THE ORGANIC ACIDS 153 



XXXII. THE ORGANIC ACIDS 



Place a pint of beer in a large flat-bottomed dish and leave 

 exposed to the air for a week. Observe that the beer becomes 

 sour in taste and acid to litmus. Vinegar has, in fact, been 

 produced, the alcohol having been oxidised to acetic acid by 

 atmospheric oxygen : — 



CgHgO + 02 = C2H4O2 + H2O. 

 This oxidation is effected by a microscopic fungus, — the vinegar 

 plant, — the germs of which exist in the air ; and it is desirable 

 to partly immerse in the beer a few beechwood shavings, upon 

 which the fungus will develop, and which will therefore pro- 

 mote the chemical change. 



Neutralise some vinegar with caustic soda, evaporate to a low 

 bulk, slightly acidify with sulphuric acid and distil over the 

 acetic acid, being careful to stop the process when charring 

 commences. Note that this volatile organic acid has all the 

 properties of an inorganic acid ; it is sour in taste, it turns blue 

 litmus red, and it neutralises alkalies and decomposes carbon- 

 ates with the formation of salts. 



Acetic acid is also obtained by the distillation of wood. If 

 the aqueous liquid thus obtained (p. 104) be neutralised with 

 soda, the wood spirit evaporated off and the residue distilled 

 with sulphuric acid, impure acetic acid — " pyroligneous acid " 

 — is obtained. 



Just as acetic acid is obtained from ethyl alcohol by oxida- 

 tion, so other organic acids may be produced from the alcohols 

 with which they correspond in number of carbon atoms. Thus 

 methyl alcohol, CH^O, yields formic acid, CHgOg, and butyl 

 alcohol, C4H^qO, yields butyric acid, C4HSO2. To a mixture of 

 sulphuric acid and bichromate of potassium (a strong oxidising 

 agent) add a few drops of butyl alcohol. Warm gently in a 

 test-tube till action is complete, then boil and notice the 

 cheese-like odour of the product of oxidation — butyric acid. 



