EXPERIMENTS IN CHEMISTRY AND PHYSICS 21 



Carbohydrates. — Starch and sugar ^ are common examples of 

 this group of substances. 



Starch Test. — If the substance to be tested is a solid, break or crush it 

 and add water to it. Pour over it a few drops of iodine solution diluted 

 with water. 2 Notice the color of the iodine, a dark brown; after it has 

 touched the material supposed to contain starch, note any change in color. 

 If starch is present, it will turn dark blue. 



Grape Sugar. — There are several forms of sugar commonly 

 used as food ; for example, cane sugar, beet sugar, and grape sugar, 

 the latter commonly known as glucose. Glucose, or grape sugar, is 

 manufactured commercially by pouring sulphuric acid over starch. 

 It is used as an adulterant for many kinds of foods, especially 

 in sirups, honey, and candy. 



Test for Grape Sugar. — The presence of grape sugar is determined by 

 the following test : Place in a test tube the substance to be tested and heat 

 it in a little water so as to dissolve the sugar. Add to the fluid twice its 

 bulk of Fehling's solution,-'' which has been previously prepared. Heat 

 the mixture, which should now have a blue color, in the test tube. If grape 

 sugar is present in considerable quantity, the contents of the tube will turn 

 first a greenish, then yellow, and finally a brick-red color. Smaller amounts 

 will show less decided red. This change also appears if Fehling's solution 

 is boiled with cane sugar. A more accurate test is obtained by placing the 

 substance believed to contain grape sugar in a test tube containing Fehling's 

 solution and allowing the mixture to remain over night in a moderately 

 warm room. If grape sugar is present, a red deposit or precipitate (copper 

 oxide) will be found in the tube the next morning. 



Organic Fats and Oils. — Tests for fats : Rub the material believed to 

 contain oil several times on paper and hold the paper to the light. If oil 

 is present, the paper will show a translucent grease spot. Try this with 

 several different nuts and decide which has the most oil. 



A second test for oil is as follows: Heat the substance to be tested in an 

 oven on a piece of paper. If oil is present, the paper will show a grease spot. 



Third test : Reduce the substance to small pieces and pour benzine, ether, 

 or other volatile oil over it. Allow the benzine or ether to evaporate; the 

 oil that remains is the extracted oil from the substance tested. 



1 The chemical formula for starch is CgHiqOs ; that of grape sugar, CoHioOe. 



2 Iodine solution is made by simply adding a few crystals of the element iodine 

 to 95 per cent alcohol ; or, better, take by weight 1 gram of iodine crystals, f gram 

 of iodide of potassiimi, and dilute to a dark brown color in weak alcohol (35 per cent) 

 or distilled water. 



3 To make Fehhng's solution (so called after its discoverer), add to 35 grams of 

 copper sulphate (blue vitriol) 500 cubic centimeters of water. Put aside until it is 

 completely dissolved. Call this solution No. 1. j u- 



To 160 grams of caustic soda and 173 fframs of Rochelle salt add 500 cub\c 

 centimeters of water. Dilute to 1 liter. Call this solution No. 2. 

 For use mix equal parts of solution 1 and 2. 

 The following formula is also convenient : — 



I. Copper Sulphate : 9 grams in 250 c.c. water. 

 IT. Sodium Hydroxide : 30 grams in 250 c.c. water. 

 III. Rochelle Salt : 43 grams in 250 c.c. water. 

 For use add to equal parts I, II, and III, two parts of water. 



