Flame Test Forensics
A one-day, hands-on lab for Chemistry and Honors Chemistry (Unit 2: Quantum). Students run flame tests on metal chlorides, try to identify a mystery sample, and discover for themselves where the flame test falls short. The lab takes about 30 minutes, which leaves time for cleanup and a debrief in a 45-minute period.
Lab Synopsis
A storage shed behind a fireworks supplier has burned, and investigators need to know what metal is in a residue found at the scene, labeled Unknown X. Students dip soaked wooden splints into seven known metal chloride solutions, hold each one in a Bunsen burner flame, record the color, and match it to a representative wavelength. Then they test Unknown X.
The twist: two of the known salts give nearly identical flame colors, and Unknown X is one of them. Students have to decide how confident they can really be, which leads them to a real limitation of the flame test and to the reason chemists use spectroscopes. Post-lab questions ask them to write a claim, evidence, and reasoning argument, calculate the frequency and energy of the emitted light, and draw a Bohr model to explain why a flame contains more than one wavelength. For the background, see the Emission Spectra lesson.
Skills Students Practice
- Performing a flame test safely and correctly with a Bunsen burner
- Recording observations and matching colors to representative wavelengths
- Explaining emitted light with ground states, excited states, and electron transitions
- Calculating frequency and photon energy with c = λν and E = hν
- Modeling electron transitions with a Bohr model
- Identifying variables and controls in an experimental design
- Writing a claim, evidence, and reasoning argument from their own data
- Recognizing the limits of a method and proposing a better test
Materials
Equipment
- Chemical splash goggles
- Bunsen burner, gas hose, and striker
- Wooden splints, soaked in water (one per test)
- Beaker of water for used splints
- Test tube rack with labeled test tubes
Solutions (0.5 M unless noted)
- Lithium chloride, LiCl
- Sodium chloride, NaCl
- Potassium chloride, KCl
- Calcium chloride, CaCl2
- Strontium chloride, SrCl2
- Barium chloride, BaCl2 (0.1 M)
- Copper(II) chloride, CuCl2
- Unknown X
The teacher version includes solution preparation masses, the recommended unknown, station setup, a debrief plan, troubleshooting tips, and a full answer key.
Downloads
- Flame Test Forensics, Student Version (PDF)
- Flame Test Forensics, Teacher Version with Answer Key (PDF)
NGSS Alignment
This lab builds toward these Next Generation Science Standards (NGSS) performance expectations:
- HS-PS4-1: Use mathematical representations to support a claim regarding relationships among the frequency, wavelength, and speed of waves traveling in various media.
- HS-PS4-3: Evaluate the claims, evidence, and reasoning behind the idea that electromagnetic radiation can be described either by a wave model or a particle model, and that for some situations one model is more useful than the other.
Disciplinary core ideas: PS4.B Electromagnetic Radiation (atoms of each element emit and absorb characteristic frequencies of light, which allows the element to be identified; electromagnetic radiation can be modeled as a wave or as photons)
Science and engineering practices: Planning and Carrying Out Investigations; Developing and Using Models; Using Mathematics and Computational Thinking; Engaging in Argument from Evidence
Crosscutting concepts: Patterns; Energy and Matter
