CHEMICAL SLEUTHING
Grades 9 - 12

The sun's energy reaches the earth as electromagnetic radiation, the same phenomenon that carries radio and television signals, radar, microwaves, X-rays, ultraviolet rays, and the very light and colors that stimulate the retinas of our eyes. The energy of electromagnetic radiation depends on its frequency. Visible light comes to us in a range of wavelengths. This lesson introduces the concept that molecules, like individual atoms, absorb unique amounts of energy due to the rotational, vibrational, and electronic characteristics of chemical bonds. This information can be used to identify the type of bonds within a molecule and ultimately to identify specific substances. Through exciting experiences, including flame tests and the construction of a simple diffraction spectrograph with which to measure sodium ion emissions, students will use the Bragg equation to compute the wavelength of the line spectra produced.

"THE WORLD OF CHEMISTRY: "Molecular Fingerprints" (#9)"

Students will be able to:

• construct a simple flame spectrograph;
• measure the wavelength of a strong electronic transition of sodium;
• compute the wavelength of the light viewed using the Bragg equation, and
• identify possible sources of error in their determination of the wavelength of the sodium flame emission.

Pre-viewing demonstration

• one gas burner
• one fine spray plastic bottle
• a water-soluble salt solution
• goggles
• a prism
• sunlight or a bright light source as from a slide projector

Post-viewing activity
per group of two to four students

• safety goggles
• two meter sticks
• one diffraction grating
• one cardboard piece with narrow slit
• one 50-ml beaker
• one gas burner
• one platinum or nichrome wire loop held by a test tube holder
• one watch glass
• one M sodium chloride, NaCl
• six M hydrochloric acid, HCl
• one cardboard piece with a narrow slit ( width of slit = 1mm or less; slit height= 2cm)
• one diffraction grating

Before the students arrive, set up a Bunsen burner. Dissolve, in water, a water-soluble salt of the ion to be examined. Place the solution in a plastic bottle that will produce a fine spray when squeezed. After the students are seated, ask for a volunteer to assist in a demonstration. Have the student wear goggles.

Light the burner and have the student squeeze the bottle so that some of the spray goes into the flame. The color associated with the particular ion should appear. (You may want to have a variety of salt solutions prepared.) Discuss with the students the reasons that the characteristic colors that appear. (This is a good way to review the previous work they have done with the concepts of electromagnetic spectrum, wavelengths, and the ground and excited states of the electrons in the atom.) You may want to ask your students to read previously assigned reports on these topics or to display illustrations of the concepts being discussed.

You may also want to have students hold a prism in a light source to demonstrate the production of color. Discuss the categories of the entire electromagnetic spectrum, and present Einstein's relationship, E = h v, showing that energy and frequency are directly proportional.

Have another student look around the room and spot something red (or substitute another color of the rainbow). Ask the students why we see red. Through the discussion, students should understand that they see red because red light is entering the eye. Ask them, "Where is the light coming from?" (The ceiling lights and the sun are giving off white light, and they are seeing white light striking the "red" object.) Ask the students, "Why doesn't the object appear white?" (Explain that all the colors except red are being absorbed, and only the red is being reflected to your eye.) Ask the students to explain how objects can absorb only certain colors and not others. (Everything is made of either atoms, ions, molecules, and somehow light must be interacting with these particles in such a way that only certain colors are absorbed. Also when light is emitted it has specific distinctive properties.) Tell the students that these distinctive properties will be explored in the following film segments and the accompanying activity.

The focus for viewing is a specific responsibility or task(s) students are responsible for during or after watching the video to focus and engage students' attention. As the students are watching the video, they should focus on identifying the four characteristics of chemical bonds that causes molecules to absorb energy. This information will be used to determine the type and number of bonds within a molecule and to identify the specific substances that comprise the molecule itself.
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START the video, The World of Chemistry: "Molecular Fingerprints" (#9) at the visual cue of a person lying on her back with a bright light focused there. Audio cue: "The reason is that when a molecule absorbs radiation, it's raised from one energy level to the next."
PAUSE at the words, "The more complicated a molecule is, the more energy states it has." A quantum model of the compound appears on the screen. Discuss students' responses to the focus question posed regarding the four characteristics of chemical bonds. The four internal motions are: kinetic (the molecular movement in space from one location to another), rotational, vibrational, and electronic (the energy from its electrons which exit at specific energy levels).

Ask the students to watch the video carefully to see if they were correct.

RESUME the film at the paused point to verify their answers.
PAUSE after the words, "... vibrational, rotational, and electronic," spoken by the demonstrator, Don Showalter. Confirm your students' answers and ask them to define the terms: kinetic, vibrational, rotational, and electronic.

Instruct the students to watch carefully the upcoming on-screen demonstration. Sound waves are used to illustrate that certain packages of energy will be absorbed or ignored by particular molecules. Tell the students that only certain packages of energy will be absorbed by a hand-held beaker during this demonstration. Ask them predict if the sound frequencies must be lower, higher, or equal to the frequency of energy within the beaker molecules before it will react to the sound.

RESUME the video.
PAUSE again after the audio cue: "Lower or higher frequencies, even though they were very intense, wouldn't work." Discuss why the beaker broke and

RESUME the video to have the students confirm or revise their predictions.
PAUSE the video at the audio cue: "Its energy is not the exact amount needed to make the molecule vibrate at the next higher level."

