LAB 2: FLAMES AND ATOMIC SPECTRA:
BURNERS, COMBUSTION, AND EXCITATION OF ELECTRONS
PURPOSE: To become familiar with the operation of common laboratory burners.
To determine the effect of oxygen on flames.
To observe the colored spectrum characteristic of common ions.
To identify an element from its atomic spectrum.
SAFETY CONCERNS:
Always wear safety goggles.
Tie back long hair when using flames.
BURNERS:
In 1855 the German chemist Robert Bunsen popularized a burner that mixed air with flammable
gas to produce a hot, smokeless flame. This became the forerunner of the gas stove. Today, all
laboratory burners are commonly referred to as Bunsen burners even if they are modifications of
his design. The common laboratory burner is made from a metal tube attached to a base
connected to a gas source or gas outlet valve on the laboratory workbench. The amount of gas
entering the burner can be controlled by turning the gas inlet or gas adjustment valve at the
base of the burner. The original Bunsen burner design did not have a gas adjustment valve but
most modifications, such as the Tirrill burner, do. Do not use the gas inlet valve to turn the
burner off, as gas will still be flowing from the main gas source and cause potential leakage. The
flow of gas fuel should be started and stopped by opening or closing the source at the bench.
Depending on your burner type, the supply of air entering the barrel of the burner is regulated
either by adjusting air vents at the base or by screwing the barrel up or down on the base. When
turning knobs a general rule is: “righty tighty, lefty loosey”, which means that turning a knob
right will usually turn the flow off and turning a knob to the left will usually turn the flow on.
COMBUSTION:
The most common fuels we burn to produce heat and light belong to a group of chemical
compounds called hydrocarbons. Hydrocarbons are made from the elements carbon, C, and
hydrogen, H. Wood for our campfires, gasoline for our cars, and candles for our birthday cakes
are all composed of hydrocarbons. When hydrocarbons are combusted (burned) the original
pattern of carbons and hydrogens in the compound is broken and each of the elements, C’s and
H’s, combines with oxygen from the air. Carbons, C, combine with oxygen, O, to form new
compounds called oxides of carbon. Either carbon monoxide, CO, or carbon dioxide, CO2, are
formed depending on the number of oxygens, O’s, available. Hydrogens, H’s, combine with
oxygen, O, to form the familiar oxide of hydrogen, H2O, called water.
When there is an abundance of oxygen present in the burning of a hydrocarbon, then there is
enough oxygen, O, to allow each atom of carbon, C, to join with two oxygens, O, to make carbon
dioxide, CO2, a colorless, odorless, gas. This is called “complete combustion” of a
hydrocarbon fuel because each carbon atom has been completely oxidized. Each carbon has
bonded with the maximum number of oxygens possible.
CH110 Lab 2: Flames & Atomic Spectra (W14)
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Without enough oxygen present some carbons, C’s, may find only one oxygen, O, with which to
combine to form the colorless, odorless gas carbon monoxide, CO. Other carbons, C’s, may not
find any oxygen at all and be left as elemental black carbon soot, C. Elemental carbon soot, C,
and carbon monoxide, CO, are the results of having incomplete oxidation of the hydrocarbon
fuel. Burning a hydrocarbon without enough oxygen to completely oxidize each carbon is called
“incomplete combustion”.
When a laboratory burner is burning hydrocarbon fuel with plenty of oxygen in the process of
complete combustion it makes a loud roaring noise and appears as a clean blue flame with two
distinct blue cones in the middle. This flame is nonluminous (does not produce light) and is
called a “hot flame”.
When a laboratory burner is burning hydrocarbon fuel with an insufficient supply of oxygen in
the process of incomplete combustion it produces solid carbon (C) and gaseous carbon monoxide
(CO). This is called a “cold flame”. The tiny, hot, glowing pieces of solid carbon become
incandescent and produce a very quiet and billowy yellow, luminous (light-producing) flame.
Objects heated with a luminous flame become coated with black soot (unburned carbon). The
carbon monoxide produced by incomplete burning of methane is a toxic, colorless, odorless gas.
Hot air balloons are equipped with two kinds of burners to provide the hot air needed to lift the
balloon off the ground. The preferred burner burns very cleanly so as not to fill the balloon with
soot, but it is also quite noisy. The other burner is nick named the “cow burner” gets used when
flying over animals that may be startled by noise. It is very quiet but produces more soot.
