Discovering C.A.S.T. (Cohesion, Adhesion and Surface Tension)

Elise S. Greene                Teachers Acad. For Math and Science
                               10 West 35th Street
                               Chicago IL 60616
                               (312)808-0100

Objectives:

This lesson is designed for students in grades 6th-8th.  As a result of 
participating in the experiments, the students will: 
1.  observe the properties of C.A.S.T. by viewing an inverted glass;
2.  define specific vocabulary terms regarding C.A.S.T.;
3.  demonstrate the concepts of C.A.S.T. by participating in the experiments;
4.  compare, contrast and analyze experiment data.

Materials Needed:

Demonstration set: 1 fluted wine glass, string or rope, water, hard plastic cut  
                   to fit the opening of the glass

Per team of 4 students
 
Station I                           Station II
2 regular drinking glasses,         2 beakers or plastic cups,
water,                              water absorbent string or rope,
several pieces of small cork        book, water

Station III                         Station IV
3 or 4 capillary tubes of           paper card with a hole in it,
different diameters, a small        liquid detergent/soap, shallow 
beaker, food coloring, water        tray or sink, scissors, water

Station V
small bottles of different
size openings, screen or cheese
cloth, rubberbands, water

Strategy:

1. Advance preparation: Assemble fluted wine glass with string tied around the 
   stem.  
   Make sure that the plastic has been cut to fit a little larger than the 
   opening of the glass.
   Test the experiment several times to make sure that the water does not spill  
   from the wine glass once it has been inverted.
2. Spark the interest of the students by asking what will happen if you pour 
   the water in the glass with the plastic over it and turn it upside down. 
   Show the students the display.
3. Discuss the definitions with students for the following words: convex, 
   concave, adhesion, cohesion, and surface tension.
4. Have a student pass around a set of worksheets that will have vocabulary, 
   questions to consider during the experiment, and experiment procedures. 
   Discuss the materials, time limits and procedures with the students.  Each 
   group of students will have 10 minutes at each station to complete the 
   discussed procedure and answer 3 questions on each experiment.  The teacher 
   will have timer ring at the end of 10 minutes.  At that time the students 
   will proceed to the next station. 
5. The students will conduct experiments at each station with a 10 minute time 
   limit.  At the end of 50 minutes, all students will have travelled to each  
   of the 5 stations.

EXPERIMENT PROCEDURES

Station I  Where Does The Cork Float?
1. Fill the glass half way with water and float the cork on the water surface:
   observe!
2. Where does the cork float?  
3. Now add more water to the glass and fill it brimfull.  Observe where the cork 
   floats now. 
4. Try to push the cork towards the edge: it will not stay there! 

Station II  Pour Water Along A String
1. Fill one of the beakers about 3/4 full with water.  
2. Stand a book about 20 cm away from the empty beaker.  
3. Show the students the string and ask: "How can I transfer the water from 
   beaker A to B without moving beaker A over or around the book?"
4. Hold one end of the string in beaker A and the other end over beaker B and 
   pour the water slowly along the string. 

Station III  The Smaller, The Stronger
1. Fill the beaker with water and place a few drops of food coloring in it.
2. Hold three or four capillaries close to each other and dip them in the water:
   observe the water level in each of the capillaries.

Station IV  The Detergent Propelled Boat
1. Fill a shallow tray with water.  
2. Cut out a boat from the paper in the form of the sketch on the board.  
3. Let the boat float on the water and place a drop of liquid detergent in the 
   center opening of the boat. 
4. Observe the movement of the boat. 

Station V  The Inverted Bottle
1. Use a rubberband to fasten the screen over the open end of the bottle.
2. Pour in water through the screen.
3. Invert the bottle: observe                  

Performance Assessment:

Students will be assessed on completing their experiments and on answering the 
worksheet questions to consider during the experiment.  The teacher will direct 
an oral review.  The teacher will monitor the work in progress by circulating 
around the work stations as the students conduct the experiments.  Each student 
that actively participates in the experiment process will receive a full lab 
grade. 
 
Conclusion:

Cohesion, adhesion, and surface tension all work together to make molecules of a 
particular substance stick together.  Cohesive forces are apparent between like 
molecules.  For instance, once the string is wet at station II, the water can 
cling to the already present water molecules and slide down the string to the 
beaker.  Likewise, adhesive forces are present in this experiment.  The water 
molecules are attracted to the string molecules.  Station IV students are made 
aware that the soap touching the water is breaking the surface tension, 
eliminating the attracting force in the rear of the boat and propelling it 
forward. 


                                    Worksheet
                              DISCOVERING C.A.S.T.
                    (COHESION, ADHESION, AND SURFACE TENSION)   
   
Define the following:

1. Convex

2. Concave

3. Adhesion

4. Cohesion

5. Surface Tension

Questions to consider during the experiments:

Station I   1. Why is the cork moving towards and sticking to the side of the 
               glass (with the glass half filled)?
            2. Where is the water level highest in the half filled glass?
            3. Why can we fill the glass more full without overflowing the water?
Station II  1. Why is it necessary for the string to be wet?
            2. What forces were holding the water to the string?
            3. Is it possible to pour other liquids along the string?
Station III 1. Will the water level in the capillaries change when the tubes are 
               either moved higher or lower in the beaker?
            2. What makes the water go up the tube in the first place?
            3. Where do we find an application of this capillary action in daily 
               life? 
Station IV  1. Why does the paper boat move forward only when the soap touches 
               the water? 
            2. What made the paper boat be pulled forward? 
            3. What would happen if we touched the soap to the side of the boat? 
Station V   1. What causes the water not to spill out of the inverted bottle or 
               glass?