Docstoc

Flame Tests - Download as DOC

Document Sample
Flame Tests - Download as DOC Powered By Docstoc
					                                          Flame Tests
Introduction
    According to the Bohr theory of the atom, electrons may occupy only specific energy levels.
When an atom absorbs sufficient energy, an electron can "jump" to a higher energy level. Higher
energy levels tend to be less stable, however, and if a lower energy level is available, the electron
will "fall back", giving off energy in the process. The difference in energies between the two levels
is emitted in the form of electromagnetic radiation. If the wavelength of the released photon is
between 400 and 700 nm, the energy is emitted as visible light. The color of the light depends on
the specific energy change that is taking place.
    White light is a continuous spectrum in which all the wavelengths of visible light are present. An
excited atom, however, produces one or more specific lines in its spectrum, corresponding to the
specific changes in energy levels of its electrons. Because each element has a distinct electron
configuration, each has a unique line spectrum.
    Flame tests are a quick method of producing the characteristic colors of metallic ions (we will
talk more about ions later). The loosely held electrons of a metal are easily excited in the flame of
a lab burner. The emission of energy in the visible portion of the spectrum as those electrons
return to lower energy levels produces a colored flame. The color is a combination of the
wavelengths of each transition, and may be used to determine the identity of the metal.
In this investigation you will perform flame tests on seven metallic ions, and use your results to
determine the identity of three unknowns.

Safety Precautions: Goggles, apron, closed toe shoes, hair tied back. Use appropriate safety with
                    open flame (burner).

Purpose: determine what color flame is associated with specific metallic ions and identify the
         metallic ions in three unknown salts using a flame test

Pre-Lab (answer BEFORE beginning the lab!)

   1.   What does a flame test indicate about the energy changes taking place among the electrons
        in a metallic ion?
   2.   What wavelengths correspond to the visible spectrum? Which color has the shortest
        wavelength? The longest?

Materials
Goggles                        lab burner
Apron                          striker
cotton swabs (Q-tips)          7 known ionic salts
deionized water                3 unknowns


Procedure
    1. Rotate among the 7 lab stations. There you will find ionic salts.
    2. Set up and light the lab burner, if it is not already lit. You should have to do this only once.
    3. For each test, dip the end of the swab in the deionized water, then dip it in one of the salts,
       and hold it in the flame. Observe the color that is given off and record these observations.
    4. Obtain samples from three unknown salts, I, II, and III . . . and predict the metal ions
       present in each salt (We know they all look the same. That's why we need a flame test to
       find out the actual metal ions present!) Hint: Each unknown is a mixture of two salts.
    5. Repeat step 3 and record observations of the colors emitted.
                                             DATA
                 Salt                            Color (observations)

   A            BaCl2

   B            CuCl2

   C             LiCl

   D              KCl

   E             NaCl

   F            CaCl2

   G            SrCl2


             Predicted
Unknown                                         Color (observations)
             Metal Ions

    I

   II

   III


Questions

   1. What metallic ions are present in the unknown solutions, I, II, and III?
   2. What is the reason for all salts being chlorides?
   3. Summarize the process that causes the colors that are seen in a flame test.
   4. What is the relationship between the colors you saw and the lines of the electromagnetic
      spectrum produced by the metals?
   5. When a glass rod is heated, a yellow flame is observed around the point of heating. What
      does the yellow flame indicate? Why is it observed when glass is heated?
   6. What other equipment could you use in this investigation if burners were not available?
   7. How are metallic salts used in fireworks?

				
DOCUMENT INFO
Shared By:
Categories:
Stats:
views:68
posted:4/3/2011
language:English
pages:2