Molecular Forces At Work: Creating Soap Bubbles

Laz, Jerome                               Everett School 
                                          650-4550
                          

Objective:

To make chemistry interesting to students by showing them it is part 
of the real world, rather than being confined to reagent bottles and 
test tubes in the classroom laboratory.
The student will be able to: 
1.  discover how the terms "surface tension," "cohesion," and 
    "adhesion" are related to soap bubbles,
2.  hypothesize the outcome of the experiments, 
3.  analyze experimental data and conditions,
4.  relate the polarity of the water molecule to the behavior of soaps 
    and detergents.

Background:
This project is based upon molecular force and the degree of surface 
tension, which depends on the amount of energy in the intermolecular 
forces.  Liquids that produce strong intermolecular forces have a 
strong surface tension.  Water molecules form hydrogen bonds between 
each other and have strong intermolecular force; as a result, a strong 
surface tension is created. 
Soap bubbles are made up of soap molecules and water molecules.  A soap 
bubble has a polar end and a nonpolar end.  Water is a polar molecule.  
The polar end of the soap molecules are attracted to each other.  The 
nonpolar ends of the soap molecules stick out from the water and help 
hold bubbles together. 
        
Apparatus needed:

overhead projector       silk thread               glycerin
Petri dishes             detergent                 distilled water
razor blades             cheesecloth
needles                  screening
alcohol                  2 small glass plates
pepper                   soap bubble solution
aluminum foil            pipe cleaners or bubble wands
beakers                  pans for bubble solution

Recommended Strategies:

1.  Float a razor blade or needle on water in a Petri dish on the 
    overhead projector.  Surface tension supports the object.

2.  Make a loop of 5" of silk thread.  Float in a low, wide container 
    of water.  Touch a bit of wet soap to the water inside of the silk 
    loop and notice that it becomes a circle.

3.  Sprinkle black pepper over the surface of water in a Petri dish.
    Add a drop of detergent to the center and watch the pepper spread 
    out. 

4.  A hollow "boat" made of a 2" by 1" piece of aluminum foil, partly 
    filled with a few drops of alcohol and with a 1/2 " length of 
    lightweight cotton wick (such as is used in a cigarette lighter) 
    over the stern, will skim over water.

5.  Fill a wide mouthed jar with water, cover mouth with cheesecloth, 
    and fasten securely.  Now invert the jar quickly.  If you punch a 
    small hole in the cheesecloth with a pointed pencil, water runs out 
    for an instant, but surface tension with the aid of atmospheric 
    pressure "seals" the hole. 

6.  Support a piece of window screen over a pan.  Slowly pour water 
    onto the screen.  Plain water will bead up.  Detergent water will 
    fall through the openings.

7.  Recipe for making soap bubbles:
     85% distilled water
     10% detergent (Joy recommended)
      5% glycerin

Students make bubbles and observe irridesent color, relative thickness 
of the top and bottom of the bubble, movement of water within the 
bubble, and longevity of bubbles.