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In this experiment, you will apply the concept of quantized atomic energy and interpret the relationship between color and wavelength. Elements will then be identified using a flame test and the electron configuration of known elements will be determined.
· CaCl2 solid (SDS)
· LiCl solid (SDS)
· KCl solid (SDS)
· NaCl solid (SDS)
Equipment from Home
· Matches or lighter
· Distilled water
· Permanent marker
· Cell phone camera
Equipment from Kit
· inoculating loop
· 10 mL graduated cylinder
· 150 mL beaker
· Stir rod
· 4 test tubes
· test tube rack
· weigh boats
· tealight candle
When hydrogen and other gaseous substances are heated they will emit light with a few characteristic frequencies. This is because atomic energy levels are quantized, meaning electrons orbiting atoms populate discrete energy levels. In contrast, matter will absorb discrete frequencies of light, which is the exact opposite of emission. When light is absorbed by a molecule, an electron undergoes a transition from a lower energy level to a higher energy level. The energy that is absorbed is given by Einstein’s equation, E = h. Therefore, light with a frequency, , is absorbed and other frequencies are transmitted. Frequency is directly related to wavelength by the equation: c = where c is the speed of light and is constant in a vacuum.
Color of Different Wavelengths
Electrons of different elements absorb and emit different amounts of energy. The energy is associated with a certain frequency and wavelength. The colors that we observe are associated with specific ranges of wavelengths as shown in Table 1. By observing the color of light emitted by a substance when its atoms absorb and emit photons the identity of the element can be determined. This is how fireworks emit different colors of light. For example, copper produces a blue flame and barium a green flame.
Since colors are indications of the wavelength of the energy that is emitted, flame tests can be used to identify certain elements. Not all metal elements give flame colors, but for Group 1 compounds, flame tests are usually by far the easiest way of identifying which metal you have got. For other metals, there are usually other easy methods that are more reliable – but the flame test can give a useful hint as to where to look.
Part 1: Preparing solutions
1. Obtain all necessary supplies from the list of materials. Put on the safety goggles. Read all safety information in the Safety Data Sheets (SDS)
2. Put 4 test tubes in the test tube rack and label them 1, 2, 3, and 4.
3. Use the 10-mL graduated cylinder to measure 10 mL of distilled water. Pour the water in an empty 150 mL beaker.
4. Turn on the scale and place a piece of weighing paper on it. Zero the scale with the paper on it.
5. Measure 7.6 g of CaCl2 on the scale and add it to the beaker of water.
6. Use the stir rod to mix until the majority of the solid has dissolved.
7. Pour the solution into test tube 1. Discard the remaining solution and rinse the beaker with distilled water.
8. Repeat steps #2 – #4 for the next solution.
9. Measure 9.3 g of LiCl on the scale and add it to the beaker of water.
10. Use the stir rod to mix until the majority of the solid has dissolved.
11. Pour the solution into test tube 2. Discard the remaining solution and rinse the beaker with distilled water.
12. Repeat steps #2 – #4 for the next solution.
13. Measure 3.5 g of KCl on the scale and add it to the beaker of water.
14. Use the stir rod to mix until the majority of the solid has dissolved.
15. Pour the solution into test tube 3. Discard the remaining solution and rinse the beaker with distilled water.
16. Repeat steps #2 – #4 for the next solution.
17. Measure 3.6 g of NaCl on the scale and add it to the beaker of water.
18. Use the stir rod to mix until the majority of the solid has dissolved.
19. Pour the solution into test tube 4. Discard the remaining solution and rinse the beaker with distilled water.
20. Take a picture of your solutions in the test tubes and include with your lab report.
Part 2: Flame Tests
CAUTION: Wear your safety glasses and choose an area that is well-ventilated. Know the location of your fire extinguisher. The fuel source should be on a flat surface. Never leave any burning fuel unattended!
21. Obtain the tealight candle from the kit. Make sure the tealight candle is away from any flammable materials and that you are in a well-ventilated area.
22. Use a match of lighter to ignite the fuel source.
CAUTION: Lit matches and open flames can cause a fire or burns to skin and/or clothing. Be sure you have your safety glasses on!
23. Hold the inoculating loop at the very end of the non-looped end in order to avoid burns. Heat the looped end in the flame until its loop is faintly orange and any coating is burned off.
CAUTION: The loop will be hot for several minutes so do not touch the loop!
24. Dip the loop into the CaCl2 solution (test tube 1).
25. Move the looped end of the inoculating loop into the flame. Make observations about what is happening, noting any color changes. The color change will be most apparent around the edges of the flame.
26. Repeat Steps #23 and #24 as needed to ensure that you are able to see the colors.
27. Record your observations in the Results table.
28. Rinse the inoculating loop with distilled water and pat it dry with a paper towel.
29. Repeat Steps #24 – #28 for the LiCl solution (test tube 2).
30. Repeat Steps #24 – #28 for the KCl solution (test tube 3).
31. Repeat Steps #24 – #28 for the NaCl solution (test tube 4).
32. Extinguish the flame and return the cover.
33. Clean and dry all equipment and return to your lab kit. Make sure the inoculating loop has time to cool before you touch it.
34. Any unused chemicals and solutions can be disposed of in the sink. Make sure to rinse with plenty of water.
35. Wash your hands.
1. What is the difference between absorbance and emission of light? What happens to an electron when a molecule absorbs a photon of light?
2. If a flame test on a solution shows the color blue, what range of wavelengths does this correspond to?
3. If a flame test on a solution shows the color red, what range of wavelengths does this correspond to?
4. Calculate the corresponding wavelength () for each frequency listed below. (SHOW CALCULATIONS)
a. 5.45 x 1014 Hz
b. 6.90 x 1016 Hz
5. Calculate the corresponding frequency () and energy (E) for each wavelength of visible light listed below. (SHOW CALCULATIONS)
a. 425 nm
b. 685 nm
|Solution||Observations of color|
1. Identify the color emitted by each of the salts along with the corresponding approximate wavelength using Table 1 in the Introduction. Record your results in the table below.
2. Why does a salt compound give off light (or a colored flame) when burned?
3. Barium chloride (BaCl2) emits a green color when flame-tested. What can be said about the wavelength of light it emits?
4. Four different salt compounds were tested in this lab. For each wavelength determined in Question #1, calculate the frequency, , and the energy, E. (SHOW ALL CALCULATIONS)
a. Ca = ______________ E = _____________
b. Li = ______________ E = _____________
c. K = ______________ E = _____________
d. Na = ______________ E = _____________