SODir.M 521 



ing 2XaHO,7H. 2 O, they fuse at +6. 27 If the evaporation be con- 

 ducted as long as water is disengaged, which requires a considerable 

 amount of heat, then, on cooling, the hydroxide, XaHO, solidifies in 

 semi-transparent crystals. 28 



Caustic soda, free from water, forms a colourless crystalline mass, 

 which eagerly absorbs moisture and carbonic anhydride from the air. 29 

 Its specific gravity is 2-1 3 ; 30 it is easily soluble in water, with 

 disengagement of a considerable quantity of heat. 31 A saturated 

 solution at the ordinary temperature has a specific gravity of about 

 1*5, contains about 45 per cent, of sodium hydroxide, and boils at 

 130 ; at 55 water dissolves an equal weight of it. 32 Caustic soda 

 is not only soluble in water but in alcohol, and even in ether. 

 Solutions of sodium hydroxide produce the sensation of soaping on the 

 skin, because the active base of soap consists of caustic soda. 33 



27 By allowing strong solutions of sodium hydroxide to crystallise in the cold, impuri- 

 ties such as, for instance, sodium sulphate may be separated from them. The fused 

 crystallo-hydrate 2NaHO,7H,O forms a solution having a specific gravity of T405 (Hermes). 

 Let us remark that, according to certain determinations, less water namely, only 

 NaHO,3H 2 O enters into the composition of the crystallo-hydrate. The crystals on dis- 

 solving in water produce cold. 



!B In solid caustic soda there is generally an excess of water beyond that required by 

 the formula NaHO. The caustic soda used in laboratories is generally cast in sticks, 

 which are broken into pieces. It must be preserved in carefully closed vessels, because it 

 absorbs water and carbonic anhydride from the air. 



29 By the way it changes in air it is easy to distinguish caustic soda from caustic 

 potash, which in general resembles it. Both alkalis absorbs water and carbonic adhydride 

 from the air, but caustic potash forms a deliquescent mass of potassium carbonate, whilst 

 caustic soda forms a dry powder of efflorescent salt. 



50 As the molecular weight of NaHO = 40, the volume of its molecule = 40/2'13 = 18-5, 

 which very nearly approaches the volume of a molecule of water. The same in general 

 refers to the compounds of sodium for instance, its salts have a molecular volume ap- 

 proaching the volume of the acids from which they are derived. 



31 The molecular quantity of sodium hydroxide (40 grams), on being dissolved in a 

 large mass (200 gram molecules) of water, develops, according to Berthelot 9780, and 

 according to Thomsen 9940, heat-units, but at 100 about 13000 (Berthelot). Solutions of 

 NaHO + wH 2 O, on being mixed with water, evolve heat if they contain less than 6H 2 O, 

 but if more they absorb heat. 



> 2 The specific gravity of solutions of sodium hydroxide at 15/4 is given in the short 

 table below : 



NaHO, p.c. . . 5 10 15 20 80 40 



Sp.gr. . . . 1-057 1-113 1-169 T224 1'331 T436 



1000 grams of a 5 p.c. solution occupies a volume of 946 c.c. ; that is, less than the water 

 serving to make the solution (see Note 18). 



00 Sodium hydroxide (and other alkalis) is capable of hydrolysing saponifying, as it 

 is termed the compounds of acids with alcohols. If RHO (or R(HO)w) represent the 

 composition of an alcohol that is, of the hydroxide of a hydrocarbon radicle and QHO an 

 acid, then the compound of the acid with the alcohol or ethereal salt of the given acid 

 will have the composition RQO. Ethereal salts, therefore, present a similitude to 

 metallic salts, just as alcohols resemble basic hydroxides. Sodium hydroxide acts on 



