434 PRINC 1TLF.S OF CHEMISTRY 



Whilst retaining essentially the methods of Thomsen, Ostwald 

 determined the variation of the sp. gr. (and afterwards of volume) pro- 

 ceeding in the same dilute solutions, on the saturation of acids by 

 bases, and in the decomposition of the salts of one acid by the other, 

 and arrived at conclusions of just the same nature as Thomsen did. 

 Ostwald's method will be clearly understood from an example. A 

 solution of caustic soda, containing an almost molecular (40 grams) 

 weight per litre, had a specific gravity of 1 '04051. The specific gravities, 

 of solutions of equal volume and equivalent composition of sulphuric and 

 nitric acids were 1*02970 and 1-03084 respectively. On mixing the 

 solutions of NaHO and H 2 SO 4 there was formed a solution of ]S"a 2 SO 4 

 of sp. gr. 1 '02959 ; hence there ensued a decrease of specific gravity 

 which we will term Q, equal to 1-04051 + 1-02970 2(1-02959) = 0-01103. 

 So also the specific gravity after mixture of the solutions of NaHO 

 and HXO ; < was 1-02633, and therefore Q = 0'01869. When one 

 volume of the solution of nitric acid was added to two volumes of the 

 solution of sodium sulphate, a solution of sp. gr. 1-02781 was obtained, 

 and therefore the resultant decrease of sp. gr. 



Q 1 = 2(l-02959) + 1-03084 -3(1-02781) = 0-00659. 



Had there been no chemical reaction between the salts, then according 

 to Ostwald's reasoning the specific gravity of the solutions would not 

 have changed, and if the nitric acid had displaced the sulphuric acid Q., 

 would be=0'01869 0-01103 = 0-00766. It is evident that a portion 

 of the sulphuric acid was displaced by the nitric acid. But the 

 measure of displacement is not equal to the ratio between Q, and Q. 2 , 

 because a decrease of sp. gr. also occurs on mixing the solution of sodium 

 sulphate with sulphuric acid, whilst the mixing of the solutions of 

 sodium nitrate and nitric acid only produces a slight variation of sp. gr. 

 which falls within the limits of error of experiment. Ostwald deduces 

 from similar data the same conclusions as Thomsen, and thus re- 

 confirms the formula deduced by Guldberg and Waage, and the teach- 

 ing of Berthollet. 28 



28 The participation of water is seen still more clearly in the methods adopted by 

 Ostwald than in those of Thomsen, because in the saturation of solutions of acids by 

 alkalis (which Kremers, Reinhold, and others had previously studied) there i> observed, 

 not a contraction, as might have heen expected from the quantity of heat which is then 

 evolved, but an expansion, of volume (a decrease of specific gravity, it' we calculate as 

 Ostwald did in his first investigations). Thus by mixing UNO grains oi a solution of 

 sulphuric acid of the composition SOj + 100TL.O, occupying a volume of lsir> c.e., with M 

 corresponding quantity of a solution '2(NaHO -I- 5HoO), whose volume =17'.):: <.<.. we 

 obtain not 3G08 but 8638 c.c., an expansion of 25 c.c. per gram molecule of the resulting 

 salt, Na 2 SO 4 . It is the same in other cases. Nitric and hydrochloric acids -ive a still 

 greater expansion than sulphuric acid, and potassium hydroxide than sodium hydroxide. 