The principal component of natural gas used in most laboratories is methane (CH4). Because
methane is odorless, a gaseous substance (ethanethiol) possessing a distinct odor is added to the
methane so that gas leaks may easily be detected.
CHEMICAL EQUATIONS:
A way to describe the change of one chemical compound into another is by using a chemical
equation. A chemical equation shows the number and formulas of all compounds that react
together (reactants) on the left side of a reaction arrow, and the number and formulas of all
compounds produced (products) on the right.
The chemical equation to describe the complete combustion of a common hydrocarbon fuel,
propane (C3H8), is as follows:
C3H8(g) + 5O2(g)  3CO2(g) + 4H2O(g)
This chemical equation for the complete combustion of propane can be translated into words as follows:
C3H8(g) + 5O2(g)  3CO2(g) + 4H2O(g)
one molecule of propane gas will react with five molecules of oxygen gas to produce three molecules of carbon dioxide gas and four molecules of water vapor.
If no number is displayed before a molecular formula the number is assumed to be 1. Oxygen,
in a chemical equation, is written O2 instead of just O because, like socks, they always start out
in pairs. The elements that occur in pairs when they are alone are called diatomic molecules.
The elements that occur as diatomic molecules are H, O, F, Br, I, N, and Cl.
12
CH110 Lab 2: Flames & Atomic Spectra (W14)
ATOMIC SPECTRA:
Sunlight can be split by water droplets into the spectrum of light called a rainbow. This same
phenomenon can be observed when a beam of white light is passed through a prism. These
observations led early scientists to the conclusion that white light was actually made up of all
colors and that under the proper conditions colors could be separated from one another. The type
of spectrum resulting from sunlight or white light is called a continuous spectrum because all
the colors are observed in a continuous span from red to violet.
Many compounds, when placed in a flame, emit a characteristic color. Copper burns green,
sodium burns yellow, lithium burns red, etc… The brilliant colors of fireworks displays are the
result of burning metal atoms arranged in patterns. When passed through a prism, the colored
light from individual burning compounds does not reveal a continuous spectrum but splits into a
discrete spectrum containing lines of only certain colors with gaps in between.
Hydrogen, for example, when excited by an electric discharge inside a light bulb, gives off a
pink color. When the pink light from a hydrogen bulb is passed through a prism it gets separated
into four colored lines: violet, blue, green, and red.
Neils Bohr explained the discrete spectrum by relating it to the electrons in the atoms. An atom
in its normal state has its electrons occupying the orbitals of lowest possible energy. This is
called the ground (energy) state of the atom. If energy is added to an atom, some of the
electrons in the atom absorb that energy and jump into an orbital of high energy and an excited
(energy) state is formed.
One way of viewing the energy levels in an atom is by picturing a stairway. Each stair
represents an energy level. Added energy can kick an electron up to a higher level like a ball
could be kicked up to a higher step on a stairway.
A ball that has been kicked up the stairs will fall back down the steps giving back the energy that
was added to it. Likewise, an atom that is excited is unstable and will immediately give up the
energy added and return to the ground state.
When a ball is kicked up the stairs there are options of how it may fall back down. It can fall all
the way back to the ground without hitting any of the steps in between or it can bounce off some
of the middle steps on its way back down. Electrons in an excited atom can fall back to the
ground state all at once or take shorter leaps and go from level to level. Not all “steps” in an
atom are the same height. Each time an electron moves from one level to another one, a photon
of light is given off. The particular wavelength of the photon is determined by the “height of the
step”. In some cases the energy of the emitted light is such that we can sense it with our eyes as
color. For example red light is a low energy light and blue light is higher energy light of
particular wavelengths that we can see. The further an electron falls, the higher the energy
emitted. Therefore, the color of the emitted light is an indication of the energy spacing of the
orbitals in the atom.
CH110 Lab 2: Flames & Atomic Spectra (W14)
13
POSSIBLE ENERGY TRANSITIONS OF LITHIUM:
Lithium has three electrons. In the ground state of lithium two electrons abide in the first shell
and one electron in the second shell. Each shell is made up of various orbitals; s, p, d, or f.
