.Lab25. FlameTests A A. Pre-Lab Discussion. Purpose. Equipment. Materials. Safety,I II-n

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1 '~.,,;. Name.:," _'._..--".-_-+-_-,~_,---_---:-~_._.'Date Class"_-'--.,"'-' FlameTests.Lab25 Text reference: Chapter 13, PP: ~ Pre-Lab Discussion. The normal electron conflgurationof atoms or ions of an element is known as the "ground state." In this most stable energy state, all electrons are in the lowest energy levels available. When atoms or ions in the ground state -. are heated to high temperatures, some electrons may absorb enough energy to allow them to "jump" to higher energy leveis. The element is then said to be in the "excited state." This excited conflguration is unstable, and the electrons "fall" back to their normal positions of lower energy. As the electrons return to their normal levels, the energy that was absorbed is emitted in the form of electromagnetic energy. Some of this energy may be in.the form of visible light. The color of this light can be used as a means of identifying the elements involved. Such crude analyses are known as flame tests. Only metals, with their loosely held electrons, are excited Inthe flame of a laboratory burner. Thus, flame tests are useful in the identification of metallic ions. Many metallic ions exhibit characteristic colors.when vaporized in the burner flame. In this experiment, characteristic colors of several different metallic ions will be observed, and an unidentified ion will be identified by means of its flame test. Purpose Observe the characteristic colors produced by certain metallic ions when vaporized in a flame. Iden~ an unknown metallic ion by means of its flame test. Equipment graduated cylinder, 10-mL laboratory burner test tubes, 13x 100-mm (8) test tube rack wire loop, platinum (or nichrome) glass-marking pencil safety goggles lab apron or coat Materials hydrochloric acid, Hcr (conc) Unidentified solutions 0.5 M solutions.of nitrates of: ~..I Na+ K+ (Li+) Ca2+ Sr2+tBa2-i', Cu2+ I' '\ /' II' "'"/~'. v II' ~ vi Safety,I II-n A A Handle concentrated HCI with great care. Report spills at once to your teacher and flush them with cool water and NaH'C0 3 solution. Note the caution alert symbols here "and with certain steps in the "Procedure." Refer to page xi for the specific precautions associated with each symbol. Always wear safety goggles and a lab coat or apron when working in the lab. 127

2 Procedure.. 1. Measure 5 ml of tap water in a graduated cylinder and pour the water into a 13x 100test tube. Using a marking pencil, mark the outside of the tube to indicate the level of the water. Discard the water. Using the 'marked tube as a guide, mark seven clean test tubes at approximately the same level. Place the clean tubes in a test tube rack. Set the other test tube aside.. 2. Into each of the clean test tubes, pour 5 ml of a different nitrate solution. Mark each test tube to indicate the metallic ion it contains. 3. Pour about 10 ml of concentrated hydrochloric acid into a 50-mL beaker. CAUTION: Use extreme care in 'handling this acid. To clean the wire loop, dip the loop in the acid and then heat the loop in the outer edge of the burner flame. Continue to clean the loop in this manner until no color is observed in the flame. 4. Dip the clean wire loop into one of the nitrate solutions. Place the loop in the outer edge of the burner flame and move the loop up and down (Figure 25-1).Note the color in the flame. Record your observations in the data list provided. ~ platinum or nichrome wire. Figure Clean the wire loop as described in step 3. Repeat step 4 using a different nitrate solution.. 6. Test each nitrate solution in the' same mariner, cleaning the loop thoroughly between tests. Record all your' observations in the data list. 7. Obtain a sample of an unknown solution. Perform a flame test and identify the metallic ion present by the color of the flame. ' ",':'"

3 I...:., Name, ~~~ ~~'~:-~~ ~~ "'~_' 2:..:." 5 'Flame Tests '(continued),. -:'""..' Observations, and Data Metallic Ion Color in Flame Li-l'.! dqzs'!t Unknown Conclusions and Questions 1. What inaccuracies may be involved in using flame tests for identification purposes? 2. Which pairs of ions produce similar colors in the flam~ tests? 3. Explain how the colors observed in the flame tests are produced. t ' :",..,.';.. ' ~.: " 129

4 4. Define these terms: a. quanta b. ground state c. excited state 5. What is a spectroscope? What is observed if the flame tests are viewed through a spectroscope? ;.,....;:. ~- ",, I,-. 0; 130

5 Flame Test & Spectral Analysis Lab 4 ~ Nmne:~. _ Date: DATA.SlIEET , --- I Spectrnm of Sodium Chloride Spectrum of Potassium Chloride I --..jl 1 1 Spectrum of Strontium Chloride Spectrum of Calcium Chloride 1----_1 I I Spectrum of Lith; Ut)J 8~m.;cJe Spectrum of Unknown Salt

6 - QUESTIONS' 1. From your observations, how do the bright-lliie spectra fth I., a e samp es tested differ? 2. What color appears at the far left end of the' visib!? the far right end? e spectrum. What colordo you see at ~'-- 3. Describe the visible flame CO: orvcu obsesved?~er inserting each of the four knownsalts into the burner fiame. How did m:s;: :"""'rr<::e... the bright-line spe;..i.t:1 you recorded? 4. How does the spectrum of the unkown salt compare to the spectra of the four known samples? \Vhat element does it most likely contain? 5. Why was -it necessary to use a separate nichrome wire loop for each sample? How might using asingie wire loop for all tests change your results? 6. What do stars and your glowing samples have in common? 7. Stars, like our Sun, emit large amounts of radiant energy. How might astronomers determine the temperature and composition of stars using a spectroscope?