Honors Chemistry: FLAME TESTS FOR METALS
Purpose: To observe and identify metallic ions using flame tests.
Prelab: read this Introduction and the procedure as the lights will be off during this lab
Why do we see colors in a flame test?
The electrons in an atom occupy different energy levels, as you know. When all of the electrons are at the lowest possible energy level they are said to be in the ground state. Electrons do not always stay in the ground state. Sometimes they can be promoted to a higher-energy electron shell. When an atom is in the flame, an electron in the outer shell of that atom receives energy from the flame and jumps up to a higher-energy shell position. When an electron is in a higher-energy shell it is said to be in an excited state.
Electrons in excited states do not usually stay in them for very long. When electrons lose their energy they do so by emitting a photon of light. Photons are particles with energy but no mass. Their energy is directly proportional to the frequency of the light. The photons emitted precisely match the quantum energy difference between the excited state and the ground state.
Is the flame test fun? Yes!!!! But… what is/was a flame test used for?
The light produced by very hot atoms in the gaseous state is a unique spectrum for each element. To observe the spectrum requires the use of a prism, diffraction grating, or spectroscope. (We will look at line spectrum of various elements using a spectroscope next week.) However, before complex instruments were invented, to observe elemental spectra chemists sometimes identified metals in compounds by doing a flame test.
Further explanation of the flame test:
Salts are a type of compound that include a metal and a non-metal. Sodium chloride (NaCl) is the most familiar example of a salt but others include calcium chloride(CaCl2) and copper(II) chloride (CuCl2). In flame tests, salts that are dissolved in water are evaporated using a hot flame. In the flame, the metal atoms become excited and produce their characteristic spectrum of light. However, since the observer does not use a spectroscope only one color is observed. Many metals produce a unique single color under these conditions, while other metals produce very similar colors that only practiced eye can distinguish. An experienced scientist, based on a comparison between the color observed and the known colors produced by different metallic salts can identify the salt. We will try our ability to identify an unknown salt during the flame test lab.
In this laboratory activity, the characteristic color of light emitted for barium, calcium, cesium, cobalt, copper, lead, lithium, magnesium, manganese, nickel, potassium, sodium, and strontium will be observed.
Real world use: This ability of metal atoms to produce these colors is put to use in the making of fireworks. By including different metal salts, or mixtures of metal salts, in the exploding shell of a firework, firework manufactures can produce beautiful displays in nearly all the colors of the rainbow.
Optional: If you are interested in learning more about the chemistry of fireworks refer to the below sites. (Use the document posted on our website for active links.
· NOVA from WGBH: Fireworks! (http://www.pbs.org/wgbh/nova/fireworks/)
· Chemical of the Week: Fireworks! from the University of Wisconsin (http://www.scifun.org/CHEMWEEK/fireworks/fireworks.htm)
· A Chemical and Engineering News article about Fireworks (http://pubs.acs.org/cen/whatstuff/stuff/7927sci3.html)
Limitations of the Flame Test
· The test cannot detect low concentrations of most ions.
· The brightness of the signal varies from one sample to another. For example, the yellow emission from sodium is much brighter than the red emission from the same amount of lithium.
· Impurities or contaminants affect the test results. Sodium, in particular, is present in most compounds and will color the flame. Sometimes a cobalt glass is used to filter out the yellow of sodium.
