Adhesion, Cohesion, and Surface Tension Demonstration

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How to set up the demonstrations

Two items are needed for the first part of the experiment to demonstrate the cohesive and adhesive forces of water: Pairs of glass slides for each student or every other student (to be shared), and water. We use a syringe to add a drop of water to each pair of slides before class, and add food coloring to the water to make the water more visible. Glass slides can be purchased from scientific supply companies such as Wards at relatively low cost, and will last for years if unbroken. We have used one box of slides repeatedly over the years.

Tip: The demonstration will not work well if the slides get dirty or if too much water is used. Clean the slides before use if they are dirty.

A needle, a small amount of water (without food dye), a clear glass or plastic dish or bowl, and an overhead projector are needed for the second demonstration of surface tension in water.

How to do the demonstrations

The instructor can demonstrate the cohesive and adhesive forces of water at the front of the class, but it is more enjoyable for students if they are able to try to pull the slides apart, and to actually feel the strength of the intermolecular forces of water holding the slides together.

Begin by asking students what they think might happen if a drop of water is added to a glass slide and then another slide is placed on the first, with the water drop sandwiched between the two slides. After a brief discussion, pass out the pairs of glass slides. Ask students to attempt to pull the slides apart, but only by pulling perpendicularly to the surfaces of the slides. After a few minutes during which students are likely to become animated and talkative with one another, discuss what happened during the demonstration. Ask students to carefully pass the slides to the front of the room for collection after the ensuing discussion.

To demonstrate surface tension, place a small amount of water (~1 cm deep) in a shallow glass or plastic dish (or plate), and place the dish on an overhead projector. Turn on the projector, and carefully place a needle on the water surface so that the needle floats. Students will be able to see a projection of the needle's silhouette as it floats on the water surface. After a minute, tell the students that you will agitate the water surface with your finger or a pencil. When you do so, the imbalance of forces at the surface will be disturbed, and the needle will sink to the bottom of the glass dish.

Ideas for discussing the demonstrations in class

Water molecules have a strong mutual attraction for one another, enabling water molecules to hold together strongly. Water and glass have a smaller attractive force, called adhesion rather than cohesion to distinguish the intermolecular forces between two separate bodies (glass attracting water) from those of water attracting water. Adhesion between water and glass leads to capillary rise in a glass tube. The cohesive force of water molecules is responsible for the phenomenon of surface tension.

Molecules of water are attracted equally to one another from all sides except the free, exposed surface of the water, where the attractive forces are unequal. As a result, water molecules are drawn toward the center of the liquid mass. The water surface appears to act as a very thin membrane, pulling the water together so that the surface becomes curved (as in a bead of water on a hydrophobic surface). The curvature results because a sphere has the smallest ratio of surface area to volume of any geometrical shape. It is easier to move an object that is completely submerged in water than to push it through the surface of the water because of surface tension. As a consequence, small insects can skate across pond surfaces and objects of greater density, such as needles, can float on water.

References and resources

The San Francisco Exploratorium has many tested and excellent demonstrations on-line. One of these, the Bubble Tray, describes the use of soap to create a sphere of water (i.e., a bubble) in order to demonstrate surface tension.

The Institute of Physics has a Practical Physics website with nearly 300 demonstrations, several of which are on the topic of surface tension.

Answers.com provides definitions and short descriptions of cohesion, adhesion, surface tension, and capillarity.