CARBOHYDRATES 105 



which absorbs water and leaves a residue of pure carbon. The 



general formula for all carbohydrates, with one exception, 



which need not be discussed here, is C„(H20)^. In this formula 



X may have the same value as n or it may be equal to one less 



than n. 



EXPERIMENT STUDY XX 



General Properties of Carbohydrates 



(i) (a) Dry some cane sugar at 100° C. until all hygroscopic mois- 

 ture has been driven off. (6) Heat about a gram of dry cane sugar in 

 a test tube. Notice moisture given off as the sugar decomposes. 

 What is the residue in the tube? This proves carbon, hydrogen and 

 oxygen as constituents of sugar, (c) To 5 g. of dry cane sugar in a 

 test tube add about 5 c.c. concentrated sulphuric acid (c. p.). With- 

 out heating notice decomposition due to the removal of water by the 

 sulphuric acid and the residue of pure carbon. This is a method of 

 preparing chemically pure carbon. 



(2) Make the soda lime test for nitrogen, Experiment XVIII, 2. 

 This proves absence of nitrogen. 



(3) Moore's Test. To 5 c.c. of a sugar solution add a little KOH 

 and warm. A yellow and then brown color is characteristic of car- 

 bohydrates. 



Constitution. Mixed Alcohol-aldehyde or Alcohol-ketone 



Compounds. — Without entering into a detailed discussion 



in regard to the constitution of these compounds, with the 



proofs for each of the views advanced, we may simply state the 



fundamental facts necessary for understanding their relation 



to each other. The simple carbohydrates which are known as 



mono-saccharoses undergo reactions which indicate that they 



contain both alcoholic hydroxyl groups and also an aldehyde or 



ketone group. Furthermore each carbon atom but one has one 



and only one alcoholic hydroxyl joined to it. This remaining 



carbon atom is in the condition of either an aldehyde or ketone 



group. The group characteristic of aldehydes and ketones is 



H R 



I I I 



- C = O, i.e. R — C = O in aldehydes and R — C = O in 



ketones (see p. 42). 



