Experiment 60 Flame Test by stariya

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									                                              Brentwood High School
                                                    Chemistry

Name____________________________________ Period ________Date ______________________________

Experiment #60 – The Flame Test and Visible Light from Gas-Filled Discharge Tubes

Learning Target: I can classify and identify an element in a compound by the distinctive color an
element emits when heated in a flame or by an electric discharge.

Materials:   Bunsen Burner Lithium chloride          Calcium chloride          Copper (II) chloride
             Cotton swabs      Sodium chloride       Barium chloride           Cobalt (II) chloride
             Distilled water Potassium chloride      Strontium chloride        Iron (III) chloride
             Electric Discharge Tubes: H2, He, Ne, Ar, Kr, Hg, I2, Xe          Diffraction Glasses

Pre-Phase: Read and highlight, in colors

Flame Test Lab Instructions
Downloaded on November 16, 2009 from
http://www.kent.k12.wa.us/staff/carriewattles/chemistry/flametestlab_inst.htm

By placing atoms of a metal into a flame, electrons can be induced to absorb energy and jump to an excited
energy state, a quantum jump. Then, they return to their ground state by emitting a photon of light (the law of
conservation of energy indicates that the photon emitted will contain the same amount of energy as that
absorbed in the quantum jump). The amount of energy in the photon determines its color; red for the lowest
energy visible light, increasing energy through the rainbow of orange yellow green blue indigo, and finally
violet for the highest energy visible light. Photons outside the visible spectrum may also be emitted, but we
cannot see them.

The arrangement of electrons in an atom determines the sizes of the quantum jumps, and thus the energy and
colors of the collection of photons emitted, known as emission spectrum. In this way the emission spectrum
serves as a „fingerprint‟ of the element to which the atoms belong. We can view the emission spectrum of
colors all at once with the naked eye. It will appear to be one color, which we will carefully describe. It is also
possible to view the separate colors of the emission spectrum by using a spectroscope. Spectroscopes have
prisms or diffraction gratings that bend light of different energies differently. Low-energy red light is bent the
least, and high energy violet light bends the most. This allows us to see the various distinct colors of the
emission spectrum of a sample.

In this lab we will record the flame test color of several metals by capturing droplets of solutions of salts, or
ionic compounds of those metals with a platinum wire loop, then placing the loop into a Bunsen burner flame.
The cations form metals in the reducing portion of the flame.

Cobalt blue glass filters are often used when viewing mixtures of metals to screen out light that is yellow in
color. The human eye sees yellow very well, since it is in the middle of the spectrum visible to the eye. Colors
at the edges of the visible spectrum, especially violet, are more difficult to see. Cobalt glass absorbs light in the
yellow wavelengths, but is transparent to light of higher energy (this is why it looks blue!). Viewing a yellow
flame through cobalt glass will allow us to see if there is any higher energy light present.

We will observe the emission spectra of hot materials and observe their characteristic spectra using diffraction
glasses.
      Hazards: There is a potential for eye injury when handling chemicals. Use eye protection.
      Most salts in this experiment are poisonous! Avoid contamination of skin and clothing. Clean
      up any spills. In the event of contact skin, wash immediately with running water in the sink.
      Exercise caution when handling a hot Bunsen burner and an open flame. Keep flammables away
      from the bench. Don the safety goggles/glasses during the experiment. Exercise caution in
      handling electric discharge tubes. They pose an electric shock hazard and may become
      considerably hot.

Experimental Phase:

   1. The demonstrator will use a cotton swab into distilled water. He will dip the moistened swab in the
      sample of lithium chloride crystals so that a few crystals stick to the cotton. He will put the crystals on
      the swab into the flame of a Bunsen burner. Observe the color of the flame and record it in your data
      table.

   2. At the corresponding station, use a clean swab for each sample, Step 1 is repeated for each of the
      metallic chlorides found at the station (sodium chloride, potassium chloride, calcium chloride, barium
      chloride and strontium chloride, copper chloride, cobalt (II) chloride, and iron (III) chloride). Be sure to
      record the color of the flame in your data table, Table 1.