Orbitals are areas that represent where an electron may be found within a shell.
The one electron in the 2s orbital of lithium can be “kicked” up to higher levels by added energy.
Each “up” arrow on the following diagram shows the 2s electron getting kicked to a higher
orbital. Each “down” arrow show the possible jumps the electron can make on its way back
down.
4s
3p
3s
2p
2s
1s
Colored lines in a spectrum are emitted when the electrons fall. The crimson color of lithium
burning in a flame is the sum of the individual colors produced.
IDENTIFICATION OF ATOMS:
The patterns created by excitation of atoms by electricity or flame are not the result of a chemical
reaction. The kind of color given off by each atom is a physical property so exact that it can be
used to identify the element. The line spectrum caused by the excitation of neon atoms by
electricity is unique and makes the red glow of a neon sign unique from that of a sodium or
mercury street lamp. The specific pattern of the hydrogen spectrum is used like a fingerprint to
identify burning hydrogen gas emitted from stars.
In this laboratory exercise you will view the emissions from lamps containing elements in the
gas phase excited by electricity, and you will view the emissions of elements excited by the
energy of flames. By comparing the spectrum of known samples you will be able to identify an
element sample unknown to you.
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CH110 Lab 2: Flames & Atomic Spectra (W14)
NOTES:
PROCEDURES:
1
ACTIONS:
I. LABORATORY BURNER:
1. Using the drawing of the laboratory burner provided on the
report sheet (Box I) as a beginning, draw in and label the
following:
A. Gas source valve
B. Gas inlet valve
C. Air inlet
II. COLD FLAME:
2. Light your burner1 by turning on the gas2 at the source and
quickly igniting the gas and air mixture at the top of the
burner tube using a match or striker.
3. With the burner lit, close off the air supply by turning the
metal air inlet tube to the right (screw it down). On the report
sheet (Box IIA) draw a picture and describe with words the
appearance, colors, and sound of this flame formed when
there is a limited supply of oxygen.3
4. With tongs, hold a clean, dry porcelain evaporating dish in the
middle of the oxygen deficient flame. In the space provided
on the report sheet (Box IIB), describe and complete the
drawing of the resulting dish.4
You may have to adjust
the gas inlet valve to allow
enough steady gas flow to
keep the burner going
without blowing out the
flame.
2
The fuel we are burning is
a hydrocarbon gas made
from the elements carbon,
(C), and hydrogen, (H).
When hydrocarbons are
burned or combusted the
carbons, C, combine with
oxygen, O, to form carbon
monoxide, CO, or carbon
dioxide, CO2 and the
hydrogen, H, combines
with oxygen, O, to form
water, H2O.
3
You now have a flame that
has very little added
oxygen.
It would be
similar to the flame of a
candle that is not equipped
with an additional air inlet
source. This is called a
“cold” flame.
4
III. HOT FLAME:
5. Open the air inlet of the lit laboratory burner by turning the
metal air inlet tube to the left (unscrew it) to increase the
amount of oxygen supplied. Allow enough oxygen to enter
the tube to make a blue cone about 5-6 cm (2 in) high. Use
the report sheet diagram (Box IIIA) and describe with words
the appearance, colors, and sound of this flame formed
when there is an abundant supply of oxygen.5
6. A. Hold a piece of iron wire with your forceps in the flame
about 1.5-2 inches above the top of the blue cone (zone 1).
Observe how brightly the wire glows and record your
observation in Box IIIA. Predict the temperature of this part
of the flame. 6
7. B. Move the piece of iron wire with your forceps to the top
outside edge of the blue cone (about ¼ to ½ inch above it)
(zone 2). Describe (Box IIIA) the glow of the wire and predict
the temperature of this part of the flame. 6
8. C. Move the piece of iron wire with your forceps to the inside
of the blue cone (zone 3) . Describe (Box IIIA) the glow of the
wire and predict the temperature of this part of the flame. 6
CH110 Lab 2: Flames & Atomic Spectra (W14)
The black soot you’ve
collected on the porcelain
dish is the element carbon,
(C). It occurs when there is
not enough oxygen present
to combine with all of the
carbons in the fuel so some
free carbons get left alone.
Some of the carbons that do
get oxygen get only enough
to form carbon monoxide,
CO.
5
You now have a flame that
has a lot of added oxygen.
