Experiment 5.
Flame Test and Chemical Fingerprinting
Learning Objectives:
1. To study the characteristic colors produced by certain metallic ions when vaporized in a flame.
2. To identify unknown metallic ions by means of their characteristic flame color.
3. To study the similar chemical properties of the elements that belong to the same group on the periodic table.
Background:
In 1913, Bohr proposed the quantized shell atomic model. His model was a huge success for the hydrogen atom. He concluded that the electron of the hydrogen atom can only be allowed on certain energy levels called orbits. Bohr observed four distinct bands in the hydrogen atom’s emission spectrum. These four bands were related to photons of different colors corresponding to quantized energy transitions.
The normal electron configuration of 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” absorb energy (heat or other forms of energy), it causes some electrons “jump” to higher energy levels. The atom is then said to be in the “excited state.” This excited configuration is unstable, and the electrons “spiral down” to lower energy levels or to its ground state. During these electronic transitions, electrons release their energies in the form of electromagnetic radiation. Some of this released energy may be in the form of visible light. The color of this light can be used as a means of identifying the element. This analysis is known as a flame test. In the case of the hydrogen atom’s emission spectrum (higher energy level to the lower energy level transitions), the red color is due to the photons released when excited electrons fall from the 3rd energy level to the 2nd energy level. All the electronic transitions that fall to the second energy level (Balmer Series) are visible to humans. Other transitions do exist but are not visible to the human eye.
To perform a flame test of a particular metallic element, its chloride is dissolved in water to prepare an aqueous solution. The solution is then vaporized in the blue flame of a Bunsen burner. This test works well for common main group metal ions, and was perfected by Robert Bunsen (1811 – 1899).
Many metallic ions exhibit characteristic colors, e.g. sodium is yellow-orange, and lithium is crimson.
There are three parts in this experiment:
Part A. Emission spectra of the elements using spectral tubes. Part B. Flame tests.
Part C. Chemical Fingerprinting.
Materials:
Set of metal chloride 0.50 M solutions (LiCl, NaCl, KCl, CaCl2, SrCl2, and BaCl2). Bunsen Burner, wooden splints, striker, cobalt glass plates, well-plates, 0.10 M of (NH4)2CO3, 0.10 M of (NH4)2SO4, and 0.10 M (NH4)2HPO4 aqueous solutions.
Note: The chloride salts are preferred since they are easily volatilized in the Bunsen burner flame.
Beaker (250 to 500 mL) half-filled with tap water.
Safety:
1. Wear goggles and an apron at all times
2. No eating or drinking is allowed in the chemistry laboratory.
3. Notify your instructor immediately if you touch or accidently ingest any of these solutions.
4. If you experience problems while lighting the Bunsen burner turn then gas line off and ask yourinstructor for help.
5. Make sure you wash your hands before you leave the laboratory.
PRELAB QUESTIONS NAME:
1. Identify the initial state and final state of the electron when it absorbs energy.
2. Identify the initial state and final state of the electron when it releases energy.
3. Identify the process by which electrons that surround an atom produce light (or electromagnetic radiation or energy).
4. Identify the reason chloride salts are used in the Bunsen burner flame.
5. Identify the chloride salt which is shown to have a crimson color when placed in the flame of a Bunsen burner.
Part A. Spectrum Analysis
(Courtesy of rit.edu; creative commons)
Procedure:
1. Obtain the spectral tubes of Hydrogen, Helium, Neon, Oxygen and any others available to you and place them in the power supply. Then turn on the power and observe the emission spectra of the samples with a spectroscope and compare the spectra to the chart provided above.
2. Record your observations and compare to the chart above.
Part B. Flame Test
NOTE: Place couple of splints in the unknown solution before all other tests have been completed.
PROCEDURE:
1. Using a striker, light the Bunsen burner and adjust it so that it has a hot blue flame. Partially fill a 250-mL of beaker with tap water.
2. Take a wooden splint, dip it into the LiCl (lithium chloride) solution, and then hold the wooden splint in the hottest part of the burner flame. Observe the color of the flame. Try not to let the wooden splint catch on fire. Extinguish the splint in the beaker that contains water.
