Carbonic acid. Molecular model of the weak acid known as carbonic acid (H2CO3). This acid forms when carbon dioxide dissolves in water, with this process playing a key role in processes such as ocean acidification and the erosion of limestone rocks. Carbonic acid can form two kinds of salts, the carbonates and the bicarbonates. Atoms are represented as spheres and are colour-coded: carbon (grey), hydrogen (white) and oxygen (red).