Forensics Lab Flame Tests

Transcription

1 Forensics Lab Flame Tests Name Per Due Date Introduction The fundamental particles that make up the building blocks of matter are known as atoms, each of which is shown on the periodic table of the elements. The most abundant elements in the universe are hydrogen and helium. These two light elements make up about 80% and 18 % of all the matter in the universe respectively. Despite comprising only a very small fraction the universe, the remaining heavy elements are very important. When solutions of these heavy element metals are heated in a Bunsen burner flame, they give off characteristic colors. These characteristic colors are from the elemental composition of the metal being heated. For example, sodium makes the flame turn bright orange this is the same orange color made by sodium street lamps and many fireworks. You can be an analytical chemist or forensic scientist in this activity, by finding out what color flames different metals make, and working out the identity of some unknown metal solutions. Qualitative analysis is a chemical analysis designed to identify the elements found in a substance or mixture. By placing atoms of a metal into a flame, the electrons of those atoms will absorb energy and jump to a higher energy level. This jump to a higher energy level is called a quantum jump. These electrons then return to their lower energy level and in the process give off energy in the form of light. The amount of energy the electron gives off as it returns to a lower energy level determines the color of the light given off. Red is the lowest energy visible light, increasing energy through the rainbow of orange yellow green blue indigo, and finally violet for the highest energy visible light. The arrangement of electrons in an atom determines the sizes of the quantum jumps, and thus the energy and colors emitted when the electrons return to their original state. The collection of colors a particular atom gives off is called is known as its emission spectrum. In this way the emission spectrum serves as a fingerprint of the element to which the atoms belong. We can view the emission spectrum of colors all at once with the naked eye. It will appear to be one color, which we will carefully describe. (Note that you have already viewed the separate colors of the emission spectrum by using a spectroscope in the last lab.) L_06x02_Flame Tests Page 1

2 The Flame Test The flame test is used to visually determine the identity of an unknown metal based on the characteristic color the metal turns a flame. Some typical color descriptions are: yellow-green scarlet crimson red-orange orange-red orange-yellow reddish purple violet purple emerald green pale green yellow blue/azure The value of the flame test is limited by interference from other brighter colors and by ambiguities where certain different metals cause the same flame color. Sodium, in particular, is present in most compounds and will color the flame yellow. Cobalt blue glass filters are often used when viewing mixtures of metals to screen out light that is yellow in color. The human eye sees yellow very well, since it is in the middle of the spectrum visible to the eye. Colors at the edges of the visible spectrum, especially violet, are more difficult to see. Cobalt glass absorbs light in the yellow wavelengths, but is transparent to light of higher energy (this is why it looks blue!). Safety Wear your goggles, lab apron and acid gloves at all times. The 6.0 M HCl is corrosive and irritating to the skin, nasal passages and eyes and can damage clothing. The salt solutions, with the exception of NaCl are toxic. Handle all of the solutions with care. Clean up your lab area when finished and wash your hands thoroughly when done. Be aware of all safety precautions when using a flame. L_06x02_Flame Tests Page 2

3 Materials Procedure Safety glasses Solutions of the following salts: 3 Spot plates barium nitrate 6.0 M HCl copper (II) nitrate Nichrome wire loop strontium nitrate Acid gloves lithium nitrate Bunsen burner potassium nitrate dh 2 O calcium nitrate 7 unknown solutions sodium chloride Lab apron 1. Set up the Knowns. a. Put on your goggles and lab apron. b. Make sure your spot plates are clean and dry. c. Using a china marker, label seven of the wells on one spot plate with each of the known metal ions. d. Place several drops of each known metal salt solution into its corresponding labeled well. 2. Cleaning your wire loop. a. Place a few drops of 6.0 M HCl into each of four wells in the second spot plate. b. Dip the flame test wire into the first well of HCl, then hold it in the flame for 10 seconds. c. Place the wire into the second well of HCl then into the flame for 10 seconds. d. Repeat the process in the 3rd and 4th well of HCl. This procedure must be repeated before each new test. 3. Conduct the flame tests a. Dipping the clean loop into the metal salt solution. b. Insert the loop into a flame at the tip of the inner cone (the hottest part) and observe the color. c. Record your observation of the color produced in Table Set up the Unknowns. a. Using a china marker, label seven of the wells on the third spot plate number 1 through 7. b. Place several drops of each unknown solution into its labeled well. c. Test each of these unknowns as described in step 3, above. d. Record your results in Table Using the results of your flame tests, complete Table 2 by listing the identities of each of the unknown metal salts. L_06x02_Flame Tests Page 3

4 Questions 1. Which two elements are the most common in the universe? What are their percentages? 2. Explain how interference affects the results of a flame test. 3. Why is cobalt glass sometimes used when conducting a flame test? 4. Explain how ambiguities affects the results of a flame test. 5. Why do you think the chemicals have to be heated in the flame first before the colored light is emitted? 6. Colorful light emissions are applicable to everyday life. Describe where else have you observed colorful light emissions. 7. What is the characteristic flame color for Sodium, Lithium, Barium, Copper, and Calcium? 8. What element is commonly used in streetlamps? 9. What is qualitative analysis? 10. What are the 7 spectral colors listed in order from lowest to highest energy? 11. Which color is the lowest energy? Which color is the highest energy? 12. What is an emission spectrum? 13. What color is bent the most by a spectroscope? Which color is bent the least? 14. What color(s) do humans see best? What color(s) do humans see poorly? Explain why this is so. 15. What precautions should be taken when using hydrochloric acid? 16. Why is it important to use a clean wire loop for each test? 17. Why can't flame test be used to positively identify an element with 100% accuracy while a spectrum can? 18. List the pairs of ions that had similar colors in a flame test. Include the color with each pairing. L_06x02_Flame Tests Page 4

5 Forensics Lab Flame Tests Name Per Due Date Table 1 Known Metal Salt In A Solution Of Observed Color Barium BaNO 3 Copper CuSO 4 Strontium SrNO 3 Lithium LiNO 3 Potassium KNO 3 Calcium Ca(NO 3 ) 2 Sodium NaCl Table 2 Unknown Salt #1 #2 #3 #4 #5 #6 #7 Observed Color Unknown Identity L_06x02_Flame Tests Page 5

6 Your Answers 1a 1b a 7b 7c 7d 7e a 11b 12 13a 13b 14a 14b 14c L_06x02_Flame Tests Page 6