MnSTEP Teaching Activity Collection > MnSTEP Activities > Properties of Cations: Flame Test Lab

Properties of Cations: Flame Test Lab

Sharon M. Welter, Shakopee MN, based on a number of flame test labs and resources

Summary

The flame test is used to visually determine the identity of an unknown metal or metalloid ion based on the characteristic color the salt turns the flame of a bunsen burner. The heat of the flame converts the metal ions into atoms which become excited and emit visible light. The characteristic emission spectra can be used to differentiate between some elements.

Learning Goals

Students will:
1.develop skills in careful qualitative observations and recording experimental data.
2.understand the relationship between color emitted by atoms of metal compounds and their electron structure.
3.use color emitted by metal atoms in a flame (photon) to identify the metal in an unknown salt compound.
Vocabulary: photon, spectral lines, ground state, excited state

Context for Use

This is lab activity used during the unit on the Periodic Table. The activity takes 45-60 minutes depending on if time is allotted for work time at the end of lab. Students should have experience with a bunsen burner and basic lab safety. This activity can be easily adapted to grades 6-12 grade or as a demonstration.

Subject: Physics:Optics, Chemistry:General Chemistry:Atomic Structure
Resource Type: Activities:Lab Activity
Grade Level: High School (9-12), Middle (6-8)

Description and Teaching Materials

This lab will be conducted after the introduction of the Alkali and Alkaline Earth Metals of the Periodic table. The lab will be introduced with a powerpoint on the mechanics of the release of energy from the atom. (down loaded file) The student will conduct flame tests on a number of unknown salt compounds. Closure strategies will occur next day (due to time) with a discussion and a short quiz.
Student Handout for Flame test (Microsoft Word 38kB Aug2 09)
Power Point introduction (PowerPoint 136kB Aug2 09)
Salt Compound list (Microsoft Word 43kB Aug2 09)

Teaching Notes and Tips

I use the wood splint method since I do not have wire loops, but how to use both are included.
Wooden Splint or Cotton Swab Method:
Wooden splints or cotton swabs offer an inexpensive alternative to wire loops. To use wooden splints, soak them overnight in distilled water. Pour out the water and rinse the splints with clean water, being careful to avoid contaminating the water with sodium (as from sweat on your hands). Take a damp splint or cotton swab that has been moistened in water, dip it in the sample to be tested, and wave the splint or swab through the flame. Do not hold the sample in the flame as this would cause the splint or swab to ignite. Use a new splint or swab for each test.
Classic Wire Loop Method:
First, you need a clean wire loop. Platinum or nickel-chromium loops are most common. They may be cleaned by dipping in hydrochloric or nitric acid, followed by rinsing with distilled or deionized water. Test the cleanliness of the loop by inserting it into a gas flame. If a burst of color is produced, the loop is not sufficiently clean. The loop must be cleaned between tests.
The clean loop is dipped in either a powder or solution of an ionic (metal) salt. The loop with sample is placed in the clear or blue part of the flame and the resulting color is observed.
Materials:
1.Bunsen burner
2.flint striker or matches
3.Water soaked wood splint
4.6 unknown metals (salts) from Group I & II
Safety:
1. Wear goggles at all times.
2. Place used splints in water fill waste container.
3. Clean up any and all spills.
4. Clean up lab station.
5. Report any accidents to teacher.
Normally this lab was done by have the student's test 4 or 5 known substances then identify one unknown. The student's identified the compound but did not understand that the test was a property of the metal atom.

Assessment

Assessment is based on the answers to the questions at the end of the lab, the lab discussion, and the quiz in the following days.

Standards

8.2.1.2.1 Identify evidence of chemical changes, including color change, generation of a gas, solid formation and temperature change.
9.2.1.1.2 Describe how experimental evidence led Dalton, Rutherford, Thompson, Chadwick and Bohr to develop increasingly accurate models of the atom.
9.2.1.1.3 Explain the arrangement of the elements on the Periodic Table, including the relationships among elements in a given column or row.
9.2.1.2.4 Relate exothermic and endothermic chemical reactions to temperature and energy changes.

References and Resources

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