This is called a “hot”
flame.
6
Temperature of a flame:
Color of a
Temp
Hot Wire:
°C
no light
< 500
not quite red
500-550
dark red
650-750
bright red
850-950
yellowish red
1050-1150
not quite white 1250-1350
white
> 1450
15
9. With tongs, hold a clean, dry, porcelain evaporating dish above
the tip of the blue cone of the oxygen abundant flame. In the
space provided on the report sheet (Box IIIB), describe with
words and complete the drawing of the resulting dish.7
10. Hold an index card on the metal burner tube in your hot flame
just until the paper begins to burn. Quickly quench the burning
card with a stream of running water. In Box IIIC Draw a
picture of the burnt card and explain what happened. 7
11. Extinguish your hot flame. Push a straight pin through the
paper immediately below the head of a match. Hang the
bottom of the match in the center of the barrel of your unlit
burner, supported by the pin. Carefully light the burner
without lighting the match. Use the drawing and describe the
results of the match in the flame on the report sheet (Box IIID).
IV. FLAME TESTS: 8
1. Clean a platinum or nichrome wire containing a small loop by
rinsing with a few drops of 1.0 M HCl and then heating it in
the hottest part of a “hot” flame9 until no color is present.
7
There is less of the black
sooty carbon
formed
because there are enough
oxygens
available
to
combine with all of the
carbons to make gaseous
carbon dioxide, CO2.
Fewer C’s get left over
alone as C, and fewer C’s
get turned into carbon
monoxide, CO.
8
To avoid contamination
of wires and burners your
instructor may choose to
set up sample stations
through which students
can rotate. Each station
can be set up for a
particular sample, with
wires and burners for use
with that sample only.
9
2. Wet the clean platinum or nichrome wire with deionized water.
Dip the wet wire into some solid to be tested until the wire is
coated with the compound.
3. Hold the wired, coated with the solid to be tested, in the hottest
part of your flame.
4. Observe the immediate color produced by each burning sample
and record it on your report sheet.
V. IDENTIFICATION OF AN UNKNOWN:
1. For an unknown sample provided (either gas or solid) record
the color and observations from the appropriate excitation
process; (line spectrum of an electrically charged gas or flame
test for a powdered solid.)
2. Identify the sample by comparing your results to those of your
known samples, and record it on your report sheet.
VI. EXCITATION OF GASES:
12. Observe, and record on your report sheet, the color given by
the various electrically charged gases.
13. For each electrically excited gas, view the sample through a
prism (spectroscope) and with colored pencils sketch the
pattern of the discrete line spectrum you observe.10
16
CH110 Lab 2: Flames & Atomic Spectra (W14)
Remember
that
the
hottest part of a hot flame
is just above the blue
cone.
10
Put the colors in order as
they appear and leave the
appropriate sized spaces
between the colored lines.
If you do not have the
appropriate colored pencil
simply label the color
present.
LAB 2: FLAMES & ATOMIC SPECTRA
PRE LAB EXERCISES:
NAME_____________
DATE______________
1.__
A Hydrocarbon can best be described as
A a compound made from carbon and water.
B. a water soluble carbon compound.
C. a compound made of primarily carbon and hydrogen.
2.__
At what place should we shut off the laboratory burner?
A. The air inlet valve
B. The gas source
C. The gas inlet valve
D. All of these are fine.
3.__
Incomplete combustion is most likely to result from
A. running a car engine in a closed garage.
B. cooking on a barbeque grill indoors with the windows and doors closed.
C. cooking on a propane grill outside.
D. Both A and B.
E. All of these.
4.__
Automobile carburetors allow air to mix with the gasoline fuel that provides energy for
the car to move. A carburetor needs to be adjusted to maintain the proper oxygen to fuel
ratio. If the oxygen to fuel ratio is high (meaning lots of O’s compared to C’s) the result
is
A. lots of CO2, little if any CO, and little if any C.
B. lots of CO2, lots of CO, and little if any C.
C. little if any CO2, lots of CO, and lots of C.
D. little if any CO2, little if any CO, and lots of C.
5.__
What is the name for the series of colored lines we see when we look at a light bulb
through a spectroscope?
