Instructor Information
JCE Classroom Activity: #36
Putting UV-Sensitive Beads to the Test
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Terre Trupp Union-Endicott High School, Endicott, NY 13760
[email protected] This Activity explores the temperature behavior of UV-sensitive beads and investigates the effectiveness of sunscreens. The beads can be ordered conveniently (1). The Activity uses the sun as its UV source. If bright sunlight is not available, use a long bulb fluorescent blacklight and warn students of proper precautions. Blacklights are available at gift and home improvement stores (2). Hand-held lamps are also available at discount stores around Halloween. Incandescent blacklights do not work well. Before introducing the Activity, instructors may wish to give each student several UV beads tied together with string and challenge them to identify the unique property that the beads possess as homework for the following day. To melt the beads into flat disks to provide an even test surface, place them on a Be Safe! If a blacklight is used, cookie tray covered in aluminum foil, spacing them far enough apart so that they will caution should be exercised benot touch when they melt. Bake the beads in an oven preheated to 350 °F. The beads cause UV radiation can be harmful to eyes and skin. Students will become colorless and will flatten to about the size of a dime. After 10–15 min- should not shine the UV source in utes, remove the beads from the oven and allow them to cool. their eyes and should limit exposPart I allows students to make initial observations of the beads. On the basis of ing their skin to the blacklight. these observations, students will likely choose purple, red, or orange for use in Parts II and III because these colors appear fastest and are most intense. In Part II students will discover that the intensity of bead color varies inversely with temperature. In Part III, students investigate the effectiveness of different sunscreens (3, 4 ). It is unlikely that they will be able to complete all three parts in one class period. A QuickTime movie that shows the UV beads changing color is available in this issue of JCE Online.W
Integrating the Activity into Your Curriculum This Activity can be used to show applications of chemistry in the real world. Part II can be used as an extension of a kinetics unit or to enhance a discussion on temperature-dependent reactions. Part III can be used as an extension of a unit on atomic structure. The sunscreen testing can also enhance consumer science topics similar to those found in the ChemCom curriculum.
Background Information about pigments used in UV-sensitive beads was not available from the vendor (1a) and the pigments appear to vary. Beads obtained at different times have been found to produce somewhat different colors. The pigments are most likely photochromic spirocompounds. The orientation of the molecular structure of these complex organic compounds changes on exposure to UV light (5–8). Different orientations have different colors: white or colorless in visible light and brightly colored in UV light. The intensity of color produced varies inversely with temperature, perhaps because increased molecular motion at higher temperature interferes with the orientation shift. perforated
Answers to Questions 1. Answers will vary. For example, a student might choose purple because it turns an intense, easily seen color. 2. See above. 3. Sunscreens with lower SPF values are less effective in blocking out UV radiation, those with higher values more effective. Answers will vary depending on the SPF values students choose to test. 4. Answers will vary, depending on sunscreens tested. Differences in ingredients may influence effectiveness. 5. Some possible answers are that UV beads can be worn and used to determine UV exposure and can be used to test materials meant to block UV such as sunglasses. UV-sensitive materials might be used to make sunglass lenses or windows to absorb UV radiation.
This Activity Sheet may be reproduced for use in the subscriber’s classroom.
fold here and tear out
About the Activity
References, Activities, and Resources (WWW sites accessed March 2001) 1. (a) Educational Innovations; http://www.teachersource.com/; 1-888/912-7474. (b) Another source is SolarActive International; http://www.solaractiveintl.com/index_newa.htm; 818/996-8690. 2. Spencer’s and Home Depot are possible sources. 3. Lawrence, G. D.; Fishelson, S. UV Catalysis, Cyanotype Photography, and Sunscreens; J. Chem. Educ. 1999, 76, 1199–1200; Blueprint Photography by the Cyanotype Process; J. Chem. Educ. 1999, 76, 1216A–1216B. 4. Test Your Risk from UV Radiation—Information on UV Radiation; http://cimss.ssec.wisc.edu/wxwise/class/uvpretst.html 5. Optical Products, Technical Overview; http://www.ppg.com/chm_optical/fototech.htm 6. Osterby, B.; McKelvey, R. D.; Hill, L. J. Chem. Educ. 1991, 68, 424–425. 7. Prypsztejn, H. E.; Negri, R. M. J. Chem. Educ. 2001, 78, 645–648. 8. Barachevsky, V. A. Int. J. Photoenergy 1999, 1, 1–5. JCE Classroom Activities are edited by Nancy S. Gettys and Erica K. Jacobsen
JChemEd.chem.wisc.edu • Vol. 78 No. 5 May 2001 • Journal of Chemical Education
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JCE Classroom Activity: #36
Student Activity
Putting UV-Sensitive Beads to the Test Terre Trupp Union-Endicott High School, Endicott, NY 13760
[email protected] Ultraviolet (UV)-sensitive beads contain pigments that change color when exposed to sunlight or other sources of UV radiation. Exposure to UV rays is harmful to skin cells. Sunscreens contain substances that absorb UV radiation and their sun protection factor (SPF) indicates how effective they are. Could you use these beads to let you know when you need to apply sunscreen?
