Passport to Chemistry Adventure Mount Holyoke College ©2010 Grades 3-6 Chemistry Kit #7 Luminescence PARENTS: These experiments are meant for you to do with your child. Please read the CAUTIONS below and do the experiment with your child. Sign the passport to indicate that you or another adult will do the experiment with your child and make sure that they are safely using the kit. If you cannot do this, please return this kit to the library. The organizers of the Passport to Chemistry Adventure Program, Mount Holyoke College, and the public libraries cannot be held responsible for accidents resulting from misuse of these kits, failure to provide adult supervision, or ignoring the general and specific cautions below. GENERAL CAUTIONS: Always work in adult/child teams. Always wear safety glasses when performing experiments. Never eat or drink while experimenting. Do not eat or drink anything in the experiment. Follow all instructions and read all label warnings. Wash your hands after all experiments. SPECIFIC CAUTIONS: Make sure the water is not so hot that it scalds the experimenter. RECOMMENDATIONS: Get your child a notebook for their explorations. Encourage your child to draw and write down what s/he sees in his/her notebook. We recommend reading the instructions first yourself and then going through them with your child, skipping parts when you see his/her interest decline. You will get to revisit these ideas many times as your child gets older. ___________________________________________ Luminescence is the emission of light! Have you wondered how glow sticks work? In this chemistry kit, we’ll explore the chemistry that happens inside glow sticks and other light producing reactions. 1. What is luminescence? Luminescence [loo-muh-nes-uhns] is the emission of light. Chemical reactions make products that are different molecules (substances) than the starting molecules. Sometimes the reaction makes energy too. Usually this energy is in the form of heat, but sometimes the energy is in the form of light. In chemiluminescence [kem-uh-loo-muh-nes-uhns] reactions, one of the products is light. Can you think of an insect that does this kind of reaction to make light? Lightning bugs (fireflies) use a chemical reaction to make light! Except when animals use this chemistry it is called bioluminescence [bahy-oh-loo-muh-nes-uhns]. In this kit you will perform reactions that create light! First, we must understand how the light is created. Do you remember that atoms make up molecules and that atoms are made up of protons, neutrons, and electrons? The Page 1 of 6 Passport to Chemistry Adventure Mount Holyoke College ©2010 Grades 3-6 Chemistry Kit #7 Luminescence protons have a positive charge and they are crammed together with neutrons in the nucleus. Electrons have a negative charge and are spread out in orbitals or regions where you are likely to find an electron. The different orbitals or regions have different energies. The energies of each orbital can be shown in a diagram as seen below. In the energy diagram furthest to the left, two electrons are shown at the lowest energy level (bottom) indicating that they are in the lowest energy orbital. Higher energy levels are also shown, but those orbitals do not have any electrons in them. So what happens during luminescence that creates the light? First a molecule that has all of its electrons in the lowest possible energy levels, called the ground state, becomes excited. In order to become excited and be in an excited state the molecule gains some energy and one or more of its electrons move into higher energy levels as shown below. Since this excited state requires more energy than the molecule likes to have the electrons in the higher energy levels move back into the lower energy levels. In order to move back down to the lower energy levels (ground state) some energy must be given off. This energy is given away in the form of light (the squiggly line in the picture)! In the Light and Sound kit, you will learn that light is made up of packets of energy. In the first experiment you will build a model of the picture above. You will build both the ground state model and the excited state model while it is giving off light (energy). These models will help you think about what is happening in the other experiments in this kit. The models will also help you remember what luminescence is really all about. Step 1. Attach the two energy levels to one wall support using the notches. Then, attach the second wall support to the other end of the energy levels. The notched spaces should fit snugly together. Make sure the Velcro dots are on the top of the energy levels so they can hold the pompoms (electrons). Page 2 of 6 Passport to Chemistry Adventure Mount Holyoke College ©2010 Grades 3-6 Chemistry Kit #7 Luminescence Step 2. Now collect 2 of the pom-poms, these will be the electrons. Attach these the lower energy level. You have just made a model showing the electron energies in the ground state of a molecule. . Step 3. The dye molecule in the light sticks you will be using shortly goes to the excited state after reagents are mixed in the tube. Move one electron from the lower energy level to the second energy level. You now have a representation of an excited state. Step 4. The molecule prefers to be in the ground state. To get there, it releases energy in the form of light. Move the excited electron from the second energy level back to the first energy level. At the same time, the molecule is releasing light. To do this, you can send the pipe cleaner (representing the light) out from the molecule like a lighting bolt. Really, it does not look like a lighting bolt but this makes it extra fun! Models are just approximations. You now have a model for luminescence! Do your models resemble the ground state and the excited state shown in the picture? Good Job! Draw a picture of your models in the ground state and in the excited state in your notebook. Now you are ready to make light! 