Summary

In this lab, students will examine three different ways to excite electrons to produce visible light found in the electromagnetic spectrum. The students will then see that this visible light has a specific color, wavelength, and energy. They will use their knowledge of the speed of light and Planck's constant to examine the energy involved in the emission of light. Finally, the students will then apply their knowledge of the emission spectrum to how the composition of stars is determined.

Grade Level

High School

NGSS Alignment

This lab will help prepare your students to meet the performance expectations in the following standards:

• HS-PS4-4: Evaluate the validity and reliability of claims in published materials of the effects that different frequencies of electromagnetic radiation have when absorbed by matter.
• HS-ESS1-2: Construct an explanation of the Big Bang theory based on astronomical evidence of light spectra, motion of distant galaxies, and composition of matter in the universe.

Objectives

By the end of this lab, students should be able to:

• Identify that all elements have unique line emission spectra.
• Calculate the energy associated with the lines of color that make up an emission spectrum.
• Diagram and describe how line emission spectra are produced by an atom.
• Apply their knowledge of emission spectra to determine the composition of a star.

Chemistry Topics

This lab supports students’ understanding of:

• Line emission spectra
• Electromagnetic spectrum
• Atomic structure

Time

Teacher Preparation: First time through: 30-60 minutes
(When solutions are made you can store them so prep would be 30 minutes)
Lesson: 90-120 minutes

Materials

• Student handout

Part 1:

• 5 power supplies for gas discharge tubes (one for each of the unknowns)
• 5 gas discharge tubes: hydrogen, helium, neon, argon, and krypton
• Rainbow glasses or diffraction gratings for each student (class set of 32)
• Colored pencils (~10 packages)
• Calculator

Part 2:

• Pliers (I use 2 at the station so more students can participate)
• Wint-O-Green Lifesavers, 2 bags for all classes (this is the only kind of lifesaver that works well – no imitations can be used)
• 7 labeled 250-mL beakers containing ~50-100 mL of saturated solutions of NaNO₃, CuSO₄, K₂SO₄, LiCl, and Sr(NO₃)₂, and two unknowns selected from the previous list of solutions
• Wooden splints (coffee stirrers work fine) for each solution
• Striker/matches/lighter to light the Bunsen burner

Safety

• Always wear safety goggles when handling chemicals in the lab.
• Students should wash their hands thoroughly before leaving the lab.
• When students complete the lab, instruct them how to clean up their materials and dispose of any chemicals.
• Always use caution around open flames. Keep flames away from flammable substances.
• Always be aware of an open flame. Do not reach over it, tie back hair, and secure loose clothing.
• Open flames can cause burns. Liquid wax is hot and can burn the skin.
• When lighting the match and wooden splint, be cautious with the flame.
• An operational fire extinguisher should be in the classroom.
• Do not consume lab solutions, even if they’re otherwise edible products.
• Food in the lab should be considered a chemical not for consumption.

