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salts of carbonic acid H2CO3.. There exist normal carbonates with a CO32− anion (for example, K2CO3), acid carbonates (hydrocarbonates or bicarbonates) with an HCO 3− anion (for example, KHC0 3), and basic carbonates [for example, Cu2 (OH)2CO3, the mineral malachite]. Only the normal carbonates of alkali metals, ammonium, and thallium are soluble in water. These solutions exhibit an alkaline reaction owing to the considerable degree of hydrolysis that occurs. Calcium, strontium, barium, and lead (divalent) normal carbonates are poorly soluble. Acid carbonates dissolve readily in water. As a rule, carbonates decompose upon heating (CaCO3 = CaO + CO 2), even before reaching the melting point; carbonates of alkali metals and thallium are exceptions. Hydrocarbonates are converted into normal carbonates (2NaHC03 = Na 2CO3 + H 2O + CO2) upon heating. Strong acids effect the decomposition of normal and acid carbonates with the liberation of CO2(K2CO3 + H 2SO4 = K 2SO4 + H2O +CO2).
Normal carbonates are widely distributed in nature and constitute one of the groups of minerals. Certain natural, normal, and basic carbonates are extremely valuable metallic ores, for example, carbonates of zinc, lead, copper, iron, and manganese. Nonmetallic raw materials, such as limestone CaCO3, magnesite MgCO3, and witherite BaCO3, are used in construction, in the manufacture of refractory materials, in the chemical industry, and for other purposes. Soda (Na2CO3 and NaHC0 3) and, to a lesser degree, potash K2CO3 prepared from synthetic carbon-ates are widely used in technology. Hydrocarbonates fulfill avery important physiological role as buffers.