Textbooks once said carbonic acid couldn't exist on its own. They were wrong
For much of the twentieth century chemists believed carbonic acid lived only fleetingly in solution. Pure, it turns out to be surprisingly durable; it is water that tears it apart, almost instantly. Deep inside icy moons such as Ganymede and Titan, dense layers of it may even separate the ice from the rock below.
The name itself has wandered. When Lavoisier's circle overhauled chemical names, they called carbon dioxide carbonic acid in 1781, because it came from burning charcoal and turned litmus red. Once the definition of an acid changed, the label moved to the molecule H2CO3, which was assumed to have no independent existence. Studies since about 1990 showed otherwise: it has its own Raman signature and survives about 20 milliseconds at body temperature. Gaseous carbonic acid was made in 1987 by heating ammonium bicarbonate, and solid forms followed from irradiated ices at extremely low temperatures.
Why did isolating it take so long? Calculations suggest the pure molecule is metastable and should linger as a gas at room temperature, but just two water molecules cut its predicted half-life to around a minute, and the water released by each breakdown speeds the next. Under high pressure the solid crystallises into flat pairs of molecules locked by unusually strong hydrogen bonds, with all three carbon-oxygen bonds almost the same length. That stability has even prompted suggestions of using solid carbonic acid to lock away carbon.
In ordinary water most dissolved carbon dioxide never becomes carbonic acid at all; at 25 degrees the ratio is roughly 1.7 thousandths. Left alone, the conversion is slow, but living things use the enzyme carbonic anhydrase to reach equilibrium rapidly, which is central to how aerobic organisms handle the carbon dioxide of respiration. At the pH of about 7.2 found in cells, the acid is close to half dissociated.
The same chemistry has planetary consequences. Carbonated drinks are simply water with carbon dioxide forced in under slight pressure, and irrigation and wastewater plants use it to acidify water. As industry has added carbon dioxide to the air, more has dissolved into the seas, and estimates suggest average ocean surface pH has already fallen by about 0.1 since pre-industrial times.
Source: Carbonic acid
