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

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

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?