PRACTICAL EXERCISES n 



2. Cane-Sugar. Perform Trommer's test with a sample of a solution. 

 A blue liquid is obtained, which is not changed on boiling. Now put 

 the rest of the solution in a flask. Add ^th of its bulk of strong hydro- 

 chloric acid, and boil for a quarter of an hour. Again perform Trom- 

 mer's test. Remember that excess of alkali must be present after the 

 acid is neutralized. The test now shows much reducing sugar. The 

 cane-sugar has been ' inverted ' i.e., changed into a mixture of 

 dextrose and levulose. 



3. Starch. (i) Cut a slice from a well-washed potato ; take a scraping 

 from it with a knife, and examine with the microscope. Note the starch 

 granules with their concentric markings, using a small diaphragm. 

 Run a drop of dilute iodine solution under the cover-slip, and observe 

 that the granules become bluish. Examine also with a polarization 

 microscope. (2) Rub up a little starch in a mortar with cold water, 

 then add boiling water and stir thoroughly. Decant into a capsule or 

 beaker, and boil for a few minutes. After the liquid has cooled, perform 

 the following experiments: 



(a) Add a few drops of iodine solution to a little of the thin starch 

 mucilage in a test-tube. A blue colour is produced, which disappears 

 on heating, returns on cooling, is bleached by the addition of a little 

 sodium hydroxide, and restored by dilute acid. 



(b) Test the starch solution for reducing sugar by Trommer's test. 

 If none is found, boil some of the mucilage with a little dilute sulphuric 

 acid in a flask for twenty minutes, and again perform Trommer's test. 

 Abundance of reducing sugar will now be present. 



4. Dextrin. Dissolve some dextrin in boiling water. Cool. Add 

 iodine solution to a portion ; a reddish-brown (port- wine) colour results, 

 which disappears on heating. As a control, the same amount of iodine 

 should be added to an equal quantity of water in another test-tube. 

 The colour returns on cooling. The colour is also bleached by alkali, 

 restored by acid. Excess of iodine should be added for the bleaching 

 experiment (i.e., more than enough to give the maximum depth of tint). 

 If too little iodine has been added, there may be no restoration of the 

 colour by the acid. The addition of a little more iodine to the acid 

 solution will then cause the port-wine colour to return, and this may 

 be again bleached by alkali, and will now be restored by acid. 



5. Glycogen. See p. 715. 



6. Molisch 's Test for Carbo-Hydrates. This is a general test for carbo- 

 hydrates. It is also given by proteins which contain a carbo-hydrate 

 group. Put a drop of dextrose solution in a test-tube. Add a drop 

 of a 10 per cent, solution of a-naphthol in methyl alcohol, and then 

 o'5 c.c. of water. Then cautiously allow I c.c. of pure concentrated 

 sulphuric acid to run under the mixture, and shake gently. A violet or 

 reddish colour appears. 



Fats. 



1. Take a little lard or olive-oil, and observe that fat is soluble in 

 ether or warm alcohol, but not in water. Put a drop of the ethereal 

 solution of fat on a piece of paper, and note that it leaves a greasy stain. 



2. Put a little alcohol in a test-tube, and then a drop of phenol- 

 phthalein solution and a drop or two of dilute sodium hydroxide to give 

 the solution a red colour. Add a few drops of an ethereal solution of 

 the lard or olive-oil. If the red colour persists, the fat is neutral; if it 

 disappears, the fat contains free fatty acids. 



3. Saponification. Melt some lard in a porcelain dish, and pour it 



