Cohesion Plates

Introduction

Mesmerize your students by “fusing” two plastic plates together with a few drops of ordinary water! Is this the newest superglue?

Concepts

• Atmospheric pressure
• Cohesive forces

Materials

Safety Precautions

Although this activity is considered non-hazardous, please follow all normal laboratory safety guidelines.

Disposal

Materials can be reused indefinitely and may be saved for future demonstrations.

Procedure

1. Fill one 100-mL beaker with water (tap or distilled).
2. Fill a second 100-mL beaker with water and add two to three drops of dish soap. Stir the solution slowly to prevent the formation of bubbles.
3. Obtain the two beveled plastic cohesion plates.
4. Place one cohesion plate on the tabletop with the beveled side up.
5. Hold the second plate in your hand with the beveled side down (see Figure 1).
6. Place the second plate onto the plate on the tabletop and quickly pull it back up. (This shows students that the plates do not stick together naturally.)
7. Using a Beral-type pipet, add one or two drops of water to the center of the plate on the tabletop. (Optional: Tell students that this is a fast-acting new glue.)
8. Quickly press the second plate onto the plate on the tabletop (making sure the water spreads out between the plates) and then pick the plate back up. (The bottom plate will be pulled up with the top plate as they appear to be glued together with water.)
9. Firmly press the beveled-sides together making sure the water spreads out completely between the plates.
10. Attempt to pry the plates apart by holding onto the center of the plastic edges and pulling them perpendicular to the face of the plates. Do not twist or slide the plates. (It will take an unexpectedly large force to pry the plates apart in this manner. However, with the right amount of force, the plates will come apart.)
11. If the plates have not come apart, slide the plates apart and dry them off with a paper towel.
12. Repeat steps 4–8 using the soapy water. (The soapy water will not act very glue-like. The plates will not stick together.)
13. Rinse the plates with water and dry them completely.

Student Worksheet PDF

12819_Student1.pdf

Teacher Tips

• Glass plates are more difficult to pull apart than plastic plates because water and glass have a higher adhesive strength than water and plastic. However, for safety, beveled plastic plates are used for this demonstration. Beveled glass plates are available at some craft stores. Use extreme caution when handling glass plates for this demonstration. They may crack when they are pulled apart and will easily shatter if dropped. Wear safety glasses and protective gloves when using glass plates for this demonstration.

Answers to Questions

1. Do the plates naturally stick together?
   

   No, the plates simply come apart.

2. When the liquid is added to the plastic plates and the plates are pressed together, what happens?
   

   The plates stick together. When the top plate is picked up, the bottom plate goes with it.

3. What happens when the soapy liquid is used? Are the plates “glued” together?
   

   No, the plates come apart as easily as they did when they were dry.

4. Explain the demonstration in terms of cohesive forces, adhesive forces and air pressure.
   

   See Discussion section for an appropriate explanation.

Discussion

Liquid water has an unusually strong surface tension because of its strongly polar water molecules. A surface tension develops because the molecules at the interface between the water and air are attracted to the water molecules below them but not to the air molecules above. The attraction of like molecules toward each other is known as cohesion. Since there are no water molecules above to attract the cohesive attraction from below, the water molecules at the surface are attracted to each other with a stronger net force. The unbalanced forces on the surface water molecules form somewhat of an elastic “skin” on the surface of the water. The cohesive strength of water is proportional to the amount of surface area.

Different molecules, such as water and plastic, are also attracted to each other by a force known as the adhesive force. This force depends on the composition of the different molecules and typically is weaker than a cohesive force between like molecules.

During the demonstration, a small amount of water spreads out along the surface of the plastic plate when the two pieces are pressed together. As a consequence, all the air is driven out from between the plates too. When the plates are pulled perpendicular to the face of the plates, a combination of cohesion and adhesion prevents the plates from coming apart. Cohesion maintains the surface tension of the water and prevents the outer water “skin” along the edges of the plate from breaking, and thus prevents the flow of air between the plates. Unbalanced air pressure on the outside surfaces of the plates compared to interior surfaces also helps keeps the plates together. The adhesive force between the water and the plastic also helps “glue” the plates together.

Soapy water does not act as a good “glue” for these plates. Soap interferes with the cohesive forces of the water and thus decreases the surface tension of the water. The lower surface tension allows air to enter between the plates more easily, and the cohesive forces, adhesive forces and unbalanced air pressure forces lose the ability to hold the plates together.