PLANT LIFE. CELLULOSE, STARCH AND SUGAR 403 



The solution is decolorized with charcoal and evaporated to 

 crystallization in the same way as is the extract from the sugar- 

 cane. 



Properties of Sucrose. Sucrose crystallizes in four-sided 

 prisms, the form of which is seen in " rock-candy." It melts 

 at 160. It does not reduce Fehling's solution (p. 401). When 

 heated to 200 to 210 it begins to decompose, slowly losing water 

 and leaving a brown, soluble mass called caramel, used in color- 

 ing whiskey and soups. 



When boiled with water, to which a trace of an acid catalyst 

 has been added, it is hydrolyzed, giving a mixture of the two 

 monosaccharides, glucose and fructose: 



H 2 O - CeHiaOs + C 6 Hi 2 O 6 . 



This mixture of glucose and fructose is called invert sugar 

 and is found in many sweet fruits and in honey. Each sugar 

 interferes with the crystallization of the other, by lowering the 

 freezing-point (p. 119), and so invert sugar is added in making 

 " fondant " candy and candy that is to be " pulled," both of 

 which are intended to remain soft for some time. With the same 

 object in view, vinegar, lemon juice, or cream of tartar is added 

 to a syrup made from cane-sugar, in order that the acid contained 

 in them may produce some invert sugar and so give a less crystal- 

 lizable mixture (icing for cakes). Prolonged heating has the 

 same effect. 



Exercises. 1. What inference do you draw as to the composi- 

 tion of tapioca, sago, and rice from the facts that they are plant 

 products and when boiled with water and cooled give a jelly-like 

 mass? How should you confirm your inference? 



2. (a) Why does a concentrated solution of sugar boil at a 

 temperature far above that of boiling water? (b) In evaporation 

 why is the boiling-point lower in a vacuum than in air? 



