AP Chemistry @ RIS Exp.: SpectroscopyPg. 1

Spectroscopy

Purpose:Observe and record the spectra of several different light sources and to make measurements of these spectra

Problems:Do all light sources produce the same variety of colors in their spectrum? What does a colored filter do to the spectra that is seen in a spectroscope? How is the wavelength of light related to the visible color that is seen in a spectroscope?

Overview:When solids are heated until they glow, their atoms produce a continuous spectrum. But those substances that are vaporized by heating in a flame can emit light characteristic of the elements in the substance. Another method of spectrum analysis involves the application of high voltage across a gas-filled glass tube. Gas atoms that are under low pressure and excited by an electrical discharge give off light in characteristic wavelengths. The emitted light is passed through a spectroscope, which breaks light into its constituent components for analysis. A gas viewed through a spectroscope, such as the one shown in the picture above, forms a series of bright lines known as a bright-line or emission spectrum. Since each element produces a unique bright-line spectrum or pattern, spectroscopy is a valuable branch of science for detecting the presence of elements. A unknown substance is identified by comparing the wavelengths of its emission spectrum to the spectrum produced by a known substance.

Procedure:
Part 1 –Light sources and colored filters
  1. Obtain a spectroscope and look through it at an incandescent light bulb. The spectrum should appear when the slit in the spectroscope is pointed just off center of the light bulb. Practice moving the spectroscope until you see a bright, clear image. Record what you see.
  2. Record what you see for the incandescent light bulb, fluorescent light and sunlight.
  3. While you look through the spectroscope, your partner should place one of the colored filters in front of the incandescent light bulb. Record what you see.
  4. Repeat step three for each of the available colored filters.
Part 2 – Flame Tests
  1. Look at a lit Bunsen burner through the spectroscope. Record what you see.
  2. Using the samples of the different substances available to you, for each sample:
  3. Dip a wet wooden splint into one of the samples; repeat until there is a decent amount of material attached to the wooden splint.
  4. Have your lab partner look through the spectroscope while you put the wooden splint into the flame of the Bunsen burner.
  5. Record the colored lines that you see in the spectroscope in your data table.
  6. Record the color of the flame.
  7. Repeat for each of the remaining samples.
Part 3 – Spectra of different light sources

CAUTION: Voltage of several thousand volts exists at the power supply and spectrum tubes. Do not touch the spectrum-tube power supply or spectrum tubes when power is applied. Use caution when handling the hot tubes!

  1. Verify that the power to the spectrum-tube power supply is turned off. Insert one of the spectrum tubes into the sockets by holding the tube at the top and bottom and press into the cups. DO NOT press on the middle of the tubes!! Helium or hydrogen is a good first choice.

The gas tubes are fragile, expensive and after being used are very hot!

SO BE CAREFUL!!

3.Turn on the power supply. If an open door is near point the spectroscope away from the open door since daylight will affect the observed gas spectrum. Adjust the spectroscope until the brightest image is oriented on your scale. Some of the spectrum tubes produce light so dim that you must be very close to them to get good observations of the spectral lines. Record in the data table the bright lines of the observed spectrum.

Turn off the power supply and, using caution, carefully remove the hot spectrum tube. Carefully lay the hot tube off to the side until it cools.

4.Repeat Steps 2 and 3 using all the other spectrum tubes, including the one marked unknown. Record your observations in the data table below.

Part 4 – Measurement of the wavelength of the colored bands in spectra

Use the following equation to calculate the wavelength for each color you observed. Add a column to your data table showing these calculated values. Convert your answers for each color into nanometers (nm).

(Use 1.896 x 10-6 m as the value for d)

  1. Plug in the light source and look through the diffraction grating. You should see the colors to the sides of the bulb. If you do not, then rotate the diffraction grating 90º. The distance from the diffraction grating to the light source is the “L” value (record to the nearest 0.1cm).
  1. Have your lab partner move the thin rod (which may be a paper clip or a piece of thick colored paper) until it seems to be in the middle of the orange color band that you see in the spectroscope. The distance from the light source to this point is the “x” value (record to the nearest 0.1cm).
  1. Repeat for each of the other major colors that you see in the spectra (red, orange, green, blue).
Data Table

Create a data table that will allow you to keep track of the pertinent information in each part of this experiment.

For each observation that you make with the spectroscope, draw a band like so in your data table and then draw in the colored lines you see in the order and spacing that you see them in the spectroscope.

Questions:

1.Describe what the red colored filter does. Then state a rule for how colored filters work.

2.a) Compare what happens when you put a same amount of LiCl and then NaCl into the Bunsen burner flame.

b)How could you use the information from Part II if you were given ten different unlabeled bottles of chemicals?

3.Compare the color of light emitted by each spectrum tube to the colors observed through the spectroscope. Explain any differences.

4.How do the intensities of the observed spectral lines compare?

5.How were you able to use the results of this experiment to identify the unlabeled gas tube

6.Compare the light emitted by the gas tubes with the light emitted by each compound in Part2.

7.How could the procedure in Part 4 be used in conjunction with a telescope?

8.There are two types of spectra: Line spectra and continuous spectra. Which spectra of those you observed were line spectra? Was there an example continuous spectra? If so, what was it?