Pre-Lab Exercises Lab 2: Spectroscopy 1. Which color of visible light has the longest wavelength? Name Date Section 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? Page 1 of 6
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. Page 2 of 6
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. Page 3 of 6
Lab Station Check Out # Name Lab Partner Date Section 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 Page 4 of 6
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? Page 5 of 6
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 SrCl 2 HBO 3 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. Page 6 of 6