Prepare the students for discussion of the next on-screen demonstration exploring the application to molecular emissions of energy. by asking them to focus on the film to tell how these absorbed bundles of energy tell us something about the molecule.

RESUME the video.
PAUSE again at the verbal cue, "That's what a spectroscope does." Discuss the function of a spectroscope and, if possible, provide spectroscopes for student inspection and use.

Ask your students to watch the video carefully to identify what molecules were used with the spectroscope and why certain frequencies in the visible spectrum were no longer visible with the use of these molecules.

RESUME the video.

STOP the film at the audio cue, "Each compound has its own unique spectrum, a 'molecular fingerprint.'"

Discuss with the class that the molecules of a chlorophyll sample were used and that the sample modified the radiated light frequencies passing through it. The frequencies that were a match to the energy patterns of the chlorophyll were absorbed. The others were radiated to the screen surface. A graph of the patterns absorbed and radiated is made which is as unique to that particular molecule as our fingerprints are to us. This discussion is a good introduction to the activity which follows.

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Note to the Teacher:
Simple spectroscopes can be constructed from cardboard tubes. One end should have a small slit and the other end should have a grating that can be provided by a small piece of acetate film. An example is available in the Teacher's Guide to the video series, The Structure of the Atom.

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Distribute the materials and the worksheet needed for this activity. Have students set up the apparatus shown in the diagram provided for them on their worksheet. To save time, the apparatus can be set up prior to the class.

Instruct the students to do the following: Pour approximately 15 ml of HCl into a beaker. Cover the beaker with a watch glass when it is not being used. Clean the wire loop by first dipping it into the HCl and then heating it in the flame of a gas burner. Continue to dip and heat the wire until no color comes from the wire when heated. Dip the clean wire loop into the NaCl solution.

Place the wire loop in the burner flame. Observe the flame through the slit in the cardboard and the diffraction grating. You should see a series of lines to the left and to the right of the slit. Select the brightest line to the right of the slit and have a partner record and label this position on a meter stick as position A. Repeat this procedure on the left side of the slit and record this as position B. Measure the distance from the diffraction grating to the slit and record this as distance Y. Record your data and the results of your calculations on the worksheet provided.

This topic of light and energy emissions encompasses many fields of study for further investigation.

Students can remain in their small groups and find out the meaning of "PCBs," the structural formula of a PCB, and the properties that made PCBs useful in electrical equipment such as transformers. They could also investigate why the use of PCBs in transformers is now banned. This could be explored through trips local energy companies.

A number of businesses in the area may use incandescent bulbs, neon, argon, or fluorescent lamps. Visits to these sites can be made, viewed through class-made spectroscopes and reported to the rest of the class for further study.

A local sign company can be toured or its workers invited to share some of the techniques used to construct signs illuminated with energized gas.

Invite an optometrist to discuss the effects of ultraviolet light on the eye.

Art:
Neon signs cost less to operate than ordinary lights. Artists have begun to express their ideas in neon sculptures. In the southwest, neon lights shaped like cacti are very popular. Gather information on the construction of neon signs and design a neon sculpture for your bedroom. Include all of the information that an artist would need to sculpt your creation. Then perhaps try to do it yourself or have it made.

Photography:
Report on how black and white photograph works, which chemicals are involved, and how they react to help produce an image.

Physics:
Investigate and report on how lasers work.

Biology:
UV light can kill microorganisms. Thirty years ago clothes dryers had ultraviolet lamps in them to kill germs as the clothes were dried. Research the reasons for their removal.

Forensic Science:
Have students contact local law enforcement agencies to determine if their state allows DNA fingerprinting as courtroom evidence. Have them investigate and discuss safeguards on the use of DNA fingerprinting in criminal proceedings.

Math:
Research the contributions of J.R. Rydberg and the development of mathematical formula for the frequencies of spectral lines.

Internet:
Early Instruments World Wide Web: http://physics7.berkeley.edu/newjfy.php

The above provides information on instruments used in the field of physics to study optics, heat, and electromagnetism. As students review the electromagnetic spectrum, the data gathered can be presented in an oral or written report.

OpticsNet World Wide Web: http://www.osa.org/index.html
This site provides information on all aspects of optical physics and engineering. Students' working with the light spectra of several of the elements may lead to additional questions that can be explored at this site.


Note to the Teacher:
Incorporation of the on-line material can occur as students are asked to research data relevant to a unit that is about to begin. They can share what they have discovered in small rotating groups, formulate questions that can be answered as the topic unfolds within the unit, or record additional questions that arise as they explore the Internet. These questions can be saved and answered as the class progresses through the material.

Other:
Women Scientists Students can research the life and contributions of the following female scientists whose work has contributed to the concepts explored in this lesson:

Marie Sklodowska Curie: radioactivity Joan Maie Freeman: sub-atomic particles Irene Curie Joliot: radioactivity Marie Goeppert Mayer: nuclear shell structure


Bibliography
Holographic Diffraction Grating, Arbor Scientific, 1996. Catalogue # 33-0980 @ $3.00 each. Phone number: 1-800-367-6695.

Chemistry, by Edward L. Waterman, Edward L. and Stephen Thompson, Addison-Wesley, New York, 1995.

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Master Teachers: Joyce Dul-Jacunski and Elizabeth Marquez

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Note to Teacher:

This lesson should be presented after the following concepts have been explored: electromagnetic radiation, atomic energy levels, and wavelengths.

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