72 CHEMISTRY FOR AGRICULTURAL STUDENTS 



In chemical formuloe and equations a small figure placed after a symbul 

 is held to refer to that symbol only. A large figure placed in front of a 

 symbol is held to govern the whole group of symbols. A point (.) between 

 two groups of symbols indicates that they are combined together. A 

 semicolon (;) is used to indicate a looser combination of compounds with 

 each other. A plus sign ( + ) between two formulae indicates that the 

 substances are mixed together, while the sign = points to reaction having 

 occurred between them and an equal weight of new substances formed. 



In volumetric determinations it is very important that the measurements 

 should be systematically entered in the notebook. The following will 

 serve as an illustration : — 



I. II. 



Weighing flask and caustic soda . 22.315 grams. 20. 169 grams. 

 Weighing flask .... 20.075 » 17.761 ,, 



Caustic soda taken . . 2.240 ,, 2.408 ,, 



Sulphuric acid used (i c.c. = .098 



grams H2SO4) . . .28.0 c.c. 30.1 c.c. 



and from this the quantity of sulphuric acid combining with a single 

 combining weight of caustic soda should then be calculated. 



Basic and acidic oxides combine to form salts. When 

 caustic soda and sulphuric acid react, each being the hydrate 

 of the respective oxides, in addition to sulphate of soda^ water 

 would also be produced, as expressed by the equation — 

 Nap.HgO + SO3.H2O = Nap.SOg + 2H2O 

 or, 2NaOH + H2S04 = Na2S04 + 2H20, 

 so that the resulting salt would weigh less than the sum of 

 the reacting substances by two combining weights of water. 

 To test the truth of this assumption, evaporate one of the 

 neutral solutions to dryness in a weighed dish on the water 

 bath, heat strongly for a few minutes over the bunsen flame, 

 cool in the desiccator, and weigh the anhydrous salt. 



