EXERCISE 7 



Some of the substances with which we are acquainted seem to be exactly the same in all parts 

 for example, glass, water, iron. Others we can see are made up of different kinds of particles for 

 example, streaks in marble, wood, ink. 



Problem. What are some of the parts that make up milk ? 



What to use. For the entire class, quart bottle of milk ; pipette ; 

 evaporating dish ; water bath ; crucible ; support ; flame. For each 

 pupil, two test tubes and rack ; funnel ; piece of filter paper ; 

 acetic acid. 



What to do. Allow the milk to stand overnight, and remove 

 the cream by pouring off or by the use of a pipette. The residual 

 milk (skimmed) is then distributed to members of class, each taking 

 about two tablespoonfuls, or milk enough to fill test tube one third 

 full. Now add acetic acid to the milk, drop by drop, until a precip- 

 itate is formed. Separate the solid substance (curd) from the liquid 

 whey by pouring into a filter paper supported by a funnel. 



NOTE i. The curd is made up largely of a substance called casein. This is 

 held in solution by the alkali naturally present in the milk. In an acid solution 

 casein precipitates out as curd. 



The teacher will collect all the whey and evaporate to dryness Whey 



over a water bath or in a double boiler. 



When all water has been driven off, assemble the solid material 



(it consists of sugar, milk albumen, and other substances) in a crucible and treat the material with a 

 flame until nothing is left that will burn. 



NOTE 2. The part remaining after burning is called ash or, sometimes, mineral matter. 



Record. Make an outline of the substances that make up milk. Opposite each item tell how each 

 is separated from other constituents. 



Questions. 1. Why does a housekeeper add baking soda to milk when it begins to turn? 

 2. Could milk be used to demonstrate reversible chemical action ? 



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