Teacher Notes

• Before this activity, students should understand how electrons’ transitions between the ground state and the excited state produce specific colored light. These activities will reinforce those concepts. The Electron Energy and Light POGIL activity is a great way to review the electromagnetic spectrum and how it is related to electrons in atoms. (And there are a lot of other great POGIL activities as well!)
• The lab is could be done over two days, or during one block period if you have the space to set up all parts of the lab at one time. I complete Part 1 the first day, Part 2 the second day, and Part 3 can be done for homework or if there’s extra time on either day (you don’t have to go in order).
• This lab is all about color, but some students may experience some level of color blindness – if so, they should be encouraged to work with group members if they need assistance with identifying colors at various points throughout this lab.
• Part 1: Line Emission Spectra
  • This activity works best in a dark/windowless room. If that is not possible, cover up as many windows as possible in the room you are working in, such as by taping paper over them, to minimize the amount of light coming in from sources other than the gas discharge tubes students will be observing. How precisely students can identify the lines of the spectra will depend on how dark you can make the room.
  • Set up the 5 power supplies spread around the room to make stations so groups of 4-6 students can congregate around the power supply. They should each contain a gas discharge tube of one of the following elements: hydrogen, helium, neon, argon, and krypton. Label each station with its element.
    • The discharge tubes quickly get very hot, so try not to leave them on for longer than necessary – and be sure students don’t touch them! Students could turn off power supplies before they rotate between stations so they aren’t running for long stretches of time without being allowed to cool off a bit.
  • Due to the darkness required for observing the spectra of the gas discharge tubes, it is strongly recommended that when the lights are off no one moves around to prevent tripping/accidents. Only when the lights are on can groups move to the next station.
  • Once in the room, hand out the rainbow glasses and have the students look at the lights in the room and record what they see.
  • Students get into groups of 4-6 and each group starts at one of the 5 stations. Have students put their rainbow glasses on and turn on the power supply, then turn off the lights for 2-3 minutes and have students observe the line emissions through the glasses.
  • Turn the lights back on and the power sources off so they can document what they saw in the lab sheet and come to a consensus within their group as to the number and color of the lines they are seeing. Then turn the room lights off/power sources on one last time (1-2 minutes) so they can confirm what they saw.
  • Turn the lights back on/power sources off and the student groups can rotate to the next station. Repeat until students have seen all 5 elements.
  • When the students have rotated through all 5 stations, turn off the power sources to allow the discharge tubes to cool. They can work on calculating the energy associated with the selected lines for each element.
    • Students are expected to know how to calculate the energy of light based on its frequency using , where E is the energy (in J), h is Planck’s constant (6.626 × 10⁻³⁴J*s), c is the speed of light (3.00 x 10⁸ m/s), and λ is the wavelength in m (so they will have to convert from nm to m, which is x 10^–9!). If you have not gone over this before, it may be helpful to complete one or two calculations together as a class first.
  • Once the discharge tubes have cooled and while students are working on calculations, remove the tubes from their power sources and place them at different stations with labels Unknown A–E. (See the answer key for unknown identification so you can be sure the wavelengths provided for each unknown match the correct gas.) Have students rotate through the 5 stations again and identify the unknowns based on their spectral line data.
    • If you are running short on time, you could have them do a subset of the unknowns instead of all five. Students can also complete unfinished calculations for homework, or you could assign fewer wavelengths per element.
• Part 2: Sparks and Flame Tests
  • Set up 8 stations for this part of the lab – this should be done in a lab, as it requires the use of Bunsen burners for 7 of the 8 stations.
  • The room should not be totally dark for this part for safety reasons, as Bunsen burners will be in use, but if it is possible to dim the lights or only turn on some of the lights in the room, that will allow students to see the sparks and colored flames better.
  • Station 1: Wint-O-Green Lifesavers
    • Lay out two pliers and sandwich bags for the students. When they are ready to complete the experiment, they retrieve 2 lifesavers from the teacher. Students should place the lifesaver in pliers, then place pliers with lifesaver into sandwich bag and crush the lifesaver with the pliers. The sandwich bag (safety shield) will collect the pieces so that they can use the pieces of the same lifesaver to repeat the experiment. They can crush the lifesavers over and over until there are no more big pieces (usually 6-8 times).
    • Students should see a bluish spark when the mints are crushed. This video shows an example of it and explains what we do and don’t know about the triboluminescence that causes the mints to spark.
    • It is important that these are Wint-O-Green lifesavers, not any other flavor, as one of the compounds responsible for the minty taste, methyl salicylate, is also believed to be responsible for the sparks!
  • Stations 2–6: Flame Tests
    • At each station 2–6, place a labeled 250-mL beaker containing 50 to 100 mL of one of the following saturated solutions (the exact concentration doesn’t matter): NaNO₃, CuSO₄, K₂SO₄, LiCl, or Sr(NO₃)₂. In each beaker place 2-3 wooden splints (coffee stirrers work well) and allow them to soak up the solution (preferably overnight).
      • The anion does not matter for this experiment, as long as the salt is soluble. You may wish to label beakers with just the relevant metal ion – something like “Sodium compound” or “Na⁺”.
      • The light purple of the potassium flame test can sometimes be hard to see, so make sure that solution is fully saturated. This is less of an issue for the other solutions, which produce notable colors even if they are less than fully saturated.
    • Have students light the Bunsen burner at the station (or have it pre-lit), place the solution-soaked wooden splint into the flame, and observe the color of the flame.
    • Remind students to remove the splint from the flame immediately if it starts burning!
  • Stations 7–8: Unknown Flame Tests
    • Repeat the procedures from stations 2–6, but with two unknown substances. For these two stations, you can choose any of the solutions used in stations 2–6, or for an extra challenge, you could mix two of the solutions together.
  • Students should complete stations 2–6 first before Unknowns A & B at stations 7 and 8.
• Part 3: Studying the Stars
  • Students will apply their knowledge of emissions spectra to identify the elements that make up stars given the spectra of a set of elements and the spectra of the stars. Color printing is strongly recommended since this section includes images of emission spectra of elements. Alternatively, you could use a projector to display the spectra.
  • If you have a subscription to Gizmo, you could replace this section with the Star Spectra Gizmo. It also goes into other astronomical details, such as the temperature and classification of stars, whether stars have planets orbiting them, etc. If this Gizmo is used, students should understand the relationship between absorption and emission spectra, since we have primarily used emission spectra thus far but the Gizmo uses absorption spectra. This video provides an overview of how emission and absorption spectra are related.