Lab 2: Spectroscopy
Chemistry 100
_____________________
Date ______________________
Section ______________________
Name
Pre-Lab Exercises
Lab 2: Spectroscopy
1. Which color of visible light has the longest wavelength?
2. List the colors of visible light from highest frequency to lowest frequency.
3. Does the light emitted by an excited element produce a continuous spectrum or a line spectrum?
4. How can atomic spectra be used to identify elements?
5. How can we determine what elements are present in stars?
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Lab 2: Spectroscopy
Chemistry 100
Chemistry 100
Lab 2: Spectroscopy
DISCUSSION
The glowing filament in an ordinary light bulb emits electromagnetic energy of many wavelengths, including (obviously)
visible light. Examining this light with a diffraction grating allows you to see the colors corresponding to the wavelengths
emitted by the light bulb. Red light has the longest wavelengths (lowest frequency) of the visible spectrum, and violet
has the shortest wavelengths (highest frequency), with all the other colors in between as seen in a rainbow.
When heated, many of the elements produce colors. Each element produces a unique color that is different from any
other element. When an excited electron falls from a higher to lower energy level, it releases energy in the form of a
photon of light. The different colors for different atoms correspond to the differences in electron energy levels in each
atom.
On an individual basis, atoms and molecules cannot be detected except with the most sophisticated of instruments. The
movement of electrons in atoms and molecules, however, is quite easy to infer from the emission of visible light as
electrons in higher excited states fall to lower energy states. For example, hydrogen gas emits lavender-colored light
when an electric current passes through it. This light is a combination of red, green, and violet emissions, and these
colors correspond to the energies of light emitted as electrons change energy levels. Viewing light through a
spectrometer allows you to not only see the spectrum, but to see it to scale. On the spectrometers we’ll be using in lab,
the numbers below the spectrum (400, 500, 600, 700, reading from right to left) indicate the wavelength of light in
nanometers (1 nanometer = 0.000000001 meter). You can disregard the numbers above the spectrum.
When a transparent colored object is placed between a light source and an observer, some of the wavelengths of light
passing through the object are absorbed while other wavelengths are transmitted. The color of the object depends on
which wavelengths are absorbed and which are transmitted. This is also true of the light reflected from opaque colored
objects. For example, an object that appears yellow is absorbing all the wavelengths except those corresponding to
yellow; these are reflected back to our eyes.
PROCEDURES
Flame Test: Using Light to Identify Elements
1. Observe flame tests of sodium, boron, lithium and strontium salts. Identify the name of the salt and the colors
emitted (given off). Be as specific as you can—there are many shades of “red” and you may need more
information than that to identify the unknowns.
2. Compare the colors of the 2 unknowns to the colors of the known elements. Record the colors emitted and the
identity of the unknown salts.
Continuous Spectra
1. Obtain a diffraction grating mounted in a 35 mm slide mount. Hold it close to one eye, and look at a light bulb
through the diffraction grating. Orient the grating so that you see two spectra to the right and left of the light
bulb. (If you see them above and below the bulb, rotate the slide a quarter turn.)
2. Focus your attention on the spectrum to the left of the light bulb. Observe the width and sequence of the colors.
Write these colors below the graph on the report sheet, and indicate their relative widths.
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Lab 2: Spectroscopy
Chemistry 100
Line Spectra
1. Using a diffraction grating, view the line spectra of hydrogen, helium, neon, and mercury. (Your instructor will
assist you with the emission tubes. They must be on for only short periods or they will overheat.)
2. Referring to the colors of the continuous spectrum you observed in part 1, record the approximate positions of
the lines of these spectra on the graphs on the report sheet. Note their colors.
Using a Spectrometer
1. Obtain a spectrometer and look through it at a light bulb. To see the spectrum, you
need to point the spectrometer at the light bulb so that the slit on the far right side is
pointed directly at the light bulb.
2. Without moving the spectrometer, look to the left and view the spectrum. Record your
observations on the report sheet as in part 1, but this time do it to scale, with 700 on
the left edge of the graph and 400 on the right edge. Include the wavelength
measurement unit, nm.
3. View the line spectra from the previous section again, this time using the spectrometer
and drawing the lines to scale. You will have to move the spectrometer back and forth
until the narrow gas tube is lined up precisely with the slit in the spectrometer.
4. Observe the overhead fluorescent lights through the spectrometer. You may see a
combination of line and continuous spectra, or just line spectra, depending on the type
of fluorescent light. Draw the spectrum on your report sheet, noting the lines. Can
you identify any elements based on the line spectrum?
Absorption Spectra
1. Again observe the continuous spectrum from a light bulb through the spectrometer and refer to the spectrum
you have recorded in the previous section.
2. Place a beaker containing red food color in water between you and the light bulb, and view the spectrum
through the spectrometer. Note carefully which wavelengths of light have been absorbed, and record your
observations.
3. Repeat step 2 with yellow and blue food color.
4. Recall your observations about green food color from experiment 1. Repeat step 2 with green food color and
see if your observations are consistent with experiment 1. Explain. Compare this absorption spectrum to that
from a green soda bottle.
5. If there is a bright white cloud outdoors, look at the light from this cloud through the spectrometer. If you look
very closely, you will see some faint dark lines in the continuous spectrum. Compare these dark lines with the
helium emission spectrum, and you will see that they coincide. Helium on the sun absorbs specific wavelengths
of light that correspond to its emission spectrum. This is how helium was discovered.
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Lab 2: Spectroscopy
Chemistry 100
_____________________
Lab Partner __________________
Date _____________________
Section _____________________
Name
Lab Station Check Out
#
REPORT
Continuous Spectra
1. Sketch the continuous spectrum as seen through a diffraction grating. Indicate sequence and relative width of
colors.
Line Spectra
1. Sketch each element’s line spectrum as seen through a diffraction grating. Indicate sequence of colors and
position of the absorption lines.
Hydrogen
Helium
Mercury
Neon
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Lab 2: Spectroscopy
Chemistry 100
Using a Spectrometer
1. Sketch the continuous spectrum as seen through a spectrometer. Make sure to include wavelengths!
Indicate scale, sequence and relative width of colors.
2. Sketch each element’s line spectrum as seen through a spectrometer. Indicate wavelengths, sequence of
colors and position of the absorption lines.
Hydrogen
Helium
Mercury
Neon
3. Sketch what you see through the spectrometer when looking at a fluorescent light. Make sure to include
wavelengths! Indicate scale, sequence and relative width of colors.
Compare the spectrum in #3 with the spectra in #2. Can you identify any elements in fluorescent lights?
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Lab 2: Spectroscopy
Chemistry 100
Absorption Spectra
Color(s) absorbed
Wavelengths Absorbed
Red Food Color
Blue Food Color
Yellow Food Color
Green Food Color
Flame Tests: Using Light to Identify Compounds
Solution
Name of Compound
Colors Emitted
NaCl
SrCl2
HBO3
LiCl
Unknown 1 ___________
Unknown 2 ___________
Post-Lab Analysis
1. How does the absorption spectrum from the green food color compare to the absorption spectrum
from a green plastic bottle?
2. In general, what causes objects to be colored?
3. Explain in some detail why red Kool-aid is red.
4. Lithium compounds are used in medicine to control mood in some psychiatric patients. The lithium
compounds are quite soluble and present in all body fluids. Suppose you needed to find out if a
patient had taken his medication recently. Propose a test to determine this.
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