Energy can be added to atoms many different ways. It can be in the form of light, an electric discharge or heat. This added or extra energy is emitted when the excited electrons in the atoms give off light and fall back to lower shells. The light emitted has wavelengths and colors that depend on the amount of energy originally absorbed by the atoms. Usually each individual excited atom will emit one type of light. Since we have billions and billions of atoms we get billions of excitations and emissions.
Different elements emit different emission spectra when they are excited because each type of element has a unique energy shell or energy level system. Each element has a different set of emission colors because they have different energy level spacings. We will see the emission spectra or pattern of wavelengths (atomic spectra) emitted by six different elements in this lab. We will then identify an unknown element by comparing the color of the unknown with the flame color of our knowns.
White light is the combination of all colors of the spectrum.”
Materials
- 6 substances: (one spatula of each)
*SrCl2 (strontium chloride)
*CaCl2 (calcium chloride)
*NaCl (sodium chloride)
*KCl (potassium chloride)
*CuSO4 x 5H2O
*LiCl (lithium chloride)
- HCl (hydrochloric acid) – concentrated
Apparatus
- 1 beaker (for hydrochloric acid (HCl)) and cover
- Bunsen burner
- 6 petric glasses
- hand spectroscope
- electric lamp
- spatulas
- crucible tongs
- flame test wire
Procedure
- Wear your googles, gloves and gown before start working
- Swich on the lamp and look at the bulb through the hand spectroscope. Look for a series of colours, one running into the next. This is the continuous spectrum.
- Hold the spectroscope up to a window which does not face the sun. This could result in permament damage to your eyes. You should see the continuous spectrum of visible light.
- Make sure your flame test wire is clean.
- Prepare needed substances.
- Light the Bunsen burner.
- Dip the flame test wire into the hydrochloric acid and next take some strontium chloride on it and put it into Bunsen fire. Watch the colour of the flame. Your partner should be watching the flame through the hand spectroscope. Look for the brightly coloured lines. There are several lines for each element and it will be propably not possible to get them all into view at once.
- Dip the flame test wire again into the hydrochloric acid to clean it from the substance.
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Repeat steps 4-8 with calcium chloride (CaCl2), sodium chloride (NaCl), potassium chloride (KCl), CuSO4 x 5H2O and lithium chloride (LiCl)
Safety guidelines
!!! - Work areas should be arranged so that a person does not need to travel through a high-hazard area while attempting to exit the laboratory during an emergency.
!!! - Eyewashes and showers must be accessible to all chemical laboratories.
!!! - Do not consume food and drink in the laboratory.
!!! - Wear your googles and gloves to protect your eyes and skin
!!! – Move in the labolatory carefully
!!! – Keep the labolatory clean in order to prevent any accidents
!!! – Pay particular attention to the protection of eyes and skin because hydrochloric acid can couse damage to your body and clothes.
!!! – Confine long hair while working in a labolatory.
!!! – Do not panic if you have done something in a wrong way, just report your teacher and do what he/she has said
!!! – Flush away all accidentally spilled substances with water and report your teacher
!!! - Neutralize the remaining substances and remove them from equipment
!!! – Look out on the fire; put it off at the end of the experiment
!!! – Wear your gown while working in labolatory
Data collection
The results obtained from the experiment are as follows:
Conclusion
When the electrons in the element are excited by heating , they jump to higher energy levels As the electrons go back down, and leave the excited state, energy is re-emitted in the form of visible light of different colours.The emission spectrum can be used to determine the composition of a material, since it is different for each element of the periodic table. The example of use the flame test is to identify the composition of stars by analysing the received light.
Evaluation
Not all the lines in the spectrum were seen. I found in the literature and internet that for lithium chloride we should see 3 lines in the hand spectroscope (blue – 490 nm, orange – 625 nm and red 700 nm), more than 5 lines for strontium chloride (blue – 405 nm, 450 nm, 480 nm, green – 550 nm, red – 650 nm), more than 12 lines for calcium chloride (blue – 420 nm, 430 nm, 450 nm, 480 nm, 500 nm, 520 nm, green – 540nm, 570nm, yellow – 600 nm, orange – 630 nm, red – 675 nm, 680 nm), one line for sodium chloride (yellow – 600 nm), 6 lines for CuSO4 x 5H2O (blue – 455 nm, 510 nm, green – 520 nm, 530 nm, 575 nm and yellow – 590 nm) and 7 lines for potassium chloride (blue – 450 nm, 465 nm, 480 nm, 485 nm, 505 nm, green – 535 nm and yellow – 590 nm). The results of the experiment weren’t the same. It might be because of the fact we saw also the spectrum of electric lamps which were switched on and the shadows of other people working at the labolatory.
Ways of improving the results:
- less shadows of other people
- more concentration during the experiment
- use of more modern equipment
- looking throug the spectroscope in the dark room in order to avoid seeing the electric lamp’s light spectrum
- the experiment should be repeated to get some more reliable results
Sources
- “Chemistry for the IB Diploma (standard and higher level)” by Geoff Neuss
- “Chemistry” (second edition) by John Green and Sandru Damji