Instructor Side
JCE Classroom Activity: #6
On the Surface: Mini-Activities Exploring Surface Phenomena by the Journal’s Editorial Staff
Integrating the Activity into Your Curriculum
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These activities can be used to introduce surface tension and surface area when discussing properties of liquids and gases. Activity 4 (not included on the Student Side) can also be used in kinetics as an example of increasing reaction rate by increasing the surface area of a solid reactant. All items used are common household materials. If the activity is to be done at home, you may need to provide some students with a disposable dropper and plastic sheet (such as overhead transparency, used report cover, or plastic “blister pack” packaging).
About These Activities Each activity has been written so that it can be done at home or in the laboratory. The activities can be done independently or in any order, but if the activities are discussed in class in the order given, the students may be better able to relate the new ideas built on those from the previous activity. 1. On Top of a Penny. This demonstrates the surface tension of water and the action of a surfactant. Other small coins with a rim similar to that on the U. S. penny can probably be substituted. Water beads up on the clean, dry coin. Students may get as many as 40 drops in an amazing lens-like bead on top of a U. S. penny. After the dishwashing liquid (detergent, a surfactant) is added, the drops spread out over the surface. As more drops are added, the water quickly overflows. 2. Circles and Squares. This activity builds on the first. Water drops bead up on the plastic (overhead transparencies or used plastic packaging work well) in round drops (a circle inside the square). Adding more water causes the drop to spread out to the sides of the square, but it remains round. The corners of the square are not filled with water. When dishwashing liquid is added to water, its surface tension is decreased and water flows over the surface until it reaches the crayon (wax) lines forming “square drops” of water. Fewer drops are needed to fill the square. Care must be taken that the water does not overflow the boundary. It may take a few minutes for the solution to flow into the corners. With some practice, you can make drops of just about any shape, even a checkerboard with two colors of water drops—an interesting and attractive effect. Unfortunately, this is not easily observable on an overhead projector. The colors do not show up well, and the inevitable “crayon crumbs” and bubbles are unattractive when projected. After the water drops evaporate, the shapes can be refilled with the same color solution. (If any squares overflow, allow the surface to dry and put a fresh layer of crayon on the line.) The residue of detergent makes the drops flow immediately into the outline. The color residue intensifies the color of the water drops. A grid from which drops have evaporated several times makes a stunning demonstration when refilled. 3. The Trouble with Bubbles. This shows that the shape of the bubble formed is always the one having least surface area—a sphere. An interesting discussion can result from asking students why the addition of dishwashing liquid allowed you to make “square water drops”, but dishwashing liquid bubbles themselves are spherical, even when created from a square film. You could also have students calculate the surface area of a sphere and cube having the same volume. 4. Fastest Fizz in Town. This activity is not included on the Student Side. To illustrate the effect of increased surface area on the rate of a chemical reaction, have students simultaneously drop whole and crushed effervescent tablets (Alka Seltzer) into water. Given their observations, you might ask why the manufacturer forms the medicine into tablets rather than selling it as a power, which would allow the patient to drink it sooner and thus get quicker relief.
Additional Activities and Demonstrations with Surfaces, Surface Area and Surface Tension http://physics.umd.edu/deptinfo/facilities/lecdem/demolst.htm#f3 Sarquis M.; Sarquis, J. Eds. Fun with Chemistry, Vol. 1, 2nd ed.; , ICE Publ. 91-005, Institute for Chemical Education, University of Wisconsin–Madison, 1991. Dust Explosion see Sarquis M.; Sarquis, J. Eds., Fun with Chemistry Vol. 2; ICE Publ. 93-001, Institute for Chemical Education, University of Wisconsin–Madison, 1993, 213, 217. Journal references include: • Somorjai, G. A.; Ruppechter, G. J. Chem. Educ. 75, 1998, 161–176. • Markham, J. R. Flying over Atoms, J. Chem. Educ. Software, Special Issue 19, in press; abstract of Special Issue 19, J. Chem. Educ. 1998, 75, 247–248. • Lorenz, J. K.; Olson, J. A.; Campbell, D. J.; Lisensky, G. C.; Ellis, A. B. “A Refrigerator Magnet Analog of Scanning-Probe Microscopy”, J. Chem. Educ. 1997, 74, 1032A. • Jasien, P. G.; Barnett, G. “Lowering the surface tension of water: An illustration of the scientific method” J. Chem. Educ. 1993, 70, 251. • Silverstein, T. P. “Polarity, miscibility, and surface tension of liquids” J. Chem. Educ. 1993, 70, 253.
