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FlameTests
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25
Text reference: Chapter 13, PP: 339-343
Pre-Lab Discussion .
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The normal electron conflgurationof atoms or ions of an element is known
as the "ground state." In this most stable energy state, all electrons are in
the lowest energy levels available. When atoms or ions in the ground state -.
are heated to high temperatures,· some electrons may absorb enough
energy to allow them to "jump" to higher energy leveIS. The element is
then said to be in the "excited state." This excited conflguration is unstable,
and the electrons "fall" back to their normal positions of lower energy. As
the electrons return to their normal levels, the energy that was absorbed
is emitted in the form of electromagnetic energy. Some of this energy may
be in .the form of visible light. The color of this light can be used as a
means of identifying the elements involved. Such crude analyses are
known as flame tests.
Only metals, with their loosely held electrons, are excited Inthe flame
of a laboratory burner. Thus, flame tests are useful in the identification of
metallic ions. Many metallic ions exhibit characteristic colors .when
vaporized in the burner flame. In this experiment, characteristic colors of
several different metallic ions will be observed, and an unidentified ion
will be identified by means of its flame test.
~
Purpose
Observe the characteristic colors produced by certain metallic ions when
vaporized in a flame. Iden~ an unknown metallic ion by means of its
flame test.
Equipment
graduated cylinder, 10-mL
laboratory burner
test tubes, 13x 100-mm (8)
test tube rack
wire loop, platinum (or nichrome)
glass-marking pencil
safety goggles
lab apron or coat
Materials
hydrochloric acid, Hcr (conc)
Unidentified solutions
0.5 M solutions.of nitrates of:
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Na+ K+ (Li+) Ca2+ Sr2+tBa2-i', Cu2+
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Safety
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Handle concentrated HCI with great care. Report spills at once to your
teacher and flush them with cool water and NaH'C03 solution. Note the
caution alert symbols here "and with certain steps in the "Procedure."
Refer to page xi for the specific precautions associated with each symbol.
Always wear safety goggles and a lab coat or apron when working in the
lab.
127
Procedure
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1. Measure 5 mL of tap water in a graduated cylinder and pour
the water into a 13x 100test tube. Using a marking pencil, mark
the outside of the tube to indicate the level of the water. Discard
the water. Using the 'marked tube as a guide, mark seven clean
test tubes at approximately the same level. Place the clean tubes
in a test tube rack. Set the other test tube aside ..
2. Into each of the clean test tubes, pour 5 mL of a different
nitrate solution. Mark each test tube to indicate the metallic ion
it contains.
3. Pour about 10 mL of concentrated hydrochloric acid into a
50-mL beaker. CAUTION: Use extreme care in 'handling this
acid. To clean the wire loop, dip the loop in the acid and then
heat the loop in the outer edge of the burner flame. Continue to
clean the loop in this manner until no color is observed in the
flame.
4. Dip the clean wire loop into one of the nitrate solutions.
Place the loop in the outer edge of the burner flame and move
the loop up and down (Figure 25-1). Note the color in the flame.
Record your observations in the data list provided.
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platinum
or
nichrome
wire.
Figure 25-1
5. Clean the wire loop as described in step 3. Repeat step 4
using a different nitrate solution.
.
6. Test each nitrate solution in the' same mariner, cleaning the
loop thoroughly between tests. Record all your' observations in
the data list.
7. Obtain a sample of an unknown solution. Perform a flame
test and identify the metallic ion present by the color of the
flame.
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128
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Observations, and Data
Metallic Ion
Color in Flame
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Unknown __
Conclusions and Questions
1. What inaccuracies may be involved in using flame tests for identification purposes?
2. Which pairs of ions produce similar colors in the flam~ tests?
3. Explain how the colors observed in the flame tests are produced.
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129
4. Define these terms:
a. quanta
b. ground state
c. excited state
5. What is a spectroscope? What is observed if the flame tests are viewed
through a spectroscope?
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130
Flame Test & Spectral
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_ Date:
Analysis Lab
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-----------------
DATA.SlIEET
1-...------, --- __
Spectrum of Potassium Chloride
Spectrnm of Sodium Chloride
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Spectrum of Strontium Chloride
Spectrum of
Lith; Ut)J
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Spectrum of Calcium Chloride
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Spectrum of Unknown Salt
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QUESTIONS'
1. From your observations, how do the bright-lliie spectra fth
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a
e samp es tested differ?
2. What color appears at the far left end of the' visib!
?
the far right end?
e spectrum. What colordo you see at
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3. Describe the visible flame
CO: orvcu obsesved ?~er inserting each of the four knownsalts
into the burner fiame. How did m:s;: :"""'rr<::e.... the bright-line spe;..i.t:1 you recorded?
4. How does the spectrum of the unkown salt compare to the spectra of the four known
samples? \Vhat element does it most likely contain?
5. Why was -it necessary to use a separate nichrome wire loop for each sample? How might
using asingIe wire loop for all tests change your results?
6. What do stars and your glowing samples have in common?
7. Stars, like our Sun, emit large amounts of radiant energy. How might astronomers
determine the temperature and composition of stars using a spectroscope?