Solar-powered Chemistry in Ultraviolet Beads


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.


Diagram of ultraviolet light and heat changing the chemical structure of a molecule
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 and Concepts

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


Materials and Equipment

  • Foam cups, 12-ounce size (4)
  • Permanent marker
  • UV beads
  • Liquid measuring cup
  • Stopwatch
  • Thermometer
  • Lab notebook
  • Microwave oven
  • Graph paper

Experimental Procedure

  1. Perform this experiment when there is sunlight outside.
  2. Label the foam cups 1, 2, 3, and 4 with the permanent marker.
  3. Place 10 UV beads into each foam 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?