This Activity Sheet may be reproduced for use in the subscriber’s classroom. JChemEd.chem.wisc.edu • Vol. 75 No. 2 February 1998 • Journal of Chemical Education
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JCE Classroom Activity: #6
Student Side
On the Surface: Mini-Activities Exploring Surface Phenomena by the Journal’s Editorial Staff When chemists describe the properties of solids and liquids, they generally talk about such things as melting point, boiling point, and density. These properties tell us a great deal about the bulk of a liquid or solid sample, but give no information at all about the sample's surface. The behavior of molecules on the surface can be very different from the molecules inside the sample. The following activities demonstrate some of the unusual properties of liquid surfaces. See if you can figure out what is happening at the molecular level in each case.
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1. On Top of a Penny. You will need one U.S. penny, dishwashing liquid, water, paper towels, and a dropper. __a. Rinse a penny thoroughly with water and dry it with a paper towel. __b. Set the penny face up on a table or lab bench. Place one drop of water on it. What does the drop look like? __c. Count how many drops you can add before the water spills over the edge of the penny. Describe the appearance of the water as you continue to add drops. __d. Dry the penny. Put one drop of dishwashing liquid on the penny. Rub it with your fingers to cover the entire surface. Wipe off any excess, leaving a film of liquid on the penny. Repeat steps b and c. How did the addition of dishwashing liquid change things? Why? 2. Circles and Squares. You will need a flat piece of transparent plastic, white paper, a crayon or wax pencil, cup, food color, water, dishwashing liquid, and a dropper. __a. Place the plastic on a sheet of white paper on a level surface. Use a crayon to draw several small squares no more than 1 cm on each side on the plastic. Make the lines thick, about 2–3 mm. __b. Half fill a cup with water. Add 2 or 3 drops of food color. Swirl to make a uniform, intensely colored solution. __c. Place a drop of the solution inside one of the squares. What shape is the drop? Add water a drop at a time until the edge of the water drop reaches the crayon lines. Be careful not to allow the water to overflow the square. Make several trials. What shape(s) do you see? __d. Now add a few drops of dishwashing liquid to the colored water and swirl to mix. Repeat step c. Make several trials. Wait about 5 minutes, and compare the shapes in the squares. How does adding dishwashing liquid change things?
Questions __1. What is surface tension? On the molecular scale, what causes it? __2. What is a surfactant? How do surfactants affect surface tension? Does this effect have any useful applications?
Information from the World Wide Web 1. http://www.epa.gov/owow/NPS/kids/TENSION.HTM 2. http://physchem.ox.ac.uk/curr-res/CDB.html 3. http://w3.mit.edu/huibers/www/s-comp.htm 4. http://www.sme.org/memb/neweek/actsoap.htm 5. http://www.exploratorium.edu/ronh/bubbles/bubbles.html
This Activity Sheet may be reproduced for use in the subscriber’s classroom. 176B
Journal of Chemical Education • Vol. 75 No. 2 February 1998 • JChemEd.chem.wisc.edu
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3. The Trouble with Bubbles. You will need dishwashing liquid, water, a small shallow container, and flexible wire (the wire from ties used for bread or trash bags works well). __a. Bend a wire so that a small circle (about 1 cm in diameter) is formed at one end. Bend another wire to form a square and another to form a triangle. __b. Pour a few milliliters of dishwashing liquid into a shallow container. Add about an equal volume of water. Dip the circle end of the wire into the dishwashing liquid solution. Remove it slowly so that a film of liquid fills the circle. Blow through the film and note the shape of bubbles formed. Repeat with the wires shaped into a square and triangle. __c. Bend wires into other shapes and repeat step b. How many shapes of bubbles can you make? What do you think is the “trouble” mentioned in the title? What causes it?
