metal (NaNO 3 , AgNO 3 ), and with bivalent metals (such as calcium, barium, lead) salts containing two acid groups—for example, Ca(NO 3 ) 2 , Pb(NO 3 ) 2 —carbonic acid, H 2 CO 3 , is bibasic , that is contains two atoms of hydrogen in the hydrate or two atoms of univalent metals in their salts: for example, Na 2 CO 3 is washing soda, a normal salt; NaHCO 3 is the bicarbonate, an acid salt. Therefore, if M′ be a univalent metal, its carbonates in general are the normal carbonate M′ 2 CO 3 and the acid carbonate, M′HCO 3 ; or if M″ be a bivalent metal (replacing H 2 ) its normal carbonate will be M″CO 3 ; these metals do not usually form acid salts, as we shall see further on. The bibasic character of carbonic acid is akin to that of sulphuric acid, H 2 SO 4 , but the latter, in distinction from the former, is an example of the energetic or strong acids (such as nitric or hydrochloric), whilst in carbonic acid we observe but feeble development of the acid properties; hence carbonic acid must be considered a weak acid . This conception must, however, be taken as only comparative, as up to this time there is no definitely established rule for measuring the energy of acids. The feeble acid properties of carbonic acid may, however, be judged from the joint evidence of many properties. With such energetic alkalis as soda and potash, carbonic acid forms normal salts, soluble in water, but having an alkaline reaction and in many cases themselves acting as alkalis. The acid salts of these alkalis, NaHCO 3 and KHCO 3 , have a neutral reaction on litmus, although they, like acids, contain hydrogen, which may be exchanged for metals. The acid salts of such acids—as, for instance, of sulphuric acid, NaHSO 4 —have a clearly defined acid reaction, and therefore carbonic acid is unable to neutralise the powerful basic properties of such alkalis as potash or soda. Carbonic acid does not even combine at all with feeble bases, such as alumina, Al 2 O 3 , and therefore if a strong solution of sodium carbonate, Na 2 CO 3 , be added to a strong solution of aluminium sulphate, Al 2 (SO 4 ) 3 , although according to double saline decompositions aluminium carbonate, Al 2 (CO 3 ) 3 , ought to be formed, the carbonic acid separates, for this salt splits up in the presence of water into aluminium hydroxide and carbonic anhydride: Al 2 (CO 3 ) 3 + 3H 2 O = Al 2 (OH) 6 + 3CO 2 . Thus feeble bases are unable to retain carbonic acid even at ordinary temperatures. For the same reason, in the case of bases of medium energy, although they form carbonates, the latter are comparatively easily decomposed by heating, as is shown by the decomposition of copper carbonate, CuCO 3 ( see Introduction), and even of calcium carbonate, CaCO 3 . Only the normal (not the acid) salts of such powerful bases as potassium and sodium are capable of standing a red heat without decomposition. The acid salts—for instance, NaHCO 3 —decompose even on heating their solutions (2NaHCO 3 = Na 2 CO 3 + H 2 O + CO 2 ), evolving carbonic anhydride. The amount of heat given out by the combination of carbonic acid with bases also shows its feeble acid properties, being considerably less than with energetic acids. Thus if a weak solution of forty grams of sodium hydroxide be saturated (up to the formation of a normal salt) with sulphuric or nitric acid or another powerful acid, from thirteen to fifteen thousand calories are given out, but with carbonic acid only about ten thousand calories. The majority of carbonates are insoluble in water, and therefore such solutions as sodium, potassium, or ammonium carbonates form in solutions of most other salts, MX or M″X 2 , insoluble precipitates of carbonates, M 2 CO 3 or M″CO 3 . Thus a solution of barium chloride gives with sodium carbonate a precipitate of barium carbonate, BaCO 3 . For this reason rocks, especially those of aqueous origin, very often contain carbonates; for example, calcium, ferrous, or magnesium carbonates, &c.
Carbonic anhydride—which, like water, is formed with the development of a large amount of heat—is very stable. Only very few substances are capable of depriving it of its oxygen. However, certain metals, such as magnesium, potassium and the like, on being heated, burn in it, depositing carbon and forming oxides. If a mixture of carbonic anhydride and hydrogen be passed through a heated tube, the formation of water and carbonic oxide will be observed; CO2 + H2 = CO + H2O. But only a portion of the carbonic acid gas undergoes this change, and