ELECTRONIC STRUCTURE: Flame Test Lab & UV Protection
INTRODUCTION:
PART I:
According to the Bohr theory of the 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 process.
The difference in energies between the two levels is emitted in the form of a photon of electromagnetic radiation.
The energy of each photon is described by the equation E = hν, where h is Planck’s constant and ν is the frequency
of the radiation. If the wavelength of the released photon is between 400 and 700nm, and the energy is emitted as
visible light. The color of the light depends on the specific energy change that is taking lace.
White light is 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 as 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.
In this investigation, you will perform flame tests on seven metallic ions, then use your results to determine
the identity of several unknowns.
PART II:
UV Beads contain different pigments that change color when exposed to ultraviolet light from any source
including the sun. The beads are all white in visible light. In UV light, depending on the pigment added to each
bead, you will see different colors. Each bead will change color about 50,000 times before the pigment will no
longer respond to UV light.
The term "light" is often used as a generic word to describe many different forms of light such as
incandescent light, fluorescent light, or sunlight, for instance. However, not all light is made up of the same energy.
Using Energy Beads, you will be able to uncover an invisible form of light energy called ultraviolet light. None of
the energy in the ultraviolet region of the light spectrum is visible to the naked eye. Just as there are many different
colors of wavelengths in the visible spectrum (red, yellow, green, blue...), so are there many wavelengths of
ultraviolet light.
First, there is long wave ultraviolet light (300 to 400 nanometers), which most of us recognize as "black
light" the light that is often used to make decorations glow in discos and theatrical productions. Long wave UV
passes easily through plastic and glass.
Short wave ultraviolet light (100 to 300 nm) is used to kill bacteria, hasten chemical reactions (as a catalyst),
and is also valuable in the identification of certain fluorescent minerals. Unlike long wave UV, the short wave UV
cannot pass through ordinary glass nor most plastics. The shortest wavelengths cannot even travel very far through
the air before being absorbed by oxygen molecules as they are converted into ozone.
UV Beads are the perfect tool for understanding how solar radiation can be harmful and to recognize
preventative measures that can be taken to reduce the risks associated with exposure to sunlight. When you expose
bare skin to sunlight, your skin will either burn or tan (which doctors warn is still not healthy for your body). UV
radiation wavelengths are short enough to break chemical bonds in your skin tissue and with over prolonged
exposure, your skin may wrinkle or skin cancer may appear. These responses by your skin are a signal that the cells
under your skin are being assaulted by UV radiation.
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With all of the SPF (Sun Protection Factor) numbers available, we want to know what SPF lotion really
works best at keeping out the sun's harmful UV rays. Since the UV Color-Changing Beads are very sensitive to
changes in UV energy, you can use the beads to determine the blocking potential of the sunscreen.
PROCEDURE:
PART I:
1. Light the burner.
2. Obtain a ~ 1 mL of each solution & each in a different well of your well plate.
3. Dip a cotton swab in a solution and put the cotton portion the hottest portion of he flame.
4. Record the color you see (under “naked eye”) when the sample is burnt, being as specific as possible
5. Repeat step 2 through 6 for all of the known samples.
6. Repeat steps 2 through 4 for all the unknowns.
7. Turn off the burner. Clean up your lab bench. Wash your hands!
8. Determine the identity of the unknowns and put them in a results table.
PART II:
1. Place 10 beads into a zipper-lock bag & use a permanent marker to write the SPF number of the sunscreen
you're testing on the outside of the bag. Be sure to set-up one bag without any sunscreen coating for
comparison purposes.
2. Apply a layer of sunscreen to the outside of the bag.
3. Expose the beads to direct sunlight and using a timer record how many seconds it takes for the beads to
change color.
4. Based on the amount of time it took for the color to change, rate the color of the beads on a scale of 1-5,
with 5 showing the most color or "burning" and 1 showing the least color. The bag without any sunscreen is
an automatic "5".
5. Return the beads to the container on the supplies table, being careful not to get any sunscreen on the beads
themselves. Dispose of the baggies in the trash and clean up your station.
POST-LAB QUESTIONS:
PART I:
1. What was happening with the electrons that caused the color to appear?
2. Put the metallic ions in order, as they would appear in the visible portion of the electromagnetic spectrum
(from smallest to greatest wavelength) based on the color seen.
3. When a clean glass rod is heated, a yellow flame is observed. What does a yellow flame indicate? Why does
it appear when the rod is heated?
4. How do you think metallic salts are used in fireworks?
5. Neon is a gas that has a reddish-orange emission spectrum. Find out why “neon lights” display a multitude
of colors.
PART II:
1. What is the wavelength range of UVB Radiation?
2. Compare the SPF Number to the level of protection the sunscreen provides.
3. What compound in the sunscreen actually provides the protection from the harmful UV rays.
4. What type of damage does UVA radiation cause to humans? UVB radiation?
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Name
RESULTS SHEET
ELECTRONIC STRUCTURE: Flame Test Lab & UV Protection
PART I:
Salt Solution
Color (Be Specific)
Ba(NO3)2
Cu(NO3)2
LiNO3
KNO3
NaCl
Ca(NO3)2
Sr(NO3)2
Unknown
Color(s) (Be Specific)
Salt Solution(s) in
Unknown
1
2
3
4
5
PART II:
SPF Number
Amount of time it took beads
to change color (sec)
Control – No Sunscreen
SPF 15
SPF 30
SPF 50
SPF 60
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Rating
(Scale of 1 – 5)