Flame Tests
Process Objectives
· To experiment with flame tests on different salts.
· To predict the identity of an unknown metal ion from a flame test.
Learning Objectives
· To list the flame color of three Group 1, three Group 2, and one Transition Group metals.
Introduction
A number of common metal ions (Li+, Na+, K+, Ca2+, Ba2+, Sr+, and Cu2+) give a distinct color to a flame. Therefore, a flame test is often used as a confirmatory test in identifying an unknown metal.
Compounds of these ions provide the beautiful colors in a fireworks display. When glass is melted in a Bunsen burner flame, sodium ions color the flame. A copper wire inserted into the flame often results in a striking flame color. While the light emitted from a few excited metal ions is beautiful, in the laboratory a simple flame test is often very helpful in identifying an unknown metal ion.
In this experiment you will observe and record the flame colors of several metals of Group 1 and 2. Review the section in the text dealing with alkali and alkaline metals.
Safety
Take the necessary precautions before beginning this experiment. Wear safety goggles, apron and gloves. Read all safety cautions in your procedures and discuss them with your teacher. It is important to use good safety techniques while conducting experiments. See pages 8 through 11.
Apparatus
Bunsen Burner
16 well plate
forceps
Nicrome Wire
Materials
hydrochloric acid, 6 - 0.1M known solutions
Procedure
1. Obtain a 5-cm length of No. 24 nicrome wire, sealed at the opening of a glass tube 10 cm long
o Use 0.5-M solutions in distilled water of the Lab grade sodium chloride and 0.5M solutions of the nitrates of barium, calcium, lithium, potassium, strontium, sodium, copper, and magnesium
o To obtain good results in this experiment, your test tubes or well plate must be exceptionally clean to avoid contamination. Make sure there is no cross contamination of solution by exchanging nichrome wires.
CAUTION: Before you use the burner in the next four procedures, check
to see that long hair and loose clothing have been confined.
2. Clean the nicrome wire by dipping it first into some 6 M hydrochloric acid in a test tube and then holding it in the colorless flame of your burner. Repeat until the wire imparts no color to the flame.
3. Place 5 drops of sodium nitrate solution into a clean well, dip the tip of the clean nicrome wire into the solution, and then hold it in the flame. .
4. Observe the color of the flame just above the wire. Heat only the tip of the wire. If you heat the glass tube into which the wire is sealed, you will break the glass. Write your observation in the table below
5. Clean the wire as before and then test a solution of sodium chloride in the same manner. Repeat the test, dipping the wire into a little dry sodium chloride. Describe what you observed in the appropriate space in the table below.
o Observation of Dry Sodium Chloride __________________________
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6. Repeat Step 2, using solutions of the nitrates of lithium, strontium, calcium, barium, and potassium, magnesium, and copper. Clean the wire thoroughly after each test. In the cases of lithium and strontium, observe which flame is more persistent and takes longer to burn off the wire. Also note the difference in the shades of color produced. When you have tested the calcium flame and then dipped the wire into hydrochloric acid and back into the flame when cleaning it, you often get an excellent flame of sodium momentarily. Record the color of the flame for each metal compound in the Data Table.
Strategy for Predicting Be sure that the wire is clean. If you are not sure of the identity after testing the unknown, retest the known solution of the metal you predict.
· If two metals are present in the same solution, the color of one flame may obscure that of the other. Record the colors of the flames in the Data Table.
1. Flame-test a mixture of the solutions of the nitrates of sodium and potassium with a clean wire. Observe the color the mixture imparts to the flame when viewed without the cobalt glasses. Repeat the test, but observe the flame as seen through the cobalt glasses. Record the colors of the flames in the Data Table.
2. Secure an unknown solution from your instructor. Test it in the flame as you did with the known solutions. Place your answer in the Data Table.
Data Table
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Metallic ion in Compound
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Color of flame
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Sodium |
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Lithium |
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Strontium |
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Calcium |
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Barium |
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Potassium |
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Copper |
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Magnesium |
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Sodium and Potassium mixture |
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Unknown Metal 1 |
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Unknown Metal 2 |
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Questions:
1. Is flame coloration a test for the metallic ion or for the nitrate ion
2. Why do dry sodium chloride and the solutions of sodium nitrate or sodium chloride all impart the same color to the flame? (Click here for Help)
3. Describe the test for sodium ions and potassium ions when both are present.
4. How would you characterize the flame test with respect to its sensitivity?(Click here for help)
5. What difficulties may be encountered in the use of the flame test for identification? (Click here for help)
General Conclusions
1. A student recorded the following results when testing three unknowns. Identify the unknowns by referring to your
Data Table.
Unknown #1 yellowish green________________________
Unknown #2 scarlet _______________________________
2. Several of the flame tests were shades of red. What should be done in the laboratory to correctly identify one of these ions?
3. During a flood, the labels from three bottles of chemicals floated away. The unlabeled bottles of white solids were known to contain the following: strontium nitrate, ammonium carbonate, and potassium sulfate. Explain how you could easily re-label these three bottles.
Correlating your facts
Describe the activity of electrons when a substance is vaporized in a flame. What is viewed through a spectroscope and how does this instrument serve in identifying substances? (You may need to research how a spectroscope works) click here for help