CHAPTER III 



THE CARBOHYDRATES 



General Properties. The carbohydrate group includes a large 

 number of compounds, very different in external appearance 

 and properties ; but they resemble each other in chemical com- 

 position, and they are so much alike in molecular structure 

 that each one of them can be transformed into other com- 

 pounds which still exhibit the properties that are common to 

 the whole group. They are composed of the elements carbon, 

 oxygen, and hydrogen, and it is characteristic of these com- 

 pounds that the hydrogen and oxygen are always present in 

 the same relative proportion as in water, i.e. two atoms of the 

 former to one of the latter, hence the name " carbohydrate." 

 The carbohydrates which occur in nature usually contain six, 

 or a multiple of six, atoms of carbon in the molecule, but this 

 is not an essential characteristic of the whole group. 



In the natural condition many carbohydrates exhibit a 

 definite organised structure; some are crystalline and others 

 are amorphous. All the crystalline and most of the amorphous 

 compounds are more or less soluble in water. Those which 

 exhibit an organised structure are insoluble. Solutions of 

 carbohydrates, when pure, are colourless, neutral to litmus, and 

 are optically active (see p. 38) ; or if not, physical isomers 

 which are optically active can be prepared from them. 



The carbohydrate group includes so many diverse com- 

 pounds that very few reactions are common to them all. The 

 following, however, are characteristic. 



Ferric chloride produces no coloration. 



Concentrated sulphuric acid dehydrates and chars them, 

 causing a brown or black coloration. In the presence of 



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