“Soda is so acidic it can dissolve a tooth overnight”
This myth got its start from a nutritionist who made the claim in the 1950s. Sodas contain acids, such as phosphoric, citric and carbonic acid. But their concentrations are lower in soda than in natural drinks, such as orange or cranberry juice. When left in soda, a tooth will not completely dissolve overnight, or even over a few days. Also, when we drink soda, we don’t tend to hold it in our mouths for long periods of time, and the saliva in our mouths helps protect the enamel.
But this does not mean that soda is harmless to teeth. High-sugar drinks can contribute to tooth decay, and acidic drinks can erode tooth enamel over time. The reason is that although enamel is hard, the substance that makes up most of it, hydroxyapatite [Ca5(PO4)3OH], is in equilibrium with its dissolved form, like any ionic solid in the presence of water. At equilibrium, most of hydroxyapatite is in solid form:
Ca5(PO4)3OH(s) → 5Ca2+(aq) +3PO43-(aq)+OH-(aq)
But when an acid is added, its free hydrogen (H+) ions neutralize some of the hydroxyl (OH–) ions, as follows:
H+ + OH– → H2O
This shifts the equilibrium to the right, causing more hydroxyapatite to dissolve, thus dissolving the tooth enamel.