Atomic Emission Spectra Investigation Report
Background Information:
What do fireworks, lasers, and neon signs have in common? In each case, we see the brilliant colours
because the atoms and molecules are emitting energy in the form of visible light. The chemistry of
an element strongly depends on the arrangement of the electrons. Electrons in an atom are
normally found in the lowest energy level called the ground state. However, they can be "excited" to
a higher energy level if given the right amount of energy, usually in the form of heat or electricity.
Once the electron is excited to a higher energy level, it quickly loses the energy and "relaxes" back to
a more stable, lower energy level. If the energy released is the same amount as the energy that
makes up visible light, the element produces a colour.
Aim:
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Observe/record the colours produced by metal salts in a flame
Use flame colours to identify unknown salts
Observe/record line spectra of hydrogen gas
Calculate frequency and energy values of specific wavelengths
Apparatus & Materials:
We need the following apparatus and materials:
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A nichrome wire
10ml HCl solution
A series of metal chlorides
2 unknown metal chlorides
A Bunsen burner
Matches
Two watch glasses
A measuring pot
Clean water
A hydrogen gas discharge tube
A spectrometer
Safety
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Wear safety goggles, lab coat and tie back long hair
Work in a well ventilated area
Don’t wear baggy clothing in the lab
Don’t touch any of the chemicals with hands. Wash well with water immediately if
you touch chemicals accidentally.
Use caution with the burner.
Collect in chemical waste container to be professionally cleared away.
Wash your hands with soap and water after you complete the day’s lab work, even if
you didn’t touch any chemicals directly.
Method:
Part A – Flame Tests:
1. Clean a nichrome wire by dipping into HCl solution. Heat the wire in the heating flame of a
Bunsen until no more colours is produced.
2. Mix a little of the salt to be tested with the acid solution on a watch glass and dip the end of the
clean wire in the solution.
3. Hold this end of the wire in the outer blue part of the flame and note any flashes of colour that
may appear and the intensity. Note that these may fade quickly.
4. Test subsequent salts in the same manner, cleaning the wire between each test as described in #1.
5. Tabulate your results making sure to include qualitative and quantitative data.
6. Use the information you have collected to identify the cations in each of the unknown samples
provided to you.
Part B – Line Spectra of Gases:
1. Go to the station where a hydrogen gas discharge tube is set up. Here, an induction coil is being
used to provide energy to “exite” the gas atoms.
2. Using the spectrometer, look at one of the fluorescent light in the room. You will see coloured
lines corresponding to specific wavelengths emitted by the white light.
3. Record the values for the violet and green lines. These should be 4360 Å and 5460 Å respectively.
Note any error in these readings so that adjustments can be made in subsequent readings.
4. Turn on the electricity to the hydrogen gas tube (for no more than 30 sec at one time to prevent
burn out).
5. Look through the spectrometer and observe/record the coloured lines that are produced.
6. Tabulate your results, including qualitative and quantitative data.
7. Process your data thoroughly, providing as much information as possible about each line found
including error and uncertainty.
Results:
Colours Produced By Metal Salts In A Flame
Metal Salts
Sodium Chloride (NaCl)
Calcium Chloride (CaCl2)
Copper(II) Chloride (CuCl2)
Potassium Chloride (KCl)
Magnesium Chloride (MgCl2)
Iron(III) Chloride (FeCl3)
Lithium Chloride (LiCl)
Unknown A
Unknown B
Unknown A might be FeCl3
Unknown B might be KCl
Colour
Bright yellow-orange
Yellow to red
Bright green
Purple-blue
Yellow and green
Bright yellow
Carmine-red
Bright yellow
Purple-blue
Line Spectra of Hydrogen Gas:
Colour
Violet
Green
Orange
Red
Wavelength/ Å
4300
5500
5800
6200
vviolet =c/λ=(3.00×108m.s-1)÷(4300×10-10m)=6.9767×1014 Hz
Eviolet=hvviolet= (4.00×10-13 kJ.s.mol-1)×( 6.9767×1014 Hz)=297 kJ.mol-1
vgreen =c/λ=(3.00×108m.s-1)÷(5500×10-10m)=5.4545×1014 Hz
Egreen=hvgreen= (4.00×10-13 kJ.s.mol-1)×( 5.4545×1014 Hz=218 kJ.mol-1
vorange =c/λ=(3.00×108m.s-1)÷(5800×10-10m)=5.1724×1014 Hz
Eorange=hvorange= (4.00×10-13 kJ.s.mol-1)×( 5.1724×1014 Hz=207 kJ.mol-1
vred =c/λ=(3.00×108m.s-1)÷(6200×10-10m)=4.8387×1014 Hz
Ered=hvred= (4.00×10-13 kJ.s.mol-1)×( 4.8387×1014 Hz=194 kJ.mol-1
Discussion:
Flame colours are produced from the movement of the electrons in the metal ions present in the
compounds. When I use the flame to heat it, the electrons gain energy and can jump into any of the
empty orbitals at higher levels, because the electrons are now at a higher and more energetically
unstable level, they tend to fall back down to where they were before. An electron which had been
excited from the 2p level to an orbital in the 7 level, for example, might jump back to the 2p level in
one go. That would release a certain amount of energy which would be seen as light of a particular
colour. However, it might jump back in two or more stages. Each of these jumps involves a specific
amount of energy being released as light energy, and each corresponds to a particular colour. As a
result of all these jumps, a spectrum of coloured lines will be produced. The colour you see will be a
combination of all these individual colours. The exact sizes of the possible jumps in energy terms
vary from one metal ion to another. That means that each different ion will have a different pattern
of spectral lines, and so a different flame colour.
