THE CHEMICAL DETECTIVE

Exploring Our Universe:
From the Classroom to Outer Space
I. Spectroscopy
Activity #6

SPECTROSCOPY: CHEMICAL DETECTIVE
NOTES TO THE TEACHER

Level: Grades 8 and up

Objectives: Students will explain the process of dispersion, and will identify substances based on the visible spectra they emit.

Materials:

Triangular prisms

Hand-held diffraction gratings

Various light sources: (Note: if the light source is a long thin tube or filament, you can avoid the need to put a slit in front of the source.)

Incandescent bulbs (continuous spectrum)

fluorescent tubes (coated tubes yield a seemingly continuous spectrum)

"black light" tube (uncoated tube for discrete spectrum)

spectrum tubes for different elements (discrete spectra)

Visible spectra for various chemical elements (see attached handout)

"Chemical Detective" activity (see attached handout)

Procedures: [NOTE: students should have previous exposure to the electromagnetic spectrum and to the concepts of wave refraction and interference.]

Use a prism or diffraction grating with an incandescent bulb to project a continuous color spectrum on a wall or overhead screen. Explain that the light is being dispersed, or separated according to wavelength. Have students identify the colors present in the spectrum; explain that there is no set number of colors in the spectrum, but that it is a continuous range of colors.

Give students hand held diffraction gratings. Have them view an incandescent source to reconfirm that white light can be separated into a continuous color spectrum. Next have them view a source which produces a discrete spectrum (gas tube, "black light") and have them explain how the spectrum they see is different. Ask them to describe any relationship that might exist between the colors viewed in the spectrum and how the light source looks to our eyes (white light has all colors, "black light" has purple, blue and green but not much orange or red).

Ask students to consider fingerprints and explain why they are important. Tell them that spectra can be used just like fingerprints: each chemical element and compound produces a unique pattern of spectral lines. This pattern of lines can be used to identify the presence of a particular element or compound in an unknown substance. Without telling them its identity, illuminate a neon gas tube. Ask students to guess what is in the tube; have them justify their guesses. Have students view the neon gas tube through their diffraction gratings and record the number of spectral lines they view and the color of each line. See whether students notice a relationship between the colors of the spectral lines and the color of the light our eye sees (most of neon's emission lines in the visible range are red and orange, so neon appears red). Give students a copy of the handout "Elemental Spectra" showing visible spectra for various elements and have them match the lines they see from the gas tube to the c

Arrange students into groups. Give each group a copy of the activity "Chemical Detective." Have each group work as a team to solve the mystery and submit a written report discussing their solution, the evidence they gathered that led them to the solution, and how they used spectroscopic techniques to solve the crime.

Discussion: All secondary school physical science texts discuss the phenomenon of light dispersion. High school physics texts usually include a mathematical description of the process by which different wavelengths of light can be separated. Yet the texts seldom offer students activities that reinforce these concepts. This is unfortunate, since a demonstration of spectroscopy techniques almost always produces an "oh, wow!' response. The viewing of a spectrum- a rainbow of colors- is always a memorable experience; it is also a vivid example of light's wave-like properties.

Extensions:

Give students diffraction gratings to take home. Have them observe light sources in their neighborhood (street lights, business signs, etc.) For each light source they observe, have them record whether the spectrum they viewed was continuous or discrete. For discrete spectra, have students record the number of lines viewed, and the colors of the lines. Have students use the sample spectra from various chemical elements to identify the composition of the light source. (NOTE: there are charts listing the visible spectra for typical light sources such neon, metal halide, sodium vapor and mercury- see Sources.

Discuss how a diffraction grating works. Develop the equation relating the wavelength of colors in a spectrum to the dispersion angle and the spacing of the lines on the diffraction grating [ lambda; = d sin theta; ]. If available, use spectrographs to measure wavelengths of simple spectral lines.

Have students who have learned triangle trigonometry do Activity #4 in Kit II: Tracing Light Through - Understanding Diffraction. This activity teaches the principles on which the FUSE spectrometer is based and tests understanding by asking students to trace light rays from a star through the spectrometer.

Have students research the process by which rainbows are formed, and explain the conditions necessary for viewing them.

Have students contact fluorescent light manufacturers (check websites like www.sylvania.com or www.ge.com) to find out how fluorescence works and what techniques they use to make fluorescent lights produce a spectrum similar to natural light (sunlight). Also, they can investigate what type of lighting is most appropriate for different situations, and how this relates to the spectral emissions.

