Flame Tests: Identifying Metals

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1 Section 8 Flame Tests: Identifying Metals What Do You See? Learning Outcomes In this section you will Produce colored flames. Identify the metal ions present in materials by the colors of light a material gives off when held in a flame. Describe how atoms create the colored light. Investigate ways of producing new colors not among those produced by the materials you test. Practice safe laboratory techniques in working with laboratory burners. What Do You Think? You may have read how royalty, suspicious of enemies, had their food tested for poison by having someone else taste it first. That s not a job anyone would enjoy for very long! How else could you test for a poisonous substance (other than by eating it)? Record your ideas about this question in your log. Be prepared to discuss your response with your small group and the class. Investigate 1. Your teacher will supply you with seven numbered wooden splints that have been soaked in various solutions. Some are harmful, and some are not. a) How could you distinguish one splint from another? 2. Light your Bunsen burner. Immediately make observations about the color of the burner flame. a) Record your observations in your Active Chemistry log. Safety goggles and a lab apron must be worn at all times in a chemistry lab. Follow all safety precautions for working with an open flame. Wear protective gloves when handling hot objects. 73 Dispose of all substances and materials as directed by your teacher

2 Movie Special Effects Wear protective gloves when handling hot objects. Tie back hair and loose clothing. Do not reach across an open flame. Dispose of all substances and materials as directed by your teacher. Wash your hands and arms thoroughly after the investigation. 3. Take one of the wooden splints and hold it in the hot part of the flame using forceps, tongs, or a fireproof glove. Note the color of the flame when the solution-soaked wooden splint is heated. As soon as the flame from the splint is no longer strongly colored, extinguish the splint by placing it in a beaker of water. a) Organize a table to record your data. b) Record your observations in your table. 4. Repeat Step 3 for the remaining six splints. a) Record all your observations. 5. Refer to your data table. The salt solutions in which the splints were soaked are metal compounds of ions of the elements lithium, barium, sodium, strontium, potassium, and calcium. Ions are atoms that have gained or lost electrons. One splint was soaked in water. a) What do you notice that was different about each splint? b) Your teacher will identify the solution each splint was soaked in. Record this information in your data table. 6. Obtain another splint soaked in an unknown solution. Hold the wooden splint soaked in the unknown solution in the burner flame. Observe what happens. a) Record your observations in your log. b) Use your observations to decide what metal ions are on the splint. Confirm your results with your teacher. c) Did you correctly identify the metal ion? Explain. d) In your log describe how you decided what the identity of your compound was, and how sure you are that you were correct. 7. Clean up your workstation and return all your equipment as directed by your teacher. 74

3 Section 8 Flame Tests: Identifying Metals Chem Talk IDENTIFYING ELEMENTS USING FLAME TESTS In this investigation you were able to create unique colors. Each color was an identifying characteristic of the solution on the splint. The colors are the result of the electron structure of each solution. When the metal ions are placed in the hot flame their electrons absorb the energy and move to higher energy levels around the nucleus. When these electrons fall back to their original level, they give off the light you see in the flame test. Since each atom has a unique arrangement of electrons, each gives a unique color. This experimental technique is called a flame test. It is the basis for identifying the composition of the salt solutions. This same effect is responsible for the burst of color in fireworks. In this experiment only the metal ions are changed. The negatively charged ion (chloride) is not affected. The flames produced in this section have interesting colors. The structure of matter is the key to the colors. The metal compounds are made of atoms that have nuclei surrounded by electrons. When electrons fall from a higher energy level to a lower energy level, they give off radiation. In this case, the electrons were able to get to a higher energy level by absorbing heat energy from the Bunsen-burner flame. Each compound has a different configuration of electrons and electron energy levels. The change in energy levels represents the color of the light given off. If you refer to the higher energy level as E h and to the lower energy level as E l, then you can state: The energy of the emitted light is equal to E h E l. The total amount of energy remains the same as it is conserved. The energy the electron gains from the heat energy of the Bunsen burner flame raises the electron to a higher energy state. When the electron drops down to the lower energy state, it gives off that energy as light energy. The process you observed in this investigation is not a chemical reaction. In a chemical reaction, the product of the reaction is different from the original materials. In the flame test, the heat did not chemically alter the metal compound on the splint. Chem Words ion: an electrically charged atom or group of atoms that has acquired a net charge, either negative or positive. electron: a subatomic particle that occurs outside of the nucleus and has a charge of 1 and mass of x g. flame test: an experimental technique or process in identifying a metal from its characteristic flame color. nucleus: the very dense core of the atom that contains the neutrons and protons. Checking Up 1. Explain what is meant by a flame test. 2. Explain how energy is conserved during the flame test. 75

4 Movie Special Effects What Do You Think Now? At the beginning of the section you were asked the following: How could you test for a poisonous substance (other than by eating it)? Suppose that you were going to try to test food samples using a flame test to see if they had been poisoned. How would you know for sure that the food had been contaminated? How do you know what color the pure food would have when it burned? What does it mean? Chemistry explains a macroscopic phenomenon (what you observe) with a description of what happens at the nanoscopic level (atoms and molecules) using symbolic structures as a way to communicate. Complete the chart below in your log. MACRO NANO SYMBOLIC Describe what you saw during the flame test. How do you know? Look back at your data table. Are there any elements that you cannot distinguish between? Why do you believe? Energy is conserved in a chemical process. This means that when you put energy in (from the flame) you will get energy out (in the form of light). Where is the energy input to produce light in your home? Why should you care? Chem Essential Questions In words, describe what is happening to the electrons as they are heated in the Bunsen burner flame. Draw a set of slides showing an electron starting at ground state, becoming excited, and falling to emit light. You will be writing a movie scene for your challenge in this chapter. Add a part to your scene that could involve a colored flame. What is the significance in the movie of using a colored flame? (For example, a red flame could mean that the characters are in danger.) 76

5 Section 8 Flame Tests: Identifying Metals Reflecting on the Section and the Challenge Most of the common fuels are primarily hydrocarbons, which do not contain metal ions. Therefore, you are most familiar with flames that are yellow-orange or occasionally a light blue. The bright colors of the excited metals are a surprise and tend to look out-of-this-world. Colors could play an interesting part in a plot line of a movie script. Perhaps an unusually colored flame could be taken as a mystical sign. A sudden change in the color of campfires could be a sign that danger is near. Consider how you might use flame tests to create unusually colored flames for your movie special effect. 1. Compare the colors you observed in your flame tests to the colors you have seen in fireworks displays. Identify what metals are used in producing the different colors in fireworks. 2. Develop a series of sketches showing how an electron of an atom can give off light. Your first sketch should have the electron in the ground state. The next sketch should show the electron excited when you apply energy to the atom and the third sketch should show how light is emitted when the electron falls to a lower energy state. 3. Preparing for the Chapter Challenge Chem to Go Name three new colors of light that could be produced by combining some of the metal salts that you tested in the lab, and identify which metal salts you would combine to produce these colors. If time and your teacher permit it, test your predictions. Inquiring Further Fireworks Fireworks have been around for centuries. They were used long before anyone knew why they produced the stunning effects they do. Investigate the manufacturing of fireworks and find the specific compounds used to produce the many colors presented during fireworks displays. Are all colors equally represented in fireworks displays, or are some colors easier to obtain than others? Research what colors are available and which, if any, do not seem possible given the range of substances readily available. 77