LAB FIVE
Name
Lab Partner(s)
Section
Date:
Flame Tests
Objective
In this experiment you will:
•
•
•
observe the flame colors emitted from selected ions;
identify metallic ions by the color emitted during vaporization of the element;
evaluate the usefulness of this method of metal identification.
Introduction
When elements are heated to high temperatures, they may be placed in an excited state.
In an excited state, the electrons move to higher energy levels. The changes in energy
that occur when the excited atoms return to their ground state causes the substance to be
luminous, or emit light. The observed colors, or spectrum, of the substance is caused by
the set of visible wavelengths of the light emitted. Since each element emits a unique set
of wavelengths, emission spectra can be used as a tool to identify the elements.
One method used to demonstrate the emission spectrum of a substance is the flame test.
Using this method, a small amount of a substance is heated and the characteristic glow of
the substance is observed. In this experiment you will perform a flame test on several
metallic salts. Based on your observations, you will develop a reference table which lists
the flame color for each metal ion. You will then perform a flame test on an unknown
substance. By comparing your observations to the data in your reference table, you will
be able to identify the metal ion in the unknown substance. Finally, you will use cobalt
glass as a tool for identifying the components of a metallic salt mixture.
Pre Lab Questions (answer on separate paper)
1. Describe the common characteristics of flame tests that would be observed if a
spectroscope were used.
2. List at least five other means of qualitative analysis that are used to identify metals.
(Use the index in your text or other reference sources.)
3. Explain the visual difference between emission spectra and absorption spectra.
4. Advertising lights are sealed tubes of various excited noble gases which emit colored
light. Using references, briefly explain the chemistry behind how these advertising
lights function.
Flame Test
1
LAB FIVE
Equipment & Reagents
Nichrome wire loop(s)
Bunsen burner
2 cobalt glass squares
Concentrated HCl
Spot plate
Compounds of Ba2+, Ca2+, Cu2+, K+, Sr2+, Li2+ and Na+
Procedure
1. Clean a nichrome wire loop by dipping it in concentrated hydrochloric acid (HCl) and
then heating it in the edge of the nonluminous portion of a well adjusted Bunsen
burner flame. CAUTION: HCl causes burns; avoid skin contact. Repeat the
dipping and heating of the flame loop until no color is emitted from the wire.
2. Dip the clean wire loop in a sample of one of the metals listed in the table and heat as
demonstrated by your instructor. Observe the color and record in your table. (Be very
specific about the particular shade of the color you observe.)
3. Repeat Steps 1 and 2 (be sure the wire is clean before testing another metal) for
compounds of barium, calcium, copper, potassium, strontium, and lithium. Observe
and record the color for each in your table. Save sodium nitrate, NaNO3, for last;
sodium is very difficult to clean from your wire.
4. Obtain a mixture of sodium and potassium compounds. With a clean loop, observe
the color. Repeat the exercise, but observe the color through two pieces of cobalt
glass. Record the results. Use the cobalt glass and observe sodium and potassium
ions separately. Record the results in your table.
5. Obtain samples of two (2) unknowns from your instructor. Using the steps above and
the data you recorded in your table, determine their identity.
6. Cleanup
Clean your lab area and glassware before being signed out.
2
Flame Tests
LAB FIVE
Name
Report: Flame Tests
Lab Partner(s)
Section
Date
Data
Metal Ion
Flame Color (be very descriptive!)
Barium
Ba2+
Calcium
Ca2+
Copper
Cu2+
Potassium
K+
Strontium
Sr2+
Lithium
Li+
Sodium
Na+
Sodium &
Potassium
Na+ & K+
Flame Color (with Cobalt Glass)
Sodium &
Potassium
Na+ & K+
Potassium
K+
Sodium
Na+
1st Guess
2nd Guess
3rd Guess
Unknown #1
Unknown #2
Flame Test
3
LAB FIVE
Post Lab Questions
1.
2.
3.
4.
5.
4
What metal ions would you predict to be contained in your unknowns? Why?
Based on your results and observations, would this method be practical to
determine the identity of metals in a mixture? Why or why not?
Explain the reason potassium was visible when using the cobalt glass.
Explain the effect of the nitrate ion in solution on the color emission.
Give at least two reasons why the flame test is sometimes invalid.
Flame Tests