The carbonates and the thiocarbonates are derivatives of carbon dioxide (or rather of carbon oxy-hydroxide, commonly called carbonic acid), and of carbon disulphide. Carbon is a tetrad, and the analogue of carbon tetrachloride would be the tetrahydroxide, C(OH)4; but this body is unstable, and its first anhydride, 0=C(OH)2, is known only in aqueous solution. However, carbonyl chloride, 0=CC12, exists; it is produced by the direct union of carbonic oxide with chlorine, when a mixture of both gases is exposed to sunlight; it was formerly known as " phosgene gas," meaning "made by light"; but it is more conveniently prepared by passing a mixture of the two gases over animal charcoal heated to redness. It condenses to a liquid, boiling at 8.40. It is immediately decomposed by water, thus: 0=CC124-2H0H = 0=C(OH)2 +2HCI; if sufficient water is present, the carbonic acid can remain in solution. The existence of the oxychloride establishes the formula of carbonic acid.
Carbonic acid is a very easily decomposable substance; if liberated, unless a great deal of water be present, it splits into its anhydride, CO2, and water: 0=C(OH)2 = CO2 4-H2O. The anhydride is a colourless gas, which condenses to a solid at about -8o°; it can be liquefied only under pressure. Carbon dioxide, or carbonic anhydride, is produced by heating a carbonate; as already remarked, all carbonates, except those of the alkaline metals, are decomposed by heat, forming oxides, and evolving carbon dioxide. It is also produced when carbon or carbon monoxide is burned with excess of oxygen. Lastly, it is produced in large quantities during the process of fermentation. Glucose, or grape sugar, either produced by the hydrolysis of starch or extracted from fruits like grapes, when mixed in dilute aqueous solution with yeast, a vegetable organism, decomposes into ethyl alcohol and carbon dioxide, thus: C6H12O6=2C2H5OH4-2CO2. The carbon dioxide being heavier than air, collects in the fermenting tuns; it is now often collected and compressed until it liquefies ; and the liquid on expansion solidifies to a snowlike solid, used for producing low temperatures.
A solution of carbonate anhydride in water contains carbonic acid, 0=C(OH)2, which is a very weak acid owing to the small extent of its ionisation. It is probable, too, that liquid carbon dioxide exists in the solution, mixed, but not combined with the water. Carbonic acid reacts with sodium, potassium, calcium, or barium hydroxide, forming carbonate of the metal: H9|=CO3. Aq + 2Na|-OH.Aq = Na2|=CO3.Aq + 2H9O ; "H2|=COs. Aq + Ca|=(OH)2.Aq = CaCO3 +* 2H2O. In such actions it is only the ionised portion of the acid which reacts, and the hydrogen ions form water; when these are removed another portion becomes ionised in order to restore equilibrium; it reacts in its turn until all has become transformed. On evaporation of the solution the alkaline carbonate is left as a white crystalline salt; hydrated sodium carbonate, Na9CO3.ioH9O, is ordinary washing-soda. All other carbonates are insoluble in water, and are consequently thrown down as precipitates on adding a solution of sodium carbonate to any ionised solution of other metals. They form flocculent precipitates, generally possessing the colour of the ion of the metal; thus copper carbonate is blue, ferrous green, cobalt pink, and so on. But with the exception of the carbonates of the metals of the sodium and calcium groups all other precipitated carbonates are " basic," that is, they are partly hydroxides, partly carbonates. Copper carbonate, for example, may be assigned the formula /O-Cu-OH-0=C\ ; it will be noticed that each atom X0-Cu-OH of copper is combined with the oxygen of the carbonic residue on the one hand, and with hydroxyl on the other. The paint known as " white lead" consists of a basic carbonate of lead, more complex than the example given above, of the formula
Many carbonates exist in the native state ; some are widely distributed minerals. Among these are Iceland or calc-spar, arragonite, limestone, chalk, and marble, all of them calcium carbonate; strontianite, SrCO3; witherite, BaCO3; spathic iron ore, FeCO3, also named clay-band when contaminated with clay, and black-band when mixed with shale. Magnesite is MgCO3 ; dolomite, a mixture of magnesium and calcium carbonates; calamine, ZnCO3; and cerussite, PbCO3. Malachite and azurite are basic caryO-Cu-OH bonates of copper, 0=C\ , and
xO-Cu-OH O O
We see here again that with weak bases, such as the hydroxides of most metals, the carbonates tend to become basic, that is, to be hydrolysed. This is why the precipitates obtained on adding a soluble carbonate to a salt of such metals are basic, and not normal carbonates.
The name "acid carbonate " is given to a double carbonate of hydrogen and a metal. Such bodies are prepared by the method which always is adopted for the preparation of double salts-by
/ONa mixture. Hydrogen sodium carbonate, 0=C<f ;
XOH the corresponding potassium compound ; hydrogen cal-O O
11 11 cium carbonate HO-C-O-Ca-O-C-OH, a ferrous carbonate of similar formula, and many others are all formed when carbonic acid and the respective normal carbonate are mixed, the mixture being kept cold. On raising the temperature of all of these, carbon dioxide escapes, and the neutral carbonate is again formed. "Acid" carbonate of sodium is the common " baking-soda ; " hydrogen calcium carbonate is a constituent of many natural waters, and is the cause of what is termed " temporary hardness" ; for on boiling the water the neutral carbonate is precipitated, and the water ceases to be "hard." The same result may be effected, paradoxical as it may appear, by the addition of lime-water ; for then sufficient calcium hydroxide is present to form normal calcium carbonate with the hydrogen carbonate, thus: Ca(HCO3)2.Aq + Ca(OH)2.Aq=2CaCO3 + 2H2O.Aq. Hydrogen ferrous carbonate is a constituent of chalybeate springs; on exposure to the atmosphere the iron is oxidised to ferric hydroxide, and the carbonic acid, being too weak an acid to form a carbonate with such a weak base as that, escapes: 2Fe(HCOg)2.Aq + 5H2O + O = 2Fe(OH)3 + 4H2CO3.Aq. The ferric hydroxide is deposited as a brown scum on the banks of the streams flowing from such wells.
Although normal hydroxide of carbon is unknown, yet if the hydrogen be replaced by ethyl, -C2H5, the compound is stable. The compound, which is produced by the action of carbon tetrachloride on sodium ethoxide, CC14 + 4Na-O-C2H5 = 4NaCl-f-C(0-C2H5)4, is the analogue of C(OH)4. It is a volatile liquid, and is named ethyl orthocarbonate. And a corresponding carbonate of ethyl, 0=C(OC2H5)2, the analogue of carbonic acid, 0=C(OH)2, is formed by treating carbonyl chloride with alcohol: 0=CC12 -f 2HO-C2H5= 0=C(OC2H5)2 -f 2HCI. These compounds are volatile, and can be weighed in the state of vapour, hence their molecular weights are known, and this is an additional proof of the correctness of the formulae ascribed to carbonic acid and the carbonates.