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 a photon of
electromagnetic radiation. The energy of each photon is described by the equation E =
hv, where h is Planck’s constant and v is the frequency of the 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 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. 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 ion.
In this investigation you will perform flame tests on seven metallic ions, then use
your results to determine the identity of several unknowns.
PRE-LAB DISCUSSION
Read the entire laboratory investigation and the relevant pages of your textbook.
then answer the questions that follow.
1.
Write out the electron configurations for each of the metallic ions to be tested in
this investigation.
Ba2+ ___________________________________________________
Cu2+ ___________________________________________________
Li+ ___________________________________________________
K+ ___________________________________________________
Sr2+ ___________________________________________________
Ca2+ ___________________________________________________
Na+ ___________________________________________________
2.
What does a flame test indicate about the energy changes taking place among the
electrons in a metallic ion? ___________________________________________
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3.
Explain why a metallic ion produces a characteristic color in a flame test,
regardless of the compound used as the source of the ion. ___________________
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4.
What wavelengths correspond to the visible spectrum? Which color has the
shortest wavelength? The longest? _____________________________________
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5.
What precautions should be taken when using 6.0 M HCl? __________________
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6.
Why is it important to use a clean nichrome wire for each test? _______________
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PROBLEM
What colors are characteristic of particular metallic ions in a flame test?
MATERIALS
chemical splash goggles
laboratory apron
latex gloves
well plate
marker pen
solutions of the following salts:
barium nitrate (Ba(NO3)2)
copper nitrate (Cu(NO3)2)
strontium nitrate (Sr(NO3)2)
lithium nitrate (LiNO3)
potassium nitrate (KNO3)
sodium chloride (NaCl)
calcium nitrate (Ca(NO3)2)
nichrome wire loop
beaker, 50-mL
hydrochloric acid (HCl), 6.0 M
lab burner
wash bottle with distilled water
3 unknown solutions
SAFETY
Wear your goggles and lab apron at all times during the investigation. The 6.0 m HCl is
corrosive and irritating to skin, nasal passages, eyes, and clothing. the salt solutions, with
the exception of NaCl, are toxic. Handle all of the solutions with care, and do not inhale
their vapors. If a solution should splash on your skin, wash the affected area with large
amounts of water and notify your teacher. When you work near the burner flame, tie back
or cover loose hair, and secure loose or bulky clothing.
PROCEDURE
1.
Put on your goggles and lab apron. Obtain a well plate and use a marker to
label seven wells with the names of the known solutions. Put a dropper
full of each known solution into its labeled well.
2.
Put on your latex gloves. Obtain a beaker with about 10 mL of 6.0 M HCl
and a nichrome wire loop. Light the burner, and adjust the flame to low.
CAUTION: Hydrochloric acid is corrosive to skin and clothing and its
vapors are irritating to lungs and eyes. “Avoid contact with the
and inhalation if its vapors. Loose hair should be tied back
and bulky or loose clothing should be secured in some
solution
or covered,
manner.
3.
For each test, be sure that the nichrome wire is clean, so as not to
contaminate the solutions. To clean the wire, first rinse it with distilled
water, using the wash bottle. Then dip it in the 6.0 M HCl solution. Place
it in the burner flame for a few moments, as shown in Figure 10-1.
Determine the color of the clean nichrome wire in the flame . This
color you should see after every trial.
4.
Sodium has a very strong color, which may affect your other tests, so test t
he sodium solution last, Dip the clean nichrome wire in one solution. Place
the wire in the burner flame and observe. Record your observations on
Data Table 1. Clean the wire and repeat this step with the next known
solution until you have tested all seven solutions. CAUTION: Do not
let these substances come in contact with your skin. If a solution should
splash on your skin, wash the affected area with large amounts of water
and notify your teacher.
is the
Figure 10-1
5.
Obtain three unknowns from your teacher. Repeat Step 4 for each
unknown. Record your observations on Data Table 2.
6.
Turn off the burner. Clean up your work area and wash your hands before
leaving the laboratory.
OBSERVATIONS
DATA TABLE 1
Flame Tests of Known Solutions
Salt Solution
Color
Ba(NO3)2
Cu(NO3)2
LiNO3
KNO3
NaCl
Ca(NO3)2
Sr(NO3)2
DATA TABLE 2
Unknown
Tests of Unknown Solutions
Color
CRITICAL THINKING: ANALYSIS AND CONCLUSIONS
1.
What metallic ions are present in the unknown solutions? (Drawing conclusions)
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2.
Summarize the process that produces the colors seen in the flame tests. (Applying
concepts) _________________________________________________________
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3.
What is the relationship of the colors you saw and the lines of the electromagnetic
spectrum produced by the metals? (Making inferences) _____________________
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CRITICAL THINKING: APPLICATIONS
1.
When a glass rod is heated, a yellow flame is observed around the point of
heating. what does this yellow flame indicate? why is it observed when glass is
heated? (Developing hypothesis) _______________________________________
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2.
The line spectrum of lithium has a red line at 670.8 nm. Calculate the energy of a
photon with this wavelength. (Making predictions)
3.
What other equipment could you use in this investigation if burners were not
available? (Designing experiments) ____________________________________
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4.
How do you think metallic salts are used in fireworks? (Applying concepts) _____
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GOING FURTHER
1.
2.
3.
Get some copper metal filings from your teacher. Work under your teacher’s
supervision. Pick up a small piece of copper filing with the wire loop, and put it in
the burner flame. Is the color of copper in the flame the same as the color of
copper nitrate solution?
Neon is a gas that has a reddish-orange emission spectrum. Find out why neon
lights can display a multitude of colors.
The invention of the spectroscope by Robert Bunsen and Gustav Kirchoff opened
up a new era of chemical investigation and research. Bunsen and Kirchoff
discovered two elements through the use of their invention. Find out which
elements they discovered, and how the spectroscope is now used.