Surface Tension

Lisa C. Ingram                 Frederick Douglass Middle Academy
                               543 N. Waller
                               Chicago IL  60644
                               (312)534-6176

Objective:

To understand the concept "Surface Tension", through several experiments.
Designed for students grades 5 through 8.

Materials Needed:

Overhead projector (teacher demonstration)
2 glasses of equal size and depth
a bucket
water
pennies 60 per group or person
bowls 2 per group or person
a box of toothpicks
liquid soap or detergent
a box of cube sugar
clear cups 1 per group or person
bubble solution
bubble wands or other bubble instruments

Strategy:

Students will be asked to brainstorm what surface and tension mean.  Also, 
discuss where they have heard the words before.  All comments will be written on 
the black board.  After all demonstrations students will comeback to these 
comments and form a definition(s).

Experiment 1:  Teacher will fill two glasses underwater.  Then take them out of 
the water, holding them rim-to-rim so no water escapes.  Stand the glasses on a 
flat surface so that one rests upside down on top of the other.  Teacher will 
slide one penny between the two rims.  When the coin is in place, water molecules 
will pull together between the rims and stop the water overflowing. 

Question:  Why did the water stop dripping, even though there is still an 
opening?

Answer:  The molecules are pulling and stretching to form a skin to the water.  
Thus no water can get out, unless the surface tension is broken.  
 
Experiment 2:  Teacher will use the overhead projector to demonstrate.  Arrange 
toothpicks in a circle in a bowl of water.  Place a cube of sugar in the center 
of the circle (place bowl on top of overhead projector).  Take another bowl, 
arrange the toothpicks in a circle again.  This time place aa piece of soap or 
liquid detergent in the center (place the bowl on top of projector).  Students 
should observe and verbally respond, the toothpicks went toward the sugar and 
they went away from the detergent.


Question:  Why did the sugar draw the toothpicks and why did the detergent repel 
the toothpicks? 

Answer:  The sugar sucks up water, creating a current that carries the 
toothpicks with it toward the center.  The soap, on the other hand, gives off an 
oily film that spreads outward.  It weakens the surface tension, and the film 
carries the toothpicks away with it.

Experiment 3:  A clear cup with a drop of food coloring, should be placed on top 
a white piece of paper.  Near by should be a bag of at least 60 pennies. 
(Teacher should set these items up, as stations, in the designated area before 
children begin)  Explanation: Tell the students that they are going to fill 
their cups up to the brim with water.  They will then have someone slide pennies 
into the color cup of water, until the color water drips on the paper.  Tell 
students not to throw the pennies, they want a drip not a splash.  One student 
in the group should be designated to count the pennies going into the water.  
They are to this until there is a drip on the paper. 

Question:  Why does it take so long for the water to drip?

Answer:  Surface tension builds around the cup, until it is broken with a penny. 

Performance Assessment:

Students will be asked to explain surface tension verbally.  They will also be 
instructed to write one or two sentences defining surface tension.