Download Flame Test Lab

Name: ____________________________________________ Hr: _________ Date: ___________
Flame Test Lab
Introduction: According to the Bohr theory of an 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 form of electromagnetic radiation (light) in the process.
The difference in energies between the two levels is emitted in the form of a photon (aka “quantum”) of electromagnetic radiation. The
energy of each photon is described by the equation E = hv, where h is Planck’s constant (6.63 x 10 -34 Js) and v is the frequency of the
radiation. If the wavelength of the released photon is between 400 nm 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 element 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.
Pre lab Questions:
1. Write the shorthand electron configurations for the following metallic ions:
a. Ba +2
__________________________
e. Na +1
__________________________
b. Cu + 2 _________________________
f.
Ca +2
__________________________
c. Li +1
_________________________
g. Sr + 2
__________________________
d. K +1
__________________________
2. What is the energy source used in this lab to excite the electrons? Classify this energy source using the terms potential,
kinetic, and/or radiant.
3. Summarize the steps required for light to be produced.
4. Which wavelengths correspond to the visible spectrum? Give your answer in METERS (not nanometers).
5. When a human looks at white light, the human just sees white, despite white light being composed of all forms of visible light.
Why?
6. A student element X and sees a blue color in the flame. Does this mean this metal only emits energy with the frequency of
blue light?
Name: ____________________________________________ Hr: _________ Date: ___________
Safety: Goggles must be worn and hair must be secured. All floppy, moppy, braided, etc…must be secured. No loose clothing. The
compounds are toxic if ingested. Wash hands after performing the lab.
Materials: forceps, lab burner, sparker, toothpicks soaked in knowns and unknowns, paper towel
Procedure:
1. Using pen, label a piece of paper towel with each of the cations used in this lab. Space these labels approximately 2 inches
apart.
2. Once complete, one student from the lab team may approach the instructor and request samples of the Knowns. You may not
receive all Knowns at one time.
3. Turn on your lab burner (remember to close the valve at the bottom when lighting then re-adjust the valve to create an inner
blue cone). Place burner in the center of the lab table.
4. Using the forceps, grip one end of a known toothpick. Position yourself so you are NOT looking at the window (light makes it
difficult to see the colors). Insert the other end of the toothpick in the flame and record WITH DETAIL in the data table the
color produced. Remember, you will be using the Cobalt Glass when viewing the KNO3 (potassium). You are welcome to look
at each known with your eyes AND through the glass if you prefer.
5. You will be allowed two of each known for initial testing.
6. Repeat step 4 for each of the knowns, remembering to use the Cobalt Glass for the KNO3.
7. Once you have gathered observations on each of the knowns, you are ready for the Unknowns. Label a piece of paper towel
with each of the Unknowns. Approach the instructor to request samples of the Unknowns. You may not receive all of the
Unknowns at one time. Three of the Unknowns are simply one of the tested Knowns. One of the Unknowns is a blend of two
of the tested Knowns. You will receive at most 3 samples of each unknown.
Suggestion: once you narrow down the identity of an Unknown, you may want to burn the Unknown and the predicted Known
simultaneously in two different burners in order to compare color.
8. Disposal: burned toothpicks are to be disposed of in the designated containers on your lab table. Nothing is to go in the sink,
and no toothpicks go into the garbage. Paper towels may go into the garbage Points will be deducted from groups who do
not follow directions.
Results:
Data Table 1: KNOWNS
Salt Solution
Cation (with
Flame Color
Flame Color through Cobalt Glass (only
charge)
required for K +1)
Ba(NO3)2
Cu(NO3)2
LiNO3
KNO3
NaCl
Ca(NO3)2
Sr(NO3)2
Data Table 2: UNKNOWNS
Unknown
Flame Color
Solution
Unknown 1
Unknown 2
Unknown 3
Unknown 4
Flame Color through Cobalt Glass
Predicted Identity of
Unknowns
Name: ____________________________________________ Hr: _________ Date: ___________
Post Lab Questions:
1. Based on your lab data, what is the predicted identity of each unknown? Were there any unknowns in which you thought it
could be something else? If so, clarify.
2. Summarize the steps required for electrons to produce light specific to this lab. Do not summarize the procedure—focus only
on the electrons.
3. What is the relationship between the colors you saw produced by each cation and the lines on the atomic emission spectrum
for each cation?
4. When a glass stirring rod is heated, a yellow flame is observed around the point of heating. Which cation can one assume is
in a glass stirring rod?
5. The atomic line spectrum of lithium has a red line at 670.8 nm. Calculate the energy of a photon with this wavelength. (You
will need to incorporate two equations AND note the unit of the given wavelength). Show all work. (FYI: you have to do this on
the quiz/test—ask if you don’t understand).
Name: ____________________________________________ Hr: _________ Date: ___________
6. You will be show the Atomic Emission Spectrum (aka “spectral lines”) for white light, sodium, lithium, and potassium.
a. Using color pencils, record the visible colors present for each near the appropriate wavelengths:
i. White light
ii. Li
iii. Na
iv. K
b. Which of the spectrums is considered “continuous”? Explain.
c. Why are more lines produced for an atom of Potassium than an atom of Lithium?
d. Compare what you saw in the flame test to the emission spectrum of these elements.