3. Carefully record your observations in the data table. Be very accurate. Your description of the color must be accurate enough to distinguish this metal ion from the other ions tested.
4. Repeat with the other solutions. Make sure you follow the order of the metals given in the datatable in Part B, e.g. lithium first and barium last as the sequence follows their location in the periodic table. Check the color of their flame tests. Record your observations for each solution. When you examine the sodium and potassium ions, first look at their color alone, then test them again, looking through a cobalt glass plate.
5. When you have tested all the known solutions and can distinguish the color of each metal ion,obtain an unknown solution. Determine which metal ion is present by performing a flame test and comparing this new data to your previous data.
6. Extinguish the splints in a beaker partially filled with water.
NOTE: Place couple of splints in the unknown solution before all other tests have been completed.
Figure 1. The flame colors with different metals present in the solutions.
DATE:
FLAME TEST.
NAME: DATA TABLE FOR
| Metal ion | Color of Flame |
| Lithium | |
| Sodium |
| Potassium (without cobalt glass) | |
| Potassium (with cobalt glass) | |
| Calcium | |
| Strontium | |
| Barium | |
| Unknown # |
Based on your observations, identify the unknowns you examined:
Unknown # is
Part C. Chemical Fingerprinting
PROCEDURE:
1. Arrange two well plates according to the figure given below.
2. Using a Beral pipet, drop 5 drops of LiCl in the wells numbered 1, 2, and 3.
3. Place 5 drops of NaCl in the wells numbered 4, 5, and 6.
4. Repeat same procedure for the other solutions.
5. Finally, place 5 drops of the unknown solution in wells 19, 20, and 21.
6. Next, add 5 drops of ammonium carbonate to wells 1, 4, 7, 10, 13, 16, and 19. Then, add 5 drops of ammonium hydrogen phosphate to wells 2, 5, 8, 11, 14, 17, and 20. Finally, add 5 drops ammonium sulfate to wells 3, 6, 9, 12, 15, 18, and 21.
7. Record your observations in the grid given on the next page.
NOTE: It may take longer for the CaSO4 to precipitate (step #12).
DATE: NAME:
DATA TABLE FOR CHEMICAL FINGERPRINTING. *
Note that the unknown is the same sample you used for flame test.
|
Solution tested |
Ammonium carbonate | Ammonium phosphate | Ammonium sulfate |
|
LiCl |
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NaCl |
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KCl |
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CaCl2 |
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SrCl2 |
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BaCl2 |
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|
Unknown |
*Key: If a precipitate is observed you will record it as PPT and for no reaction, record as NR.
Based on the results of your experiments, what metal was found in your unknown? Explain. Unknown # is .
POSTLAB QUESTIONS NAME:
1. Although flame tests are usually specific for each element, they can be misleading. Sodium may bepresent in each sample therefore a cobalt glass is needed to observe the real color emitted by potassium. Why is blue cobalt glass used instead of any other colored glass?
2. An unknown solution tested in this experiment gave a yellow green flame test. Do you expect to observe a precipitate with the reagents used in Part C of this experiment?
3. Write the electronic configuration of each metal investigated in this module. Does the electronconfiguration of each metal predict the reaction outcome observed in chemical fingerprinting?
4. Research some information about the origin of fireworks. Explain how they are made, whatchemicals are used, and what colors they burn.
5. Why do different atoms/ions emit different colors of light?
6. Colorful light emissions are applicable to everyday life. Where else have you observed colorful light emissions?
7. Determine the wavelengths for Potassium (violet, λ = 400 nm) and Strontium (red, λ = 700 nm) lightemissions. Calculate the frequencies (c = wavelength × frequency) and the energy (E = h × frequency) for each. Fill the table below with your results. SHOW YOUR WORK! c = speed of the light = 3×108 m/s and h = Planck’s constant = 6.62×10-34 J×s.
| Wavelength (nm) | Frequency (Hz) | Energy (J) | |
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Potassium |
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Strontium |
