Flame Tests for Metallic Ions

 

Subject Area

  Chemistry: atomic structure, spectroscopy
Age or Grade

  Chemistry I: grades 10, 11
Estimated Length

  One or two 45 minute class periods
Prerequisite knowledge/skills

  Students should understand atomic structure, electron configuration concepts, and the wavelike nature of light.
Description of New Content

  Students will be introduced to the concept of electromagnetic emission.
Goals

  Students will be introduced to the interplay between light and matter, and analyze data to make comparisons and form educated conclusions.
Materials Needed


 4 Plastic spray bottles
Methanol
Samples of NaCl, LiCl, KCl, CuSO4, BaCl2, and Pb(NO3)2
Cottom swabs
3M HCl
Bunsen burners

Procedure

 

 

 

Opener

Before class, fill four spray bottles with solutions prepared by dissolving 5g of NaCl, LiCl, KCl, and CuSO4, respectively, in approximatley 200 mL of methanol each (note: some salts will not completely dissolve).  

Tell the students you have made four different salt solutions, but do not reveal the identity of the salts.  Turn off the room lights, choose one solution and spray a fine mist over a lit bunsen burner.  The mist should ignite to form a large ball of colored fire, which can be regenerated by repeated spraying.  In the same manner, ignite the other three solutions.  

Ask the students to think about what they have just seen, and why the solutions produced different colors when exposed to the flame.  Explain that the cations in the various salts are responsible for the colors, and ask them why they think different cations produce different colors; what could be different about the various cations?

Development

Have the students break into their lab groups, and provide each group with labeled samples of the following salts: NaCl, LiCl, KCl, CuSO4, BaCl2, and Pb(NO3)2.  Explain that they are to run flame tests of these salts using the following procedure:

1. Dip a cotton swab in hydrochloric acid, then dip the wet swab into one of the salt samples.  Place the swab in the flame for about 5 seconds and observe the color of the flame.

2.  Repeat for each remaining salt, using a new cotton swab each time.

The students should now be able to assign characteristic colors to each of the six compounds examined.  

Next, provide the students with four unknown samples (CuCl2, Na2CO3, K2CO3, and Li2CO3), explaining that the samples are salts containing cations from the set of compounds just examined, though not necessarily paired with the same anions.  Have them conduct flame tests to identify the cation present in each unknown sample, based on their results from flame tests with the known compounds.

The expected results are summarized in the table below.

metal

flame colour

barium

light green

calcium

brick red

copper

blue/green

lead

blue/white

potassium

lilac

sodium

yellow-orange

Closure

 Return to the original question: why do different cations produce different colors when heated?  Explain that when an atom is excited thermally, its electrons are excited from the ground configuration to higher energy levels.  Upon returning to the ground configuration, the electrons emit energy in the form of electromagnetic radiation, and for some atoms, including many metals, the wavelengths of these emitted photons are within the visible range.  The reason different atoms produce different colors when heated is because their electrons emit characteristic wavelengths of light when falling back to the ground state, due to the fact that the energies of excited and ground state electrons are unique among each element as a result of their unique atomic charges and ground state configurations.  
Evaluation

  Use student feedback to improve the lesson.  Evaluate students' performance based on how successfully they were able to deduce the characteristic colors of the known compounds, as well as the identity of the cations in the unknown compounds.  
Extensions

  This lesson is readily extended into a subsequent lab practicum involving spectroscopic analysis of the colors produced in flame tests.  Using a simple spectroscope, students are able to record the emission spectra of the various metals, which serves as a good introduction to the concept of spectroscopy.
References  Schreiner, R.; Johnson, K. J. Chem. Educ. 2001, 78, 640.