8 ELEMENTARY AGRICULTURAL CHEMISTRY 



an acid, forming, in doing so, a salt and water. The most im- 

 portant alkalies are soda, NaOH, potash, KOH, and lime, CaO 

 or CaHgOj. Ammonia, NII3, or in solution (NH4)0H, also acts 

 as an alkali. A substance is said to have an alkaline reaction 

 if it turns certain vegetable colours — e.g., litmus — which have 

 been reddened by an acid, back to blue again. 



Base: — a substance, generally an oxide or hydroxide of a 

 metal, which can partially or wholly neutralise the acidity of 

 an acid, forming thereby a salt and water. The alkalies are 

 bases soluble in water, but many bases are insoluble. 



Basicity of an acid is the number of atoms of replaceable 

 hydrogen present in a molecule of the acid. Thus the basicity 

 of nitric acid, HNOg, or hydrochloric acid, HCI, is 1 ; or these 

 acids are said to be monobasic. Sulphuric acid, HgSO^, has a 

 basicity of 2, or is dibasic; phosphoric acid, II3PO4, of 3, or is 

 trihasiCf and so on. Monobasic acids can only form one kind 

 of salt with a metal, since the hydrogen must be wholly replaced, 

 if replaced at all. Thus there can only be one sodium nitrate 

 — the substance NaNOg. Dibasic acids, or acids of higher 

 basicity, can, however, form more than one salt with a metal. 

 Thus the sodium salt of sulphuric acid might be NaHSO^ or 

 NajSO^, according to whether one or both the atoms of hydrogen 

 in the acid have been replaced by sodium. The former salt 

 belongs to a class called the acid salts, and would be more 

 correctly named sodium hydrogen sulphate. 



Destructive Distillation : — the change produced when a 

 substance, generally a carbonaceous compound, is submitted to 

 a high temperature with exclusion of air, and when gases and 

 vapours are emitted, the original substance being permanently 

 destroyed by the process. A good example is afforded in the 

 preparation of gas from coal. In most cases a black residue, 

 consisting largely of carbon, is left behind. 



Endothermic and Exothermic Compounds.— By an 



exothermic compound is understood a substance in whose 

 formation heat was evolved — e.g., carbon dioxide. Most com- 

 pounds are exothermic. An endothermic compound, on the 