Try This You will need bright sunlight (UV source), UV-sensitive beads of 4–5 different colors melted into disks, white paper, 3 small beakers or drinking glasses, plastic wrap, scissors, ruler, water, ice, hot water, timer, cotton swabs, and sunscreen (one brand of 3 different SPFs, 3 brands of same SPF). Record observations at each appropriate step below. Part I: Exposing UV-Sensitive Beads ___ 1. Lay UV-sensitive disks of 4–5 different colors in a straight line on a piece of white paper. The disks should not touch each other. ___ 2. Expose the disks to the sun; observe the color change for each disk. After all disks have changed colors, take them out the sun; observe the color change for each disk. ___ 3. Repeat step 2 one or two times. Do some color changes happen faster than others? Are some changes easier to see than others? Do the colors appear and disappear in the same order? At the same rate? ___ 4. Based on your observations, which color disk is the most effective UV-sensing device? Obtain 6 more disks of this color from your instructor to use in Parts II and III. Part II: Does Temperature Influence Bead Sensitivity? ___ 5. Label 3 small beakers or drinking glasses “hot”, “cold”, and “room temperature”. Fill the one labeled hot 3/4 full of hot (not boiling) water, the cold 3/4 full of ice water, and the room temperature 3/4 full of room-temperature water. Arrange the containers in a straight line. Cut three 6-in. squares of plastic wrap. Place one square flat into each container so the wrap touches and covers the water surface. Excess wrap will extend up the sides of the container and can be folded over the container rim. ___ 6. Get three disks of the color you selected in step 4 and place one in the center of the plastic wrap in each of the three containers. Wait 3–5 minutes for the disks to come to the temperature of the water. ___ 7. Predict at which temperature the disk will change from white to colored most quickly and which will be the most intensely colored. Then, expose the disks to the sun simultaneously, observing color changes. When all disks have changed color, take the containers out of the sun. Predict which disk will lose its color first, and then observe the color changes. Repeat the test. Part III: Using UV-Sensitive Beads to Test the Effectiveness of Sunscreens __ 8. Draw 4 circles (by tracing around disks) in a straight line on a piece of white paper. The circles should not touch. __ 9. Label one circle Control and place a disk (the color selected in Part I) in it. __ 10. Take another disk of the same color and use a cotton swab that has been soaked in sunscreen to lightly coat its entire surface. Label a circle with the SPF number of that sunscreen and place the disk on it. Repeat twice using the same brand sunscreen, but different SPF values. Predict which sunscreen will be the least effective. __ 11. Simultaneously expose all four disks to the sun for two minutes. Develop a scale from 1 to 5 that explains the variations in colors observed and use it to rank the disks. Which sunscreen-coated disk had the brightest color compared to the control? Repeat your test at least once more. __ 12. Repeat steps 1–5 using three different brands of sunscreen but all with the same SPF number. Use the same control, but use 3 fresh disks for applying the sunscreen and doing the tests.
Questions ___ 1. ___ 2. ___ 3. ___ 4. ___ 5.
Which color disk did you select for use in Parts II and III? Explain why you chose this color. Describe your observations from Part II, step 7. Did your predictions match your observations? Explain. Which SPF was the least effective at blocking UV light? Most effective? Did the results match your predictions? Are all brands of sunscreen with the same SPF value equally effective? Why might they differ? What are some practical uses for UV-sensitive beads and other UV-sensitive materials?
Information from the World Wide Web (accessed March 2001) 1. Information Sheet on UV radiation; http://katipo.niwa.cri.nz/lauder/uvinfo.htm 2. Lifestyle and the Sun; http://www.wce.ac.nz/cancer/lifestyles_sun/suncont.html This Activity Sheet may be reproduced for use in the subscriber’s classroom.
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Journal of Chemical Education • Vol. 78 No. 5 May 2001 • JChemEd.chem.wisc.edu
