Soap Bubble Chemistry

Theresa Colby                   Montessori Elem School
Al Oldenburg                    Lindblom HS
Al Tobecksen                    Fenger HS

Objectives:

1. Students will understand the chemistry of soap bubble films.
2. Students will build their own model for making large soap bubbles.
3. Students will investigate with prepared geometric wire models to see the
   maximum number of planes, the maximum number of lines and the sizes of the
   angles that are produced when the planes and lines intersect.

Materials:

1. Pop-it beads strung into a long chain and in a large jar
2. Straws
3. String
4. Prepared wire and string models
5. Two strings of suckers
6. Prepared soap bubble solution
7. Buckets and trays
8. Protractors

Suggested Strategy:

For an attention-getter, let the pop-it beads pull themselves out of the jar in 
which they are contained.  The last pop-it bead is pushed into a small hole 
drilled into a racquetball.  Starting from the racquetball, count off 18 
sections of pop-it beads and separate that from the chain.  Ask the students 
what does this small piece of chain represent (ans. - a soap molecule). 

Review soap molecules and how they arrange themselves in water.  See diagram
that follows.

/\/\/\/\/\/\/\/\/\O (H2O) O/\/\/\/\/\/\/\/\/\
/\/\/\/\/\/\/\/\/\O (H2O) O/\/\/\/\/\/\/\/\/\
/\/\/\/\/\/\/\/\/\O (H2O) O/\/\/\/\/\/\/\/\/\

Present three questions: 

    1) What is the shortest possible way to connect two points?  (Ans. - a 
       straight line.)
    2) What is the shortest possible way to connect three points?  (Most people 
       would say a triangle, but that is wrong - see diagram 1 below.) 
    3) What is the shortest possible way to connect four points?  (Most people 
       would say a square, but that is wrong - see diagram 2 below.)  

Using two plexiglass plates and small rubber suction cups (first two suction 
cups, then three, then four) and an overhead projector, let the soap bubbles 
show the answers.  Some students may guess that planes of soap bubbles meet at 
120 degrees since it will be very clear on the screen; some students may surmise 
that only a maximum of three planes will ever intersect - and both guesses are 
correct!  
    
Present another question: what is the maximum number of lines that can intersect 
a single vertex in a soap bubble model and what angle(s) do these lines form? 
(Ans. - four lines maximum and the angle is 109.23 degrees - it is very unlikely 
anyone would know it or guess it.)  Bring out the models, give each group a 
protractor and tell them to go outside to find out.  (Soap bubbles are very 
sloppy.)

Before you turn the students loose, show them how to make a large bubble maker. 
Take two meters of string, double it up so it is only one meter long, run it 
through two straws and tie the ends of the string together.  Slide the straws so 
they are opposite each other, dip it into the solution, wave it in the air and 
you get really big bubbles. 

Back in the classroom - follow up!  Why do the soap bubble films assume the 
shapes that they do?  The answer is that soap film has the property that its 
surface area has a minimum value when it has reached equilibrium.  What forces 
are involved?  Answer - gravitational potential energy (GPE), surface tension, 
and the compressional energy of trapped air. 

Preparation of Bubble Solution:

85% water
10% liquid detergent
5% glycerin

Diagrams:

(See "Suggested Strategy")

       Diagram 1                            Diagram 2

           |                                \      /
      120  | 120 degrees                 120 \____/120 degrees
          / \                                /120 \
         /   \                              /      \
          120 
Shortest length connecting              Shortest length connecting
     three points.                           four points.