Spectroscopy - Flame Tests
Objectives:
• To explain the origins of a line (emission and absorption) spectrum
• To observe the flame emission colors from excited metal ions
• To identify an element from its emission spectrum
Suggested Grade Level(s):
Ninth through twelfth grade
Subject areas:
Chemistry
Astronomy
Earth Science
Timeline:
One class period (1 hour)
National Education Standards:
6.2. Physical Science (grades 9-12)
Structure of atoms
6.5. Science and Technology (grades 9-12)
Understanding about science and technology
Background:
This can also be used as an introduction studying the composition of planets and
stars as well as studying the physics of color and light. If used as an introduction
to composition of planets and stars student should be familiar with using a
spectroscope and understand what information can be obtained from the
spectroscope. This activity can also be used as a cumulating lesson after
students have learned about the structure of the atom, electrons and energy
levels as well as a basic understanding of the electromagnetic spectrum.
Materials:
Report Sheet
Platinum or Nichrome wire
Concentrated HCl
Flame source
Watchglass
Crystals of calcium chloride
CuCl2, NaCl, BaCl2, LiCl, SrCl2 and KCl salts
Cobalt glass plate
Lesson:
INTRODUCTION
The current model of the atom includes a nucleus, comprised of protons and
neutrons, surrounded by electrons. From interpretations of the atomic absorption
and emission spectra of elements, and from a detailed quantum study of electron
behavior, it is generally agreed that the energy of each electron in an atom is
quantized. This means that an electron in an atom may have only certain
discrete energies. When an electron absorbs or emits energy, it can only absorb
or emit a “quantum” of energy.
When an atom absorbs energy from a flame or an electric discharge, it absorbs
the energy necessary to excite one of its electrons to a higher energy state. We
say the atom is then in an excited state, Eλ. When the electron returns to a
lowest energy state (ground state), it emits the same amount of energy
previously absorbed, but in the form of one or more photons
E (excited state)
________________
_______e-___________
E photon
________
E photon
E (grounded state)
_____e-___________
__________________
The energy of the photon equals the difference between the two energy states
for the electron and is inversely proportional to its wavelength.
Since many energy states exist for electrons in a given atom, a large number of
excited states are also possible. Therefore, when light is emitted from a large
collection of excited atoms, a variety of electrons return to their ground state.
When these emitted photons pass thorough a prism, a line spectrum is produced,
where each lime on the spectrum corresponds to photons of fixed energy and
wavelength.
Each element exhibits its own characteristic lime spectrum because the electron
energy states in its atoms are different from those in atoms of other elements.
For example, the 11 electrons on sodium have a different set of electron energy
states than do the 80 electrons in mercury or the 56 electrons in barium.
Therefore when an electron in an excited sodium atom moves to a lower energy
state, a photon is emitted. This photon has a different energy and wavelength
from one that is emitted when an electron de-excites in mercury or a barium
atom.
The different wavelengths of the emitted photons produce different, but
characteristic, colors of light. Light emitted from an excited sodium atom is
characteristically yellow-orange, mercury emits a blue light, and barium emits a
green light.
In part A of this experiment, we will observe the characteristic colors emitted from
several “excited” elements. Flame tests are used for identification. The
probability of a given electron de-excitation process in an atom reflects the
intensity of that line of given energy in the line spectrum- the more probable the
de-excitation, the more intense the line. An analytical study of line positions and
intensities in an emission spectrum will be made in parts B and C.
1. Part A: Flame test of various elements
a. Clean the Test Loop. Dip the platinum or nichrome wire end of the
test loop into concentrated HCl (caution: avoid contact with skin.)
Heat the wire in the hottest region of the flame until there is no
visible color. Repeat this cleaning procedure as necessary.
b. Flame test for calcium in CaCl2.
In a watchglass add a few crystals of calcium chloride, to 2 or 3
drops of concentrated HCl and stir. Dip the clean wire into the
mixture and return it to the flame (view it through a spectroscope).
Correlate the color of the flame with the wavelength of the emission
using the chart on the Report Sheet. Check the appropriate
section.
c. Flame tests for Other Cations. Repeat parts 1 and 2 substituting
CuCl2, NaCl, BaCl2, LiCl, SrCl2 and KCl salts. For KCl view the
flame through a cobalt glass plate- it filters out wavelengths that
interfere with those of the excited state
2. Part B: The Spectrum of an unknown element
a. Determine the wavelengths in the Emission Spectrum of the
Unknown. You will be given an unknown sample from one that was
done in Part A. Identify the compound by comparing it to the
results of Part A
3. Post Lab Questions
a. Explain why the color of the ion in the flame tests differs?
b. Over what (approximate) wavelength range is the emission of light
from excited sodium atoms most sensitive to the eye? Hint: sodium
vapor lamps are used for public lighting of streets, parking areas,
etc
c. Over what (approximate) wavelength range is the emission of light
from excited mercury atoms most sensitive to the eye? Hint:
mercury vapor lamps are used for public lighting of streets, parking
areas, etc
d. Aerial displays of fireworks are accompanied by many colors in the
starbursts.
• What is the source of the orange-yellow color?
• What ion is responsible for the red color?
Extensions:
As an extension students can research how other sciences use this technology,
like forensics, geology and\or biology.
Evaluation:
Students were evaluated based on the post lab questions as well as the results
from part B of the lab.
Resources:
Space Foundation ‘Astronomy Principles for the Classroom’ June 25-29, 2007
Addendums:
Addendums
PRELABORATORY ASSIGNMENT
SPECTROSCOPY
NAME______________________
Date________________________
1. Distinguish between an absorption spectrum and an emission spectrum.
2. The strong line in the calcium emission line spectrum occurs at 657.3 nm,
to what color does this correspond?
3. What does it mean when we say that an atom is in an “excited state”?
Data table:
For each element, record the approximate wavelengths emitted by each ion.
Wavelength in nanometers
740
Red
Orange
Yellow
Green
520 565 590 625
Cyan
500
Blue
435
Violet
Element
380