DRAFT DOCUMENT
LAB 15
IDENTIFICATION OF ANIONS AND CATIONS IN SOLUTION
In this activity you will become a chemical detective. You will conduct analyses
to determine the ionic composition of two unknown solutions. The process of
determining the composition of a sample of matter by conducting chemical tests is called
qualitative analysis. Solutions of unknown ions can be subjected to chemical and flame
tests and the results can be compared to the results given by known ions, in the same
tests. By conducting the appropriate tests and applying simple logic, the identities of the
ions present in the unknown solutions can be determined.
To help you identify the composition of two unknown metallic solutions, you will
use flame tests to identify the cation and chemical tests (e.g. silver nitrate, lead nitrate) to
identify the anion. The flame test is based on the concept that when elements are heated
to high temperatures, some of their electrons are excited to higher energy levels. When
these excited electrons fall back to lower energy levels, they release excess energy as
light (photons). The color of the emitted light depends on its energy (blue light is more
energetic than red light). As each element emits a characteristic pattern of light energies,
excited electrons fall back to lower energy levels. The color of the light given off is
characteristic of a specific element.
Qualitative tests for anions are based upon the idea that each ion reacts in its own
characteristic way in a chemical reaction (no two ions react the same). Reactions
involving color changes, the evolution of gas, and/or the formation of a precipitate
(a solid product) are indicative of the presence of specific anions.
As you complete this activity, remember that careful observation and logical
reasoning are crucial if you are to be a successful chemical detective.
OBJECTIVES
At the end of the activity, you will be able to:
1. Perform flame tests to determine the characteristic flame colors of known cations.
2. Perform qualitative tests to characterize anions in solution.
3. Devise a schematic plan to identify anions and cations in solution.
4. Identify the composition of two unknown ionic solutions.
5. Write the formulas and names of specific ionic compounds.
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PART 1: Flame Tests for Cations
MATERIALS
Goggles
Splints
Cobalt blue glass
Bunsen burner
Solutions of: potassium nitrate
calcium nitrate
copper(II) nitrate
sodium nitrate
strontium nitrate
barium nitrate
lithium nitrate
PROCEDURE
1. Dip a wooden splint into a test tube of metallic salt solution. Remove splint after
10 seconds.
2. Wave the splint SLOWLY back and forth through the flame just above the tip of
the inner blue cone. (DO NOT LET THE SPLINT CATCH ON FIRE.) Record
your observations in the chart below.
3. After you have viewed the flame color produced by sodium nitrate and potassium
nitrate, use the cobalt blue glass to view the flame color again. Record new
observations in the appropriate space below.
4. Dispose of used splints.
DATA
Ion
Table of Flame Tests
Flame Color
sodium, Na+
sodium, Na+ (cobalt glass)
potassium, K+
potassium, K+ (cobalt glass)
calcium, Ca2+
barium, Ba2+
strontium, Sr2+
lithium, Li+
copper(II), Cu2+
__________________________
__________________________
__________________________
__________________________
__________________________
__________________________
__________________________
__________________________
__________________________
QUESTIONS
1. List several elements that could be identified based on their characteristic flame
color.
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2. Explain why some elements are difficult to identify by flame color.
3. Which element produces the most intense color?
4. Would flame tests be useful for detecting specific cations present in a mixture of
metal ions? Why?
5. What is the purpose of using cobalt glass in the identification of sodium and
potassium?
6. Problem Solve: You are enjoying a July 4th fireworks display when one of the
rockets explodes prematurely. The color of the explosive mixture is yellow,
green, and red-orange. Which three metals were most likely present in the
rocket? Support your answer with evidence from the flame tests you performed.
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PART 2: Qualitative Tests for Anions
MATERIALS:
Known anions:
5% NaCl
5% NaNO3
5% Na2S
5% NaI
5% Na2SO4
0.6 M NaHCO3
Indicators:
0.2 M AgNO3
0.2M Pb(NO3)2
3.0 M HCl
goggles
depression plates
China marker
PROCEDURE
CAUTION
READ LABELS CAREFULLY!!!!!!
Avoid getting AgNO3 on your hands and clothing.
1. Label 8 troughs on the depression plate with the anion being tested.
2. Add the following chemical reagents to each trough in order and record your
observations.
Known Anion
a)
4 drops 5% NaCl
Qualitative Tests for Anions
Indicator
Observations
2 drops 0.2 M AgNO3
___________________________
b) 4 drops 5% NaNO3
2 drops 0.2 M AgNO3
___________________________
c)
2 drops 0.2 M AgNO3
___________________________
d) 4 drops 5% Na2SO4
2 drops 0.2 M AgNO3
___________________________
e)
4 drops 5% Na2SO4
2 drops 0.2 M Pb(NO3)2
___________________________
f)
4 drops 5% NaI
2 drops 0.2 M Pb(NO3)2
___________________________
g) 4 drops 0.6 M NaHCO3
2 drops 0.2 M Pb(NO3)2
___________________________
h) 4 drops 0.6 M NaHCO3
2 drops 3.0 M HCl
___________________________
4 drops 5% Na2S
QUESTIONS
1. Underline the anion being tested in 2a-h.
2. Explain why it is necessary to test sodium sulfate with lead nitrate.
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3. Justify your observation in 2b. Explain why you did not observe a similar result
when AgNO3 was added to NaCl or Na2S.
4. Write a balanced equation for each chemical reaction that gave a positive
qualitative test.
PART 3: Determining the Composition of Unknown Ionic Solutions
PROCEDURE
1. Devise a plan to identify the composition of each unknown. List the steps on the
worksheet.
2. Draw a flow chart summarizing the expected results that would lead to a clear
identification of an unknown anion.
3. Perform the qualitative tests (chemical and flame tests) for each unknown. Record
your observations in an organized fashion.
WORKSHEET / DATA
1. Procedural Plan
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2. Flow Chart
3. DATA
ANALYSIS
1. List the anion and cation present in each unknown. Give the chemical name and
the formula of each compound.
Unknown
Cation
Anion
Name
Formula
2. Complete the chart (Compounds Formed From Ions) by legibly writing the
formula and name of each compound.
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EXTENSION
THINK and then WRITE: Use your knowledge about valence electrons, atomic radii,
ionization energy, orbital energies, the process of emitting light, and additional
reference material as needed to explain why the emitted light in the flame test for
sodium is yellow while the light emitted for strontium is bright red-orange. For full
credit you must give a thorough explanation in a well-written paragraph.
Adapted from: Addison-Wesley: Identification of Anions and Cations in Solution and Flame Tests for
Metals; Chemistry: Non-Consumable Laboratory Handbook. 4th Ed. 1995.
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Compounds Formed From Ions
Directions: 1) Write the charge of each ion.
2) Write the formula of the compound formed by the two ions.
3) Give the classical and/or Stock name of each compound.
Cl
HCl
H
Na
CO3
NO3
S
hydrochloric acid
NaCl
sodium chloride
Zn
NH4
Al
Fe2+
Pb4+
103
PO4
H2PO4
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LAB 15
PRELAB: IDENTIFICATION OF ANIONS AND CATIONS IN SOLUTION
Read the introduction to the lab activity, highlighting key information as you read. Use
the information in the introduction and your class notes to answer the following
questions.
1) What is an ionic solution composed of?
2) List three examples of ionic solutions.
3) Answer a or b
a. Why do elements emit visible light?
b. What is the relationship between emitted visible light and energy?
4) Differentiate between a qualitative test and quantitative test.
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