Molecularity: Quanta

1. Which of the atomic spectra produced by the light tubes is simplest (i.e., has fewest lines)?
2. Is there a pattern in the lines emitted by the gas in this tube? Try to describe the pattern.
3. An absorption spectrum is the complement of an emission spectrum; instead of measuring the wavelengths of light emitted by the atoms, we measure the wavelengths of light absorbed by the atoms when we pass light through them. Describe the relationship between this absorption spectrum and the corresponding emission spectrum.
4. Show that your data for hydrogen are consistent with the Rydberg Formula, and obtain a value for the Rydberg constant, R_H, from your data:

   1/l = R_H(1/4 - 1/n₂²)

   Begin by setting up a table that looks as follows:

   Wavelength, l	1/l	Guessed value of n2	Calculated value of RH

   
   Assign a different value of n₂ to each observed wavelength, then use each wavelength, n₂ pair to calculate the value of R_H. If all pairs give the same value of R_H, then you have, first, shown that the data are consistent with the Rydberg formula, and, second, you have obtained the value of R_H experimentally.
5. Is the light emitted in the flame tests continuous or discrete?
6. What color(s) is (are) transmitted by each color filter? What color(s) is (are) transmitted by each combination of 2 filters? By the combination of 3 filters?
7. The primary colors of light are considered to be red, blue, and green, so that red + blue + green gives white light. Are your experiments with filters consistent with this view? Fill in the missing colors below (each color is represented by its first letter):

   R + B + G = W
   R + B = __
   R + G = __
   B + G = __
   M + G = __
   C + R = __
   Y + B = __
   M + C + Y = __

• Nichrome wires for flame tests
• Bunsen burners
• spectroscopes
• power supplies for gas tubes
• gas tubes: H₂; He; Ne; Ar; Hg
• solutions of several common salts for flame tests: LiCl NaCl KCl SrCl₂ CaCl₂ BaCl₂
• Light filters: magenta, cyan, yellow, blue, green, red. These can be prepared by buffering a concentrated solution of Yamada's universal indicator at pH 4 or pH 7, and adjusting pH with HCl or NaOH to achieve the desired color.
• Light sticks

You will work with solutions of salts, some of which are toxic. Avoid skin contact. Clean up all spills immediately. In case of skin contact, flush with plenty of water.

Avoid looking directly at the light emitted by any of the light tubes used in this experiment. Some of them emit ultraviolet light. View the tubes using a spectroscope.

When analyzing sunlight with the spectroscope, DO NOT POINT THE SCOPE DIRECTLY AT THE SUN. You will obtain a satisfactory spectrum by simply looking out the window.

Record your observations in your notebook. Your instructor will tell you whether to carry out flame tests or observe light tubes first.

Part 1: Flame Tests

Set up a Bunsen burner, light it, and adjust for a blue (not orange) flame. Obtain a Nichrome wire and small volumes of the salt solutions. Clean the wire by holding the loop in the burner flame just above the inner pale blue cone and heating until you observe no color in the flame above the red-hot loop. Be sure to heat the wire until no color at all is emitted by the wire. Commonly, your wire will emit a yellow color indicative of the presence of Na⁺. Na⁺ is somewhat difficult to eliminate, but be sure to continue heating until all yellow color is gone.

Test each salt solution in turn by dipping the wire in the solution to obtain a film of solution spanning the loop, then holding the loop in the flame. You should perform the flame test of each solution at least three times to be sure that you obtain consistent, repeatable behavior. The flame test for potassium ion should be viewed through a cobalt blue glass filter. Be observant! Record what you see. In addition to the color observed, you should note the time required for color to first appear, the duration of the color, whether more than one color is seen and in what order, and so on. Be sure to clean the wire between flame tests by heating it until no color is seen.

Part 2: Light Tubes

NOTE: Only the row of fluorescent lights furthest from the windows should be on while you do these experiments.

Obtain a spectroscope. Begin by pointing the spectroscope out the window. Make careful observations. Record what you see. Then point the scope at one of the room fluorescent lights. Record what you see. In turn, view each of the light tubes set up in the lab, getting the spectroscope slit as close to the light tube as possible. Record your observations, being as quantitative as possible. Finally, view the tungsten light bulb that is set up in the lab, and record your observations. NOTE: The spectroscope registers all light that enters it, so when viewing a particular light source try to position yourself so that minimal light from other sources enters the scope.

Part 3: Spectroscopic Analysis of Flame Tests

NOTE: All laboratory lights should be turned off during this part of the experiment.

Once you are comfortable using the spectroscope, use it to analyze the flame tests that you analyzed visually above. Record wavelengths of all observed spectral lines for each cation solution.

Part 4: Light Filters

NOTE: All overhead laboratory lights should be off during this part of the experiment.

From the instructor, obtain magenta, cyan, yellow, red, blue, and green light filters (these are simply solutions of appropriately colored substances). Screen a flashlight or tungsten light source with each filter in turn, and use the spectroscope to analyze the light coming through the filter. Record what you observe. Perform experiments using the following filter combinations: magenta + cyan, magenta + yellow, yellow + cyan, magenta + cyan + yellow.

Part 5: Spectroscopic Analysis of Light Sticks

NOTE: All overhead laboratory lights should be off during this part of the experiment.

Obtain a light stick from the instructor and activate it. Use the spectroscope to analyze the light emitted by the stick.

Clean-up. When you have finished all of your work:

• Clean the nichrome wire by heating it in the burner flame until no color is seen above the wire.
• Be sure gas valves are completely turned off.
• Clean glassware by recommended procedures, shake off excess water, and place in the drying oven.
• Return all borrowed equipment to the instructor before leaving lab.
• Clean up your work area before leaving lab.

Pour solutions of NaCl, KCl, and CaCl₂ down the sink. Pour all remaining solutions in the container marked Metal Waste.

Preparation Questions