This experiment used to visually determine the identity of an unknown metal or metalloid ion. It
based on the characteristic colour the salt turns in the flame. The heat of the flame converts the
metal ions into atoms which become excited and emit visible light. The atom can lose energy by
emitting a photon. The colour of the photon depends on the energy difference between the upper
and lower energy levels.
Spectroscopy is the analysis of light spectra and the way in which light interacts with matter. When
light is analysed it is commonly separated into its component colours. The light source is directed on
a slit and the "beam" of light is separated using a prism or grating. The reason that the images are
lines is that the light from the lamp is focused on a narrow slit. The illustration shows the separation
of a light beam into its component colours. This produces an image of the slit which has the shape of
a line. The resulting beam of light can be broken into the colour spectrum or into its components of
the spectrum emitted by the atom. You can see the specific colours emitted by the light source. A
white light source will give a spectrum like the one shown above.
Evaluation:
My results are accurate for the method and set up I repeated the experiment for each metal salts
twice in order to minimise the impact of human error and differences. My results were also fairly
similar which indicated that there was only one drastic outlier which needed to be omitted from my
results.
Metal Salts
Sodium Chloride (NaCl)
Calcium Chloride (CaCl2)
Copper(II) Chloride (CuCl2)
Potassium Chloride (KCl)
Magnesium Chloride (MgCl2)
Iron(III) Chloride (FeCl3)
Lithium Chloride (LiCl)
Symbol
Colour
Bright yellow-orange
Yellow to red
Bright green
Bright violet
Yellow and green
Bright yellow
Carmine-red
Name
Colour
Na
Sodium
Intense yellow
Ca
Calcium
Brick red
Cu(II)
Copper(II) (non-halide) Green
K
Potassium
Lilac(violet)
Mg
Magnesium
No colour
Fe
Iron
Gold
Li
Lithium
Pink-Red
Using my table of the flame test compared to the table from Wikipedia, there is drastic outlier due
to there is no colour imparted to the flame by magnesium chloride. This indicates that neither
magnesium nor chlorine has a characteristic flame colour. A few tinges of yellow-orange sodium
colour appear as a consequence of traces of sodium impurity in the magnesium chloride solution.
Colour
Violet
Green
Orange
Red
Wavelength/ Å
4300
5500
5800
6200
Colour
Representative
Wavelength (nm)
Wavelength
Region (nm)
Violet
420
400 - 440
Blue
455
440 - 470
Blue-green
480
470 - 490
Green
525
490 - 560
Yellow-green
565
560 - 570
Yellow
580
570 - 585
Orange
620
585 - 630
Red
660
630 - 700
Using my table of hydrogen’s wavelenth compared to the table from Science Curriculum, I can find
that all my data is in the available range.
This experiment is fast and easy to perform, and does not require any equipment not usually found
in a chemistry laboratory. However, this test cannot differentiate between all elements. Several
metals produce the same flame colour. It will be good to do more times of this experiment to ensure
the data and use different nichrome wires to do each salt then I can collect more corrective data,
because the nichrome wire is hard to be completely clean. For part B, the spectrometer presents the
colour for hydrogen gas, but it is not clear enough to get an exact data for the wavelength.
It would be interesting to investigate why some of the metal salts presented the same colour but
some didn’t. It would also be interesting to find the wavelength of each colour presents in a flame by
using the spectrometer.
Conclusion:
Through this experiment, I record the colours produced by metal salts in a flame and use flame
colours to identify the two unknown salts. I also observed line spectra of hydrogen salts and use the
data I collected to calculate frequency and energy values of hydrogen wavelengths. Whilst my
results were similar to those provided in Wikipedia they were drastically less and subsequently
whilst I have been able to prove that pattern of results is supported the values remain unverified.
Marks
References:
(n.d.). Retrieved from http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch6/bohr.html
about.com. (n.d.). Retrieved from
http://chemistry.about.com/od/analyticalchemistry/a/flametest.htm
chemistry comes alive. (n.d.). Retrieved from
http://jchemed.chem.wisc.edu/JCESoft/CCA/CCA2/MAIN/FLAME/CD2R1.HTM
creative chemistry. (n.d.). Retrieved from http://www.creativechemistry.org.uk/activities/flametests.htm
Lab: Flame Tests. (n.d.). Retrieved from http://kaffee.50webs.com/Science/labs/Chem/LabFlame.Tests.html
praticalchemistry. (n.d.). Retrieved from http://www.practicalchemistry.org/experiments/flamecolours-a-demonstration,102,EX.html
wikipedia. (n.d.). Retrieved from http://en.wikipedia.org/wiki/Flame_test