A. continuous spectrum
B. discrete spectrum
C. rainbow
D. white light
6. Explain briefly IN YOUR OWN WORDS what is responsible for the various color spectra
of each ion or element. (What causes the colors?)
CH110 Lab 2: Flames & Atomic Spectra (W14)
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CH110 Lab 2: Flames & Atomic Spectra (W14)
LAB 2: FLAMES & ATOMIC SPECTRA
REPORT:
NAME___________________
PARTNER_________DATE___
I. LABORATORY BURNER
III. HOT FLAME
I.
IIIA.
Appearance
Colors
Sound
(Zone 1; color/temp)
(Zone 2; color/temp)
(Zone 3; color/temp)
IIIB.
II. COLD FLAME
IIA.
Appearance
Colors
IIIC.
Sound
IIB.
IIID.
CH110 Lab 2: Flames & Atomic Spectra (W14)
19
IV. FLAME TESTS:
Compound
Composite
Color when
Excited
Compound
Composite
Color when
Excited
Compound
Lithium
(LiCl)
Sodium
(NaCl)
Potassium
(KCl)
Strontium
(SrCl2)
Barium
(BaCl2)
Copper
(CuCl2)
Composite
Color when
Excited
V. IDENTIFICATION OF AN UNKNOWN:
Unknown #
Composite Color when Excited
VI. EXCITATION OF GASES:
(In order from Indigo - Blue - Green - Yellow - Orange - Red)
Composite Color
when Excited =
Argon
(Ar)
Helium
(He)
Hydrogen
(H2)
Mercury
(Hg)
Neon
(Ne)
20
Conclusion/Identity:
Discrete line spectrum (show proper spacing)
Indigo
Blue
Green
Yellow
Orange
Red
Indigo
Blue
Green
Yellow
Orange
Red
Indigo
Blue
Green
Yellow
Orange
Red
Indigo
Blue
Green
Yellow
Orange
Red
Indigo
Blue
Green
Yellow
Orange
Red
Indigo
Blue
Green
Yellow
Orange
Red
CH110 Lab 2: Flames & Atomic Spectra (W14)
LAB 2: FLAMES & ATOMIC SPECTRA
NAME________
RELATED EXERCISES:
1.___ The center of the blue cone of a hot flame
A. is the best area to put an item you wish to heat.
C. is the hottest part of the flame.
B. contains no fire at all.
2.___ To give the highest heat to an object where should it be placed?
A. In the center of the blue cone of a hot flame.
B. In the center of a cold flame.
C. 5-6 cm above the inner cone of a hot flame.
D. At the outer tip of the inner cone of a hot flame.
3.___ The “cow burner” on a hot air balloon described in the lab discussion is an example of
A. incomplete combustion
B. complete combustion
4.___ Driving down the highway you see black soot coming out of the exhaust. The identity
(chemical composition) of the soot is
A. dirt
B. carbon monoxide
C. carbon dioxide
D. elemental carbon
5.___ Driving down the highway you see black soot coming out of the exhaust. How was the
black soot produced?
A. hot oil and dirt are blown out.
B. lack of oxygen causes incomplete combustion of the hydrocarbon fuel where carbons
don’t have enough oxygen to convert each C to CO2.
C. excess oxygen causes complete combustion of the hydrocarbon fuel where carbons
have enough oxygen to convert each C to CO2.
6.___ Driving down the highway you see black soot coming out of the exhaust. What
specifically should be done to avoid it?
A. Clean the soot deposits out of the engine.
B. Open the hood and stand and look at the engine.
C. Adjust the carburetor so that less oxygen is mixed with the fuel.
D. Adjust the carburetor so that more oxygen is mixed with the fuel.
7.___ A particular Fourth of July Fireworks display is brilliantly red when ignited. What
element might be present in that firework to make it red?
A. Sodium
B. Barium
C. Copper
D. Lithium
8.
Translate the following word description into a chemical equation for the incomplete
combustion reaction of propane. (Hint: Model after the word translation of the complete combustion
example in the discussion section at the beginning of this lab chapter.)
Two molecules of propane gas react with seven molecules of oxygen gas (O2 remember)
to form six molecules of carbon monoxide gas and eight molecules of water vapor.
CH110 Lab 2: Flames & Atomic Spectra (W14)
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CH110 Lab 2: Flames & Atomic Spectra (W14)