PRACTICAL EXERCISES 11 



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. 608. 



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 0*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 into an alcoholic solution of potassium hydroxide previously heated 

 on a water-bath nearly to boiling. Mix well, and keep the mixture 

 gently boiling on the bath till saponification is complete. This 

 only takes a short time. Remove a little of the soap solution, and 

 drop it into distilled water in a test-tube. If unsaponified fat is 

 present it will rise to the top as drops of oil. In this case boiling 

 should be continued. If all the fat has been saponified the soap 

 solution will mix with the water and no oil-drops will separate. 



