Carbonic Acid

Carbonic Acid is used to signify both the gas carbon dioxide C02, and its compound with water H2C03. The gas occurs in the atmosphere to the extent of about .04 per cent., and is found in volcanic gases. It is always produced when carbonaceous substances burn in air or oxygen. It is one of the waste products of the animal economy, and hence occurs in expired air. Green plants, however, under the influence of sunlight, decompose the atmospheric CO2 with elimination of the oxygen. It is also generally a product of fermentative action. It may be prepared by the action of an acid upon a carbonate, as chalk or marble. It is a colourless gas with a peculiar odour. It is heavier than air (density 1.52). By cold and pressure it may be liquefied, or solidified to a white snow-like mass. Though not really poisonous it is non-respirable, and if present to the extent of 2 or 3 per cent. renders air suffocating. Lighted tapers immersed in it are at once extinguished. It is soluble in water, and water charged at high pressure gives off the gas at ordinary pressure with effervescence, e.g. sodawater, champagne, etc. Its solution in water has weak acid properties, and may be regarded as containing an acid H2C03, the salts of which are known as carbonates when both hydrogen atoms are replaced by a metal, as CaC03, and fticarbonates when only one is so replaced.