· The test cannot differentiate between all elements. Several metals produce the same flame color. Some compounds do not change the color of the flame at all
Flame Test Colors:
Symbol / Element / ColorAs / Arsenic / Blue
B / Boron / Bright green
Ba / Barium / Pale/Yellowish Green
Ca / Calcium / Orange to red
Cs / Cesium / Blue
Cu(I / Copper(I) / Blue
Cu(II) / Copper(II) non-halide / Green
Cu(II) / Copper(II) halide / Blue-green
Fe / Iron / Gold
In / Indium / Blue
K / Potassium / Lilac to red
Li / Lithium / Magenta to carmine
Symbol / Element / Color
Mg / Magnesium / Bright white
Mn(II) / Manganese(II) / Yellowish green
Mo / Molybdenum / Yellowish green
Na / Sodium / Intense yellow
P / Phosphorus / Pale bluish green
Pb / Lead / Blue
Rb / Rubidium / Red to purple-red
Sb / Antimony / Pale green
Se / Selenium / Azure blue
Sr / Strontium / Crimson
Te / Tellurium / Pale green
Tl / Thallium / Pure green
Zn / Zinc / Bluish green to whitish green
Materials
Spray bottles with Chloride salts: Water
Barium chloride, BaCl2 • 2H2O Lead (II) nitrate, Pb(NO3)2
Calcium chloride, CaCl2 • 2H2O Equipment
Cesium chloride, CsCl Cobalt Glass
Cobalt (II) chloride, CoCl2 • 6H2O Spray Bottles with solutions to spray
Cupric chloride, CuCl2 • 2H20 Tirrill Burner
Lithium chloride, LiCl
Magnesium chloride, MgCl2 • 6H2O
Manganese chloride, MnCl2 • 4H2O
Nickel chloride, NiCl2 • 6H2O
Potassium chloride, KCl
Sodium chloride, NaCl
Strontium chloride, SrCl2 • 6H2O
Safety First ! ! !
In this lab, the solutions you will be using contain harmful materials. Avoid skin contact with these chemicals. Observe all precautions, especially the ones listed below.
· Wear your safety goggles, aprons and gloves.
· Do not taste any of the substances or touch them with your hands.
· Do not spray the chemicals at anyone. Do not spray excess chemicals.
· Return or dispose of all materials according to the instructions of your teacher.
· Wash your hands.
Procedure
1. Light the Tirrill burner.
2. Take one of the spray bottles filled with 0.1 M solution of chloride salt and spray a fine mist into the flame of the Tirrill burner. Do not overspray. Observe the color of the flame.
3. Record your observations for the flame color produced by the metallic salt solution in the Data Table.
4. Repeat steps 2 & 3 for each of the remaining metallic salt solutions.
5. After viewing the flame colors produced by MgCl2, NaCl, and KCl with the naked eye, view the flame colors through a piece of cobalt glass. Record your observations in the Data Table.
6. Using the spray bottle with the mixture of NaCl, and KCl, perform a flame test. View with the naked eye and through a piece of cobalt glass. Record your observations in the Data Table..
7. Repeat Steps 2-6 as needed using unknown I and unknown II.
8. Clean up your lab equipment, lab area, and return all materials.
9. Wash your hands ! ! !
Post lab Questions
1. Is flame coloration a test for the metallic ion or for the chloride ion?
2. List the elements that produced the most easily identified colors.
3. Which elements are least easily identified? Explain.
4. Which element produces the most intense color?
5. Identify the ions in Unknown I
6. identify the IONS in Unknown II.
7. Would flame tests be useful for detecting metal ions present in a mixture of metal ions? Explain.
8. What is the purpose of using the cobalt glass in the identification of sodium and potassium?
9. What differences did you observe in the flame, from the mixture of sodium and potassium, between your naked eye and the cobalt glass? Explain some reasons for the differences
10. What difficulties may be encountered in the use of the flame test for identification?
11. Why do we see colors in the flame tests, and why are there different colors for different metal ions?
12. How does the flame test provide support for quantized energy levels? Explain your answer
Flame Test for Metals Rubric 70 points
____ 4 pts Lab is written according to basic lab instructions
____ 2 pts Rubric is included
____ 5 pts Questions answered with question clearly indicated with the answer
____ 10 pts Lab physically completed on day class is assigned (no make ups) if absent, data will be provided
____ 24 pts Post lab questions 1-12 answered completely in sentences
____ 25 pts Lab area cleaned up with no chemicals on surrounding area, sink, hood
data table
ION / FLAME COLORBarium, Ba2+
Calcium, Ca2+
Cesium, Cs1+
Cobalt, Co2+
Copper, Cu2+
Lead, Pb2+
Lithium, Li+
Magnesium Mg2+ / (cobalt glass)
Manganese, Mn2+
Nickel, Ni2+
Potassium, K+ / (cobalt glass)
Sodium, Na+ / (cobalt glass)
Strontium, Sr2+
Potassium and Sodium Mixture
/ (cobalt glass)
Unknown I / (cobalt glass)
Unknown II / (cobalt glass)