     Table 1. Data Table of Observed Color in the Flame Test

                                                  Flame Test
                            Compound                                       Flame Color
      Lithium chloride
      Sodium chloride
      Potassium chloride
      Calcium chloride
      Barium chloride
      Strontium chloride
              Transition Metal Compound                       Flame Color            Cobalt Glass Filter
      Copper (II) chloride
      Cobalt (II) chloride
      Iron (III) chloride



   3. At the stations with discharge tubes, observe and record, in Table 2, the color of the electrically excited
      gas. Describe the number and colors of the visible light lines observed through the spectroscope. For
      complex spectra, consider the four brightest lines.
Table 2. Data Table of Observed Color in the Discharge Tubes
          Element                      Observed Color          Emission Spectral Lines
             H2




            He




            I2




            Ne




            Ar




            Kr




            Hg




            Xe
Post-Phase:

Analysis:

1. Go to the course website and click on the course resource site titled “Element Absorption and Emission
   Spectra”. How does the atomic emission spectrum of a tested metal element relate to the color of the flame?



2. Explain how the presence of chlorine in the tested compounds affects the color of the flame.



Real-World Chemistry: Fraunhofer Lines

   Background and Theory : The brightest star in our sky is the Sun. Absorption lines in the solar spectrum
   were first noticed by an English astronomer in 1802, but it was a German physicist, Joseph von Fraunhofer,
   who first measured and cataloged over 600 of them about 10 years later. These lines are now known
   collectively as the "Fraunhofer lines." In the 1800's, scientists did not know that these lines were chemical in
   origin. Thus, the letters used by Fraunhofer to identify the lines have no relation to chemical symbols nor to
   the symbols used to designate the spectral types of stars. Today's astronomers use some of the designations
   simply for convenience and ease in identifying lines.

   Now we know that each absorption line is caused by a transition of an electron between energy levels in an
   atom. Each element has a distinct pattern of absorption lines. Once the pattern of the lines of a particular
   element has been observed in the laboratory, it is possible to determine whether those elements exist
   elsewhere in the universe simply by pattern matching the absorption lines. Table 3, below, matches the
   “known” Fraunhofer lines of the solar absorption spectrum to the elements that are responsible for them.

                         Table 3 -- "Known" Solar Fraunhofer Lines
                    Designation Wavelength (nm)               Origin
                    A             759.4               terrestrial oxygen
                    B             686.7               terrestrial oxygen
                    C             656.3               hydrogen (Hα)
                    D1            589.6               neutral sodium (Na I)
                    D2            589.0               neutral sodium (Na I)
                    E             527.0               neutral iron (Fe I)
                    F             486.1               hydrogen (Hβ)
                    H             396.8               ionized calcium (Ca II)
                    K             393.4               ionized calcium (Ca II)

   3.    Research the absorption spectra and the principal Fraunhofer Lines of the Sun at the course resource in
        the teacher website titled “Absorption Spectra of Sunlight” (NASA). Define a solar Fraunhofer line.
Day 94 Laboratory Rubric
          Criteria                       Level 1                  Level 2             Level 3             Score
Perform a flame test safely.    Did not perform the               Performed the flame test properly.
                                flame test properly.                                                      20
Record direct, or cobalt        Recorded correctly no      Recorded correctly       Recorded
glass-filtered, observations of more than six (6) out of   at least nine (9) of     correctly all         24
a flame test.                   twelve (12) tests.         twelve (12) tests.       twelve (12) tests.
Record the perceived plasma Recorded correctly no          Recorded correctly       Recorded
color and atomic emission       more than eight (8) out    at least fourteen (14)   correctly all
spectral lines of a gas         of sixteen (16) tests.     of sixteen (16) tests.   sixteen (16) tests.   32
discharge tube.
Relate the color of plasma to I cannot relate this.            I can relate the color of plasma to the
the atomic emission spectral                                          atomic emission spectral lines.     10
lines.
Explain the role of chloride    Did not relate this.           Relate the presence of chloride to the
in the color of plasma in a                                     color of a plasma in a flame or a gas
flame or a gas discharge                                                              discharge tube.     10
tube.
Define a solar Fraunhofer       Did not define correctly                      Define correctly a solar
line.                           this.                                                Fraunhofer line.   4
                                                                                                Total 100

								
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