Student Worksheet
LSM 3.LA.5
Activity 3.4.12Line Spectra
spectrum.Some
All substances
absorbor emit somepart of the electromagnetic
may absorb
absorbsomeofthe visiblespectrum,while other substances
substances
in the ultravioletregionor other regions.The spectrumproducedis calleda darkcan alsoemit light in differentpartsofthe
line or absorptionspectrum.Substances
spectrum.
Under
certain
conditions,the light emitted appearsas
electromagnetic
bright lines.The spectrumproducedis calleda bright-line spectrum.The purpose
of this activity is to illustratethe two main typesof line spectra-brightJine and
absorption.
Materials
overheadprojector
2 piecesof heavypaper
diffractiongrating (600lines/mm)
2 largebeakers
flat glassplateto coverbeaker
PUrewarer
and of iodine
tiny crystalsof potassiumpermanganate
spectrumtube powersupply
hydrogengastube
spectroscope
Seriousshockhazardmay
frll)l
resultfrom the use ot a high\.t
vottagesuppty.
An overheadprojectoris setup to shineits light on a white wall or board.Two
piecesof hea\y paperare placedabout 5 mm aparton the horistraight-edged
zontalstageof the projectorso that they block all of the light exceptfor a thin
strip.
. A diffractiongrating (containingabout 6000 7500lines/cm)is hung in the
centreof the verticalprojectinglensusinga pieceof plastictape.
. Placea largecleanbeakeron the slit formedby the two piecesofpaper. Shut
off the room lights and observethe continuousvisiblespectrum.
. Now fill the beakerwith somepure waterand observethe spectrumagain.
(a) Do eitherglassor waterchangethe coloursin the visiblespectrum?Do
absorbvisiblelieht?
thesesubstances
.
Add a few crystalsof potassium permanganateto the water and stir. Observe
the visiblespectrum.
(b) What effectdoesaqueouspotassiumpermanganatehaveon the visible
spectrum?
.
Warm some solid iodine in a beakerin a fume hood or by some other safe
method. Cover the beakerwith a flat glassplate.Placethe beakercontaining the
iodine vapour on the overheadprojector and observethe spectrum.
(c'ontinued)
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Chapter3 Atomic Theories
C o p y r i g h tO 2 0 0 3N e l s o n
LSM 3.LA-5
(c) Comparethe spectrumobtainedto the one for the potassiumpermanganatesolution in step5. Can gasesalsoabsorbelectromagneticradiation?
Observethe spectrum of sunlight recordedon Earth's surface(Figure 2).
Figure 2
T h e s p e c t r u mo f s u n l i g h tr e c o r d e do n
the surfaceof Earth is a dark-line
[absorption]spectrumdue to gases in
the atmospheresof Earth and the Sun
absorbingspecificparts of the sunlight.
(d) What evidenceis there that some light is being absorbed?Suggestsome
possiblegasesthat might be responsible.
Set up a hydrogengastube in a gasdischargepower supply.Switch on the
power and turn off all lights.Observethe spectrurnwith a handheld spectroscope.Draw and label a diagram of this spectrum.If tin-repermits,observethe
spectraof other gases.
(e) Under what cor-rditions
do gasesproducelight that is in the visibleregion
of the electromagneticspectrum?
(f) In this case,is the spectrumproduceda bright-line or continuous
spectrum?
(g) How are line spectraused in chemicalanalysis?
C o p y r i g h t@ 2 0 0 3N e l s o n
Chapter-l Atomic Theories 113