Sources: Flinn Scientific, 1-800-452-1261 (for hand held diffraction gratings)
Electro-Technic Products, Inc., 773-561-2349 (for gas spectrum tubes)
Arbor Scientific, 1-800-367-6695 ("Night Spectra Quest" chart for identifying typical light sources)

Level:	Grades 8 and up
Objectives:	Students will explain the process of dispersion, and will identify substances based on the visible spectra they emit.
Materials:	Triangular prisms Hand-held diffraction gratings Various light sources: (Note: if the light source is a long thin tube or filament, you can avoid the need to put a slit in front of the source.) Incandescent bulbs (continuous spectrum) fluorescent tubes (coated tubes yield a seemingly continuous spectrum) "black light" tube (uncoated tube for discrete spectrum) spectrum tubes for different elements (discrete spectra) Visible spectra for various chemical elements (see attached handout) "Chemical Detective" activity (see attached handout)
Procedures:	[NOTE: students should have previous exposure to the electromagnetic spectrum and to the concepts of wave refraction and interference.] Use a prism or diffraction grating with an incandescent bulb to project a continuous color spectrum on a wall or overhead screen. Explain that the light is being dispersed, or separated according to wavelength. Have students identify the colors present in the spectrum; explain that there is no set number of colors in the spectrum, but that it is a continuous range of colors. Give students hand held diffraction gratings. Have them view an incandescent source to reconfirm that white light can be separated into a continuous color spectrum. Next have them view a source which produces a discrete spectrum (gas tube, "black light") and have them explain how the spectrum they see is different. Ask them to describe any relationship that might exist between the colors viewed in the spectrum and how the light source looks to our eyes (white light has all colors, "black light" has purple, blue and green but not much orange or red). Ask students to consider fingerprints and explain why they are important. Tell them that spectra can be used just like fingerprints: each chemical element and compound produces a unique pattern of spectral lines. This pattern of lines can be used to identify the presence of a particular element or compound in an unknown substance. Without telling them its identity, illuminate a neon gas tube. Ask students to guess what is in the tube; have them justify their guesses. Have students view the neon gas tube through their diffraction gratings and record the number of spectral lines they view and the color of each line. See whether students notice a relationship between the colors of the spectral lines and the color of the light our eye sees (most of neon's emission lines in the visible range are red and orange, so neon appears red). Give students a copy of the handout "Elemental Spectra" showing visible spectra for various elements and have them match the lines they see from the gas tube to the c Arrange students into groups. Give each group a copy of the activity "Chemical Detective." Have each group work as a team to solve the mystery and submit a written report discussing their solution, the evidence they gathered that led them to the solution, and how they used spectroscopic techniques to solve the crime.
Discussion:	All secondary school physical science texts discuss the phenomenon of light dispersion. High school physics texts usually include a mathematical description of the process by which different wavelengths of light can be separated. Yet the texts seldom offer students activities that reinforce these concepts. This is unfortunate, since a demonstration of spectroscopy techniques almost always produces an "oh, wow!' response. The viewing of a spectrum- a rainbow of colors- is always a memorable experience; it is also a vivid example of light's wave-like properties.
Extensions:	Give students diffraction gratings to take home. Have them observe light sources in their neighborhood (street lights, business signs, etc.) For each light source they observe, have them record whether the spectrum they viewed was continuous or discrete. For discrete spectra, have students record the number of lines viewed, and the colors of the lines. Have students use the sample spectra from various chemical elements to identify the composition of the light source. (NOTE: there are charts listing the visible spectra for typical light sources such neon, metal halide, sodium vapor and mercury- see Sources. Discuss how a diffraction grating works. Develop the equation relating the wavelength of colors in a spectrum to the dispersion angle and the spacing of the lines on the diffraction grating [ = d sin]. If available, use spectrographs to measure wavelengths of simple spectral lines. Have students who have learned triangle trigonometry do Activity #4 in Kit II: Tracing Light Through - Understanding Diffraction. This activity teaches the principles on which the FUSE spectrometer is based and tests understanding by asking students to trace light rays from a star through the spectrometer. Have students research the process by which rainbows are formed, and explain the conditions necessary for viewing them. Have students contact fluorescent light manufacturers (check websites like www.sylvania.com or www.ge.com) to find out how fluorescence works and what techniques they use to make fluorescent lights produce a spectrum similar to natural light (sunlight). Also, they can investigate what type of lighting is most appropriate for different situations, and how this relates to the spectral emissions.
Sources:	Flinn Scientific, 1-800-452-1261 (for hand held diffraction gratings) Electro-Technic Products, Inc., 773-561-2349 (for gas spectrum tubes) Arbor Scientific, 1-800-367-6695 ("Night Spectra Quest" chart for identifying typical light sources)

CHEMICAL DETECTIVE
STUDENT ACTIVITY

You are a private eye who stays in business mainly by recovering lost dogs and the occasional runaway pet turtle. You have just learned that last night some devious criminal elements pulled a huge bank robbery. This could be your big break. Crack this case and you'll be famous!

You grab your diffraction grating and head to the scene of the crime. What is that strange glowing gas you see in the bank vault? Hold your breath, it's a clue. You look at the gas through the grating. The emission lines you see are like fingerprints. Now you can identify the criminal elements.

This is the "perpetrator spectrum":

Look at the visible light emission lines of the suspects, shown on the following page. Can you match the lines in the "perpetrator spectrum" to the elements whodunnit?

The president of the bank is offering a huge reward to whoever solves this case but is demanding hard evidence against the wrongdoers. Write a report to the president of the bank listing the names and aliases of the perpetrators and, most importantly, explaining in detail how you used techniques of spectroscopy to crack the case.