Colorful Flames and Emission Spectra
AP Chemistry Lab #7
Name_______________________________ Date_________________ Section_____
Just as a fingerprint is unique to each person, the color of light emitted by a metal
salt when heated in a flame is unique to each metal. In this activity, the characteristic
color of light emitted by several metal salts will be observed. Then, each group will attempt
to identify an unknown metal salt based on their earlier observations.
In addition, you will be using a diffraction grating to observe the light emitted from a
gas that has had an electric current run through it. You will be calculating the wavelengths
of the lines on the emission spectrum of the gas and comparing them to known values.
Materials
• calcium chloride,
• copper (II) chloride
• lithium chloride
• potassium chloride
• sodium chloride
• strontium chloride
• zinc chloride
• barium chloride
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tap water
laboratory burner
watch glasses (8)
wooden splints soaked in water (2)
beaker, 250-mL
scoopulas
unknown salt
Procedure
Part I—Colorful Flames
1) Create a data table with space to record the color flame for each salt used
(including your unknown).
2) Fill a 250-mL beaker about half full with water to use as rinse water.
3) Label 8 watchglasses with the formulas of the 8 metal salts (see materials list).
4) Using the correct scoopula, carefully scoop up a small amount of the first salt and
place it on the appropriate watchglass. Repeat for the remaining 7 salts.
5) Light the laboratory burner (be sure that you are wearing goggles and long hair is
tied back).
6) Obtain a wooden splint that has been soaked in water.
7) Dip the soaked end of one of the wooden splints in one of the metallic salts, then
place it in the flame. Observe and record the color of the flame. If you notice that
some of the salt has fallen on the burner, turn off the burner and clean it off (or
your data may be incorrect).
8) Immerse the wooden splint in the “rinse water” to fully extinguish it.
9) Repeat steps 7-8 with the other salts. Change your rinse water as needed.
10) Obtain your unknown salt, record its letter, and perform steps 7 and 8 with it. Try
to identify your unknown based on your data.
11) Clean up your lab station. Place any excess salts in the appropriate waste container.
Part II—Emission Spectrum
1) Create a data table with three columns, headed: color of line, X (cm), Y (cm). You’ll
need space for three different colors.
2) Set up the apparatus shown in the figure below.
3) One lab partner will view the emission spectrum of hydrogen by looking through the
diffraction grating at the center of the spectral tube. The other partner will stand
behind the power supply and move a pencil slowly along the meter stick. The student
viewing the spectrum should indicate when the pencil is at the point where the image
of the spectral line closest to the spectrum tube appears to be. Measure the
distance in cm between the tube and the image of the spectral line. Record this
distance (x) in your data table and record the color that went with it.
4) Repeat step 2 for the other spectral lines of hydrogen.
5) Using colored pencils sketch the emission spectrum of hydrogen on your paper.
Calculations (for part II) (please show one sample calculation for each)
1) Record the distance for (Y) in your data table.
2) Make a second data table with four columns: color of line (as above), Z, sin Θ ,
wavelength (cm).
3) Find the distance (Z) from the diffraction grating to the image of the spectral line
using the Pythagorean theorem, where X and Y are the sides of the triangle and Z is
the hypotenuse.
4) Find the sine of the angle Θ (shown on the diagram).
5) Find the wavelength in cm using the equation λ = d (sinΘ) , where d = 1.9 x 10-4 cm.
6) Find the percent error for each wavelength using these known values:
Red:
6.563 x 10-5 cm
Blue-green:
4.861 x 10-5 cm
Violet:
4.359 x 10-5 cm
7) Calculate the energy and frequency of one of the spectral lines observed.
Analysis
1) What is a metallic salt?
2) What were the two sources of energy used in the lab?
3) Explain what is happening inside the atoms of the metals and the gas to produce
light/colored flames.
4) Why are the colors different for each of the metals?
5) What is a diffraction grating? How does it work?
6) What is the connection between the lines in an emission spectrum and the energy
levels in an atom?
7) How did you identify your unknown?
Conclusion
Summarize the lab and your data. Suggest a source for any errors that occurred.