2. Controlling the speed of reaction: In this experiment you will perform chemical reactions that make light (chemiluminescence). Have you ever seen the sticks or necklaces that glow? These sticks use a chemiluminescence reaction to make them glow. Why don’t they glow until you activate them? In order to activate the stick you have to bend it. When the stick is bent a small glass tube that is inside breaks. The liquid that was in the glass tube can now mix with the liquid that was outside of the glass tube. These two liquids react to form light! How do the electrons gain energy to form the light? First two liquids (diphenyl oxalate and hydrogen peroxide) react producing energy absorbed by a dye molecule. Upon absorbing energy, the electrons of the dye molecule become excited and jump to higher energy levels. When these electrons relax and move back to lower energy levels light is produced. In this experiment you test how well this reaction works at different temperatures. First, make some predictions your notebook. Remember, it doesn’t matter if your prediction is correct, just take your best guess. Your guesses will get better the more you practice. Which temperature (cold, medium, or warm) will make the brightest light? At which temperature (cold, medium, or warm) will the light last the longest? Page 3 of 6 Passport to Chemistry Adventure Mount Holyoke College ©2010 Grades 3-6 Chemistry Kit #7 Luminescence Step 1. Collect the three glow sticks. Step 2. Activate each stick by bending it. When you have finished activating the last one, record the time in your notebook. This is the start time. Step 3. Place one stick in the freezer, this is the cold stick. Instead of the freezer you could place the stick in ice water, or cold water. Step 4. Pour some hot water a beaker. Hot water from the faucet is fine. Be sure it is not so hot that you scald yourself. Cover the beaker with the cap. This is the warm stick. Step 5. Keep the last stick at room temperature, this is the medium stick. Step 6. Record the color of each stick in a table in your notebook. A table helps you organize data. Your table could have headings Glow Stick Temperature, Light Seen after 15 Minutes, Light seen after 30 minutes. You may need more columns if you keep observing for even more minutes. Step 7. Wait 15 minutes. Record the time in your notebook. Step 8. Remove the sticks from the freezer and the warm water. Place the three sticks side by side. Record your observations about how much light is being produced in the table you made in your notebook in your notebook. Step 9. Place some fresh warm water in the cup. Return the sticks to the freezer and the warm water. Wait another 15 minutes (30 minutes from the start time). Record the time in your notebook. Step 10. Again, place the sticks side by side. Record your observations about how much light is being produced in the table. Repeat steps 8-10 until you can see a trend. What happened? Were you able to control the amount of light produced by changing the temperature? The heat provided to the warm stick allows the molecules to move faster. When they move faster they bump into each other more and have more opportunities to react. Each excited dye molecule gives some light. So, when the number of reacting molecules is increased because they are bumping into each other more often and more vigorously, more dye molecules are excited and the amount of light given off is increased. The stick in the freezer did not give off much light because the chemical reaction was happening slowly. Many of the bumps were too soft. There was not a lot of energy available all at once to produce light. You can take that cold stick and put it into the warm water now and see that it can glow strongly when the temperature is higher. Slowing down the reaction is a good way of having your glow stick last longer. 3. Controlling the color emitted: Now that you know how light sticks work you can determine how to make different colors of light. The main chemical reaction is the same inside of each glow stick but different dyes are added to the stick. Why don’t all dyes produce the same color of light? To answer this question, have your model Page 4 of 6 Passport to Chemistry Adventure Mount Holyoke College ©2010 Grades 3-6 Chemistry Kit #7 Luminescence of the energy levels handy. In the model the excited dye has an electron in a higher energy level state. When the excited electron returns to its ground state energy level, light is produced. What happens if the distance between the excited state energy level and the ground state energy level is changed? A different amount of energy or color of light is produced! In the Light and Sound kit, you will find the relationship between energy and color shown on the right. In this experiment you will determine which dyes are inside of typical light sticks. Common light stick colors are blue, red, and yellow. Using the diagram on the right write down these colors in order of increasing energy. The energy of the color given off is the energy of the gap between energy levels in the dye. The table below gives the relative size of the gap between energy levels in three dyes with blue, red, and yellow colors. Use this information to predict which dye was is in each color. Feel free to use the label to name the dye rather than its chemical name. Label A B C Dye Rhodamine B 9, 10- diphenylanthracene 1-choro-9,10-bis(phenylethynyl)anthracene Energy Gap Small Huge medium Dyes A, B and C were red, blue, and yellow, respectively. Were your predictions correct? Write your conclusions in your notebook. Great job young scientist! Enclosed is a stamp to place on your passport to a Chemistry Adventure Day at Mount Holyoke College. To enter the chemistry adventure day, you need a filled passport and your lab notes from each of the kit experiments. So, be sure to keep your passport and notes in a safe place. For further exploration check out these books and web sites: 1. Our web site (http://www.mtholyoke.edu/courses/magomez/ChemistryPassport/kits3-6.html) provides links to other experiments related to this kit. Page 5 of 6 Passport to Chemistry Adventure Mount Holyoke College ©2010 Grades 3-6 Chemistry Kit #7 Luminescence 2. Sitarski, Anita. Cold Light: Creatures, Discoveries, and Inventions That Glow, Honesdale, PA: Boyds Mills Press, 2007. 3. Simon, Hilda. Living Lanterns: Luminescence in Animals, New York: Viking Press, 1971. Page 6 of 6