Acids And Bases

Joseph Humphrey                Seward School
                               4600 S. Hermitage Ave.
                               Chicago, Ill. 60609
                               312-650-5890

Objective: 

 Eighth Grade Activity
WHITE WINE or GRAPE JUICE? 

Materials Needed:
 
5-250-ml beakers
iron (III) chloride solution
tannic acid solution
sulfuric acid
1-L flask, or large clear picture
3-eye droppers

Strategy:                 

Five beakers are arranged on a demonstration table.  A pale yellow liquid (white 
wine) is poured from a large flask into the first beaker.  When the liquid is 
poured into the second beaker, a blue liquid (grape wine) is produced.  Both of 
these beakers are emptied into a large flask and the entire content turns blue.  
The blue liquid is poured into the third beaker and a clear solution is produced.  
When the fourth beaker is filled, the liquid is blue again.  When the last beaker 
is filled, the liquid is clear.  When all the beakers are emptied back into the 
large flask, the entire content of the flask becomes clear again. 
 
Procedure:

1. Arrange five 250-ml beakers or clear glass tumblers in full view of the 
   class. 
2. Prior to performing the demonstration, prepare the beakers as follows: 
   a. Beaker 1: empty.
   b. Beaker 2: 2 drops of iron (III) chloride solution.
   c. Beaker 3: 5ml of sulfuric acid.
   d. Beaker 4: empty.
   e. Beaker 5: 5ml of sulfuric acid.
3. Fill a 1-L flask or large clear pitcher with tannic acid solution.
4. Develop your own "story line": the following is a suggestion.
   a. At a recent dinner party, a guest was served white wine.  (fill beaker 1 
      from the large flask, to produce "white wine" in the first beaker.)
   b. The second guest asked for grape wine.  Being a chemist, this is no 
      problem (fill beaker 2 from the flask.  A blue solution (grape wine) will 
      be produced.)...etc.

Reactions:

1. In beaker 2, tannic acid reacts with iron (III) chloride to form the blue 
   complex iron (III) tannate.
2. When the blue iron (III) tannate is poured back into the flask, more of the 
   blue complex is formed and the entire contents of the flask becomes colored.
3. When the blue "solution" is poured into beaker 3, sulfuric acid breaks up the 
   iron (III) tennate complex and forms a colorless solution.
4. Pouring the contents of beakers 3 and 5 back into the flask breaks up all of 
   the iron (III) tannate and produces a clear solution.

Solutions:

1. Tannic acid: The concentration is not critical.  About 1 tsp in 1 L of water 
   works well.  Adjust the amount of tannic acid until the solution is slightly 
   yellowish.   
2. Iron (III) chloride solution is saturated.
3. Sulfuric acid is concentrated. CAREFUL WITH THIS!

Questions for students:

1. Why does the colorless solution become blue in color?
2. Why does the blue solution become colorless again?
3. Can you think of a use for iron tannate? (Iron (II) tannate is used to make 
   ink.  It is easily oxidized in air to form the complex iron (III) tannate.)