526 PRINCIPLES OF CHEMISTRY 



the considerable basic energy which sodium possesses With the 

 soluble salts of most metals, sodium carbonate gives precipitates 

 either of insoluble carbonates of the metals, or else of the hydroxides 

 (in this latter case' carbonic anhydride is disengaged) , for in- 

 stance, with barium salts it precipitates an insoluble barium car- 

 bonate (BaCl 2 + Na 2 C0 3 = 2NaCl + BaC0 3 ) and with the aluminium 

 salts it precipitates aluminium "hydroxide, carbonic anhydride being 

 disengaged 3Na 2 CO 3 + A1 2 (S0 4 ) 3 4- 3H 2 O = 3Na 2 S0 4 + 2A1(OH) 3 

 -h 3C0 2 . Sodium carbonate, like all the salts of carbonic acid, evolves 

 carbonic anhydride on treatment with all acids which are to any extent 

 energetic. But if an acid diluted with water be gradually added to a 

 solution of sodium carbonate, at first such an evolution does not take 

 place, because the excess of the carbonic anhydride forms acid sodium 

 carbonate (sodium bicarbonate), NaHCOg. 20 The acid sodium 

 carbonate is an unstable salt. Not only when heated alone, but even 

 on being slightly heated in solution, and also at the ordinary 

 temperature in damp air, it loses carbonic anhydride and forms the 

 normal salt. And at the same time it is easy to obtain it in a pure 

 crystalline form, if a strong solution of sodium carbonate be cooled and 

 a stream of carbonic anhydride gas passed through it. The acid salt 

 is less soluble in water than the normal, 21 and therefore a strong 



80 The composition of this salt, however, may be also represented _as a combination 

 of carbonic acid, H 2 C0 3 , with the normal salt, Na-jCOs, J ast as the latter also com- 

 "bines with water. Such a combination is all the more likely because (1) there exists 

 another salt, Na2CO 3 ,2NaHC03,2H 2 (sodium sesquicarbonate), obtained by cooling 

 a boiling solution of sodium bicarbonate, or by mixing this salt with the normal 

 salt; but the formula of this salt cannot be derived from that of normal carbonic 

 acid, as the formula of the bicarbonate can. At the same time the sesqui-salt has 

 all the properties of a definite compound ; it crystallises in transparent crystals, has 

 a constant composition, its solubility (at in 100 of water, 12'6 of anhydrous salt) 

 differs from the solubility of the normal and acid salts; it is found in nature, and 

 is known by the names of trona and urao. The observations of Watts and Richards 

 showed (1886) that on pouring a strong solution of the acid salt into a solution of the 

 normal salt saturated by heating, crystals of the salt NaHCOgjNa-jCOsjSHijO may be 

 easily obtained, as long as the temperature is above 85. The natural urao (Boussingault) 

 has, according to Laurent, the s*ame composition. This salt it very stable in air, and 

 may be used for purifying sodium carbonate on the large scale. Such compounds have 

 been little studied from a theoretical point of view, although particularly interesting, since 

 in all probability they correspond with ortho-carbonic acid, C(OH) 4 , and at the same time 

 correspond with double salts like astrachanite (Chapter XIV., Note 25). (2) Water of 

 crystallisation does not enter into the composition of the crystals of the acid salt, so that on 

 its formation (occurring only at low temperatures, as in the formation of crystalline com- 

 pounds with water) the water of crystallisation of the normal salt separates and the water 

 is, as it were, replaced by the elements of carbonic acid. If anhydrous sodium carbonate 

 be mixed with the amount of water requisite for the formation of Na-jCOjEkjO, this salt 

 will, when powdered, absorb CO 2 as easily at the ordinary temperature as it does water. 



21 100 parts of water at dissolve 7 parts of the acid salt, which corresponds with 

 A'8 parts of the anhydrous normal salt, but at 100 parts of water dissolve 7 parts of 



