic acid, is unstable and spontaneously decomposes into water and carbon dioxide as follows:
- H2CO3 → H2O + CO2
Carbonates are often frequently unstable with heating, with many metal carbonates decomposing at high temperatures(sometimes quite low temperatures) to form a metal oxide and carbon dioxide, such as the simple conversion of copper(II) carbonate to copper(II) oxide.