LESSON II 

 THE CARBOHYDBATES 



1. NOTE the general appearance of the specimens of grape sugar or dextrose, 

 cane sugar, dextrin, and starch which are given round. 



2. Put some of each into cold water. Starch is insoluble ; dextrose, cane 

 sugar, and dextrin dissolve after a time, but more readily in hot water. 



3. Trommer's test. Put a few drops of copper sulphate solution into a 

 test-tube, then solution of dextrose, and then strong caustic potash. On 

 adding the caustic potash a precipitate is first formed, which, owing to the 

 presence of the sugar, rapidly redissolves, forming a blue solution. On boil- 

 ing this a yellow or red precipitate (cuprous hydrate or oxide) forms. 



4. Fehling's test. Fehling's solution is a mixture of copper sulphate, 

 caustic soda, and Eochelle salt of a certain strength. It is used for esti- 

 mating dextrose quantitatively (see Lesson XII.). It may be used as a qualita- 

 tive test also. Boil some Fehling's solution ; if it remains clear it is in good 

 condition ; add to it an equal volume of solution of dextrose and boil again. 

 Keduction, resulting in the formation of cuprous hydrate or oxide, takes 

 place as in Trommer's test. 



5. Moore's test. Add to the dextrose solution about half its volume of 

 20-per-cent. potash and heat. The solution becomes yellowish brown. Add 

 to this some sulphuric acid (25 per cent.) and the odour of caramel becomes 

 apparent. 



6. 'Fermentation test. Add a fragment of dried yeast to the dextrose 

 solution in a test-tube ; fill the test-tube up with mercury, and invert it over 

 mercury in a trough. Place it in an incubator at body temperature for 24 

 hours. The sugar is broken up into alcohol and carbon dioxide ; the latter 

 gas collects in the upper part of the test-tube. 



7. Cane Sugar. (a} The solution of cane sugar when mixed with copper 

 sulphate and caustic potash gives a blue solution. But on boiling no reduc- 

 tion occurs. 



(6) Take some of the cane-sugar solution and boil it with a few drops of 

 25-per-cent. sulphuric acid. This converts it into equal parts of 

 dextrose and levulose. It then gives Trommer's or Fehling's test 

 in the typical way. 



(c) Boil some of the cane-sugar solution with an equal volume of con- 

 centrated hydrochloric acid. A deep red solution is formed. Dex- 

 trose, lactose, and maltose do not give this test. 



8. Starch. (a) Examine microscopically the scrapings from the surface 

 of a freshly cut potato. Note the appearance of the starch grains with their 

 concentric markings. 



(&) On boiling starch with water an opalescent solution is formed, which, 

 if strong, gelatinises on cooling. 



(c) Add iodine solution. An intense blue colour is produced, which dis- 

 appears on heating, and if not heated too long reappears on cooling. 

 N.B. Prolonged heating drives off the iodine, and consequently 

 no blue colour returns after cooling. 



