FLAME TESTS INTRODUCTION According to the Bohr theory of the atom, electrons may occupy only specific energy levels. When an atom absorbs sufficient energy, an electron can “jump” to a higher energy level. Higher energy levels tend to be less stable, however, and if a lower energy level is available, the electron will “fall” back, giving off energy in the process. The difference in energies between the two levels is emitted in the form of a photon of electromagnetic radiation. The energy of each photon is described by the equation E = hv, where h is Planck’s constant and v is the frequency of the radiation. If the wavelength of the released photon is between 400 and 700 nm, the energy is emitted as visible light. The color of the light depends on the specific energy change that is taking place. White light is a continuous spectrum in which all wavelengths of visible light are present. An excited atom, however, produces one or more specific lines in its spectrum, corresponding to the specific changes in energy levels of its electrons. Because each element has a distinct electron configuration, each has a unique line spectrum. Flame tests are a quick method of producing the characteristic colors of metallic ions. the loosely-held electrons of a metal are easily excited in the flame of a lab burner. The emission of energy in the visible portion of the spectrum as those electrons return to lower energy levels produces a colored flame. the color is a combination of the wavelengths of each transition, and may be used to determine the identity of the ion. In this investigation you will perform flame tests on seven metallic ions, then use your results to determine the identity of several unknowns. PRE-LAB DISCUSSION Read the entire laboratory investigation and the relevant pages of your textbook. then answer the questions that follow. 1. Write out the electron configurations for each of the metallic ions to be tested in this investigation. Ba2+ ___________________________________________________ Cu2+ ___________________________________________________ Li+ ___________________________________________________ K+ ___________________________________________________ Sr2+ ___________________________________________________ Ca2+ ___________________________________________________ Na+ ___________________________________________________ 2. What does a flame test indicate about the energy changes taking place among the electrons in a metallic ion? ___________________________________________ __________________________________________________________________ __________________________________________________________________ 3. Explain why a metallic ion produces a characteristic color in a flame test, regardless of the compound used as the source of the ion. ___________________ __________________________________________________________________ __________________________________________________________________ 4. What wavelengths correspond to the visible spectrum? Which color has the shortest wavelength? The longest? _____________________________________ __________________________________________________________________ __________________________________________________________________ 5. What precautions should be taken when using 6.0 M HCl? __________________ __________________________________________________________________ __________________________________________________________________ 6. Why is it important to use a clean nichrome wire for each test? _______________ __________________________________________________________________ __________________________________________________________________ PROBLEM What colors are characteristic of particular metallic ions in a flame test? MATERIALS chemical splash goggles laboratory apron latex gloves well plate marker pen solutions of the following salts: barium nitrate (Ba(NO3)2) copper nitrate (Cu(NO3)2) strontium nitrate (Sr(NO3)2) lithium nitrate (LiNO3) potassium nitrate (KNO3) sodium chloride (NaCl) calcium nitrate (Ca(NO3)2) nichrome wire loop beaker, 50-mL hydrochloric acid (HCl), 6.0 M lab burner wash bottle with distilled water 3 unknown solutions SAFETY Wear your goggles and lab apron at all times during the investigation. The 6.0 m HCl is corrosive and irritating to skin, nasal passages, eyes, and clothing. the salt solutions, with the exception of NaCl, are toxic. Handle all of the solutions with care, and do not inhale their vapors. If a solution should splash on your skin, wash the affected area with large amounts of water and notify your teacher. When you work near the burner flame, tie back or cover loose hair, and secure loose or bulky clothing. PROCEDURE 1. Put on your goggles and lab apron. Obtain a well plate and use a marker to label seven wells with the names of the known solutions. Put a dropper full of each known solution into its labeled well. 2. Put on your latex gloves. Obtain a beaker with about 10 mL of 6.0 M HCl and a nichrome wire loop. Light the burner, and adjust the flame to low. CAUTION: Hydrochloric acid is corrosive to skin and clothing and its vapors are irritating to lungs and eyes. “Avoid contact with the and inhalation if its vapors. Loose hair should be tied back and bulky or loose clothing should be secured in some solution or covered, manner. 3. For each test, be sure that the nichrome wire is clean, so as not to contaminate the solutions. To clean the wire, first rinse it with distilled water, using the wash bottle. Then dip it in the 6.0 M HCl solution. Place it in the burner flame for a few moments, as shown in Figure 10-1. Determine the color of the clean nichrome wire in the flame . This color you should see after every trial. 4. Sodium has a very strong color, which may affect your other tests, so test t he sodium solution last, Dip the clean nichrome wire in one solution. Place the wire in the burner flame and observe. Record your observations on Data Table 1. Clean the wire and repeat this step with the next known solution until you have tested all seven solutions. CAUTION: Do not let these substances come in contact with your skin. If a solution should splash on your skin, wash the affected area with large amounts of water and notify your teacher. is the Figure 10-1 5. Obtain three unknowns from your teacher. Repeat Step 4 for each unknown. Record your observations on Data Table 2. 6. Turn off the burner. Clean up your work area and wash your hands before leaving the laboratory. OBSERVATIONS DATA TABLE 1 Flame Tests of Known Solutions Salt Solution Color Ba(NO3)2 Cu(NO3)2 LiNO3 KNO3 NaCl Ca(NO3)2 Sr(NO3)2 DATA TABLE 2 Unknown Tests of Unknown Solutions Color CRITICAL THINKING: ANALYSIS AND CONCLUSIONS 1. What metallic ions are present in the unknown solutions? (Drawing conclusions) __________________________________________________________________ __________________________________________________________________ 2. Summarize the process that produces the colors seen in the flame tests. (Applying concepts) _________________________________________________________ __________________________________________________________________ __________________________________________________________________ __________________________________________________________________ __________________________________________________________________ 3. What is the relationship of the colors you saw and the lines of the electromagnetic spectrum produced by the metals? (Making inferences) _____________________ __________________________________________________________________ __________________________________________________________________ CRITICAL THINKING: APPLICATIONS 1. When a glass rod is heated, a yellow flame is observed around the point of heating. what does this yellow flame indicate? why is it observed when glass is heated? (Developing hypothesis) _______________________________________ __________________________________________________________________ __________________________________________________________________ __________________________________________________________________ 2. The line spectrum of lithium has a red line at 670.8 nm. Calculate the energy of a photon with this wavelength. (Making predictions) 3. What other equipment could you use in this investigation if burners were not available? (Designing experiments) ____________________________________ __________________________________________________________________ __________________________________________________________________ 4. How do you think metallic salts are used in fireworks? (Applying concepts) _____ ________________________________________________________________________ ________________________________________________________________________ GOING FURTHER 1. 2. 3. Get some copper metal filings from your teacher. Work under your teacher’s supervision. Pick up a small piece of copper filing with the wire loop, and put it in the burner flame. Is the color of copper in the flame the same as the color of copper nitrate solution? Neon is a gas that has a reddish-orange emission spectrum. Find out why neon lights can display a multitude of colors. The invention of the spectroscope by Robert Bunsen and Gustav Kirchoff opened up a new era of chemical investigation and research. Bunsen and Kirchoff discovered two elements through the use of their invention. Find out which elements they discovered, and how the spectroscope is now used.