IDENTIFICATION OF AN ORGANIC COMPOUND 47 



3. Odour. The odour is sometimes characteristic. 



4. Appearance. The appearance under the microscope gives evi- 

 dence of homogeneity or impurity. The microscopical appearance is 

 very Useful in identifying the different kinds of starch. Many substances 

 have a characteristic crystalline structure, e.g. cholesterol, cystine, 

 osazones of carbohydrates, etc. 



5. Effect of Heat. By heating the substance firstly on platinum, 

 secondly in a small dry tube many valuable details can be ascertained. 

 The odour may be peculiar, the substance may melt, char, decompose, 

 sublime, or boil. The melting-point and boiling-point of solids and 

 liquids can be observed directly after such a preliminary examination. 



6. Detection of the Elements. By ascertaining whether the 

 substance does or does not contain nitrogen, it may be placed in either 

 of the following groups : 



Non-nitrogenous. Nitrogenous. 



Hydrocarbons. Cyanogen compounds. 



Alcohols, phenols Amides. 



Esters, ethers. Amines. 



Aldehydes. Amino acids. 



Ketones. Guanidine compounds. 



Acids and Salts. Purines. 



Fats and cholesterol. Proteins. 



Carbohydrates. I 



7. Solubility. 



(a) Alkali salts and salts of bases, ^ o> ^ 



The lower alcohols, aldehydes, acids, ketones, amides, amines 



The polyhydric alcohols and carbohydrates 



Phenols and hydroxy acids 



In general, compounds containing several OH groups 



() Aromatic acids are insoluble or very slightly soluble, but dis- 

 solve in boiling water. 

 Starch is insoluble and gives an opalescent solution with hot 



water. 

 Tyrosine, cystine and uric acid are soluble with difficulty in 



water. 



Fats, higher fatty acids and cholesterol are insoluble in water, but 

 soluble in ether. 



82 

 ; 8 



rt 



