Abstract

Objective

Use UV beads to study a chemical reaction to find out how temperature affects the rate at which UV beads lose their color.

Introduction

Though ultraviolet (UV) light can be detected with electronic devices, a simple and colorful way to detect ultraviolet (UV) light is with UV-sensitive beads. UV beads change color when they are exposed to sunlight or to another ultraviolet (UV) light source, but they remain white or off-white when indoors. Because lightbulbs in homes and businesses do not produce ultraviolet light, the UV beads are colorless when kept indoors. But if you take them outside on a sunny day, UV light from the Sun turns them a variety of colors, including red, orange, yellow, blue, and purple. Some people even make bracelets from the beads so they know when they are exposed to UV light.

UV beads are made from white or clear plastic, with a photochromic dye, which means that the dye changes color when it reacts with ultraviolet light. This is a chemical reaction, which is defined as the process in which one or more substances (reactants) are chemically changed into one or more new substances (products). The ultraviolet light actually causes the shape of the molecules to change. After interacting with the UV light, the molecules change from a colorless form to a colored form. See Figure 1. The molecule undergoes a reversible rearrangement. It is reversible because the product can convert back to the reactant (this is indicated by the two arrows, one pointing left and the other pointing right). And it is a rearrangement because the parts of the molecule were rearranged.

Figure 1. Example of a molecule that changes color when exposed to ultraviolet light. The colored form of the molecule is not very stable, so with just a little energy from room-temperature heat, the molecule reverts back to its colorless form. You can read the Wikipedia article on photochromism, listed in the Bibliography, for more details about this reaction. (Wikipedia, 2009.)

The goal of this chemistry science fair project is to investigate how temperature affects the speed with which the colored molecules return to their color-free state. In other words, how does temperature affect the rate at which the colored form of the molecules reverts to the color-free form? The reaction rate measures change per unit time. For this science fair project, the change will be loss of color from the beads. The time will be how long it takes, in seconds, for the change to occur.

Terms, Concepts, and Questions to Start Background Research

• Ultraviolet (UV) light
• Photochromism
• React
• Chemical reaction
• Reactant
• Product
• Molecule
• Reversible rearrangement
• Stable
• Energy
• Reaction rate
• Absolute zero

Questions

• Based on your research, what is photochromism?
• What are some other reactions that depend on light?

Bibliography

• Wikipedia Contributors. (2009, July 30). Photochromism. Wikipedia: The Free Encyclopedia. Retrieved September 1, 2009, from http://en.wikipedia.org/w/index.php?title=Photochromism&oldid=305133838
• National Aeronautics and Space Administration. (n.d.). The Electromagnetic Spectrum. Retrieved September 1, 2009, from http://science.hq.nasa.gov/kids/imagers/ems/index.html
• Chem4Kids.com. (n.d.). Rates of Reactions. Retrieved September 1, 2009, from http://www.chem4kids.com/files/react_rates.html
• Helmenstine, A.M. (n.d.). Factors that Affect the Chemical Reaction Rate. Retrieved September 1, 2009, from http://chemistry.about.com/od/stoichiometry/a/reactionrate.htm
• U.S. Metric Association, Colorado State University. (2007, October 9). Metric System Temperature: Kelvin and Degree Celsius. Retrieved September 15, 2009, from http://lamar.colostate.edu/~hillger/temps.htm

For help creating graphs, try this website:

• National Center for Education Statistics. (n.d.). Create a Graph. Retrieved June 2, 2009, from http://nces.ed.gov/nceskids/CreateAGraph/default.aspx

Materials and Equipment

• Styrofoam^TM cups, 12-ounce size (4)
• Permanent marker
• UV beads; available online from Carolina Biological Supply Company, item # 956209 (1 pkg.)
• Liquid measuring cup
• Stopwatch
• Thermometer; available from Carolina Biological Supply Company, item # 959876
• Lab notebook
• Microwave oven
• Graph paper

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Experimental Procedure

1. Perform this experiment when there is sunlight outside.
2. Label the Styrofoam cups 1, 2, 3, and 4 with the permanent marker.
3. Place 10 UV beads into each Styrofoam cup
4. Add 250 mL of water to each cup, using the following temperatures:
• Cup # 1: Lukewarm water (20 degrees Celsius)
• Cup # 2: Ice water (0 degrees Celsius)
• Cup # 3: Lukewarm water (20 degrees Celsius)
• Cup # 4: Hot water (60 degrees Celsius)

Note: Warm the water in the liquid measuring cup in the microwave. Use the thermometer to measure the temperature of the water. Record all data in your lab notebook.
5. Take cups 2, 3, and 4 outside and expose them to the sunlight for 1 minute. Take the ice out of cup #2 before exposing the UV beads to the sunlight.
6. Return indoors.
7. Start the stopwatch immediately after bringing the UV beads back inside.
8. Observe how long it takes the UV beads to lose their color.
1. Compare the colors in the UV-exposed beads to the beads in cup # 1 (which were not exposed to UV).
2. In your lab notebook, record the time at which the beads in cups 2, 3, and 4 lose their color and look like the beads in cup #1.
9. Repeat steps 3–8 two more times (the beads can be reused many times). This will show that your results are reproducible.
10. Graph your results.
1. Convert the temperatures to kelvin (to do this, add 273 to the temperatures in degrees Celsius).
1. Kelvin is a better unit to use in this science fair project because it is based on absolute zero rather than on arbitrary temperatures, such as the temperature at which water freezes. See the U.S. Metric Association website reference in the Bibliography for more information.
2. Graph the temperature (kelvin) on the x-axis and the time (in seconds) on the y-axis.
3. How does increasing the temperature affect how fast the UV beads lose their color?

Variations

• Try more temperatures, such as 10, 30, 40, 50 degrees Celsius (convert to kelvin: 283 K, 303 K, 313 K, and 323 K).
• Experiment with different colors of the UV beads. Do UV beads of different colors have varying rates of color loss?
• Devise a way to perform the procedure at temperatures below 0 degrees Celsius.
• Based on your results, what temperature change would result in a doubling of the reaction rate? Hint: On your graph, pick two times that vary by two-fold, such as 100 seconds and 50 seconds, and determine what the temperature difference is for those two points.
• Graph 1/time on the y-axis vs. temperature on the x-axis. Because the rate measures change per unit time, the value for 1/time is proportional to the rate. Does the rate increase in a straight line as the temperature increases?
• If you want to explore this topic further, look up the Arrhenius equation. It describes mathematically how the rate of a reaction depends on the temperature.

• For more science project ideas in this area of science, see Chemistry Project Ideas.

Credits

David B. Whyte, PhD, Science Buddies

• Styrofoam^TM is a registered trademark of The Dow Chemical Company.

Last edit date: 2009-12-07 12:00:00