Unknown Compound Identification
An unlabeled container of a white compound has been discovered in the chemistry stockroom. In order
to dispose of it correctly, you and your team are being asked to identify the compound and many of its
chemical and physical properties.
Objectives:
-Identify the compound
-Explain your identification using evidence from the experiment
Weigh out ~5 g of your unknown compound. To make your task manageable, the identity of your
unknown will be restricted to the following list of solid compounds. These are available in lab to use in
reactions with, and as comparisons to, your unknown compound.
Possible Unknowns
CaCO3
CaCl2
Ca(NO3)2
MgCl2
MgSO4
KCl
KNO3
K2SO4
K2CO3
Na2CO3
NaCl
Na2SO4
NaC2H3O2
NH4Cl
(NH4)2SO4
Safety Notes:
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The compounds used in this project are generally recognized as safe, but always treat chemicals
with care, especially when dealing with your unknown
Although your unknowns may look like salt, do not use your hands to touch or taste any of the
unknowns. Some of the possible unknowns are hazardous when ingested.
There are a few hazardous chemicals involved in this lab. Make sure to wear goggles at all times
while working in the lab.
Be especially careful when working with concentrated acids or bases. When handling these
chemicals you should be wearing gloves and goggles at all times. If you spill some please alert
the teacher immediately. If you spill on yourself please alert the teacher and immediately flush
the area with water.
When using a Bunsen burner make sure all students are wearing goggles, tie back long hair, and
watch out for loose clothing to prevent accidents.
Please let the teacher know immediately if there are any spills or accidents, or you have a
question about handling chemicals.
Equipment & techniques you may find useful:
 anion and cation
 flame tests
tests
 preparing a solution
Available Materials
Concentrated sodium
in dropper bottles
hydroxide, nitric acid,
Dilute (0.05 M) sodium
hydrochloric acid available
hydroxide, nitric acid,
hydrochloric acid
Silver Nitrate 0.1 M
Barium Chloride 0.1M
Ammonia Solutions
Solid KCl, CaCl2, MgCl2,
NaCl
Unknown substances
labeled A, B, C,D
containing one of the
following anions and
cations:
Anions: CO3-2, Cl-, NO31
,SO4-2, C2H3O2Cations (Ca+2, Mg+2, Na+,
NH4+, K+)
Bunsen burner
Wire loops (for flame
tests)
pH probe
pH paper
conductivity probe
large test tubes
Erlenmeyer flasks and
Beakers (50, 150, 250 mL)
pipets
weighing paper
Procedure
Part I: Creating Solution of your solid
1. Obtain ~5g of your designated sample (A, B, C or D) from your teacher.
2. Dissolve 1 g of your unknown substance in deionized water in a 250 mL beaker. 2040ml of water may be needed.
3. You will be conducting tests with small amounts (1-2 mL) of your solution.
4. Tests are divided up into a cation test (Part II), and anion tests (Part III).
5. After determining your cation and anion you should be able to identify your unknown
solid.
Part II. Cation Test (Flame Test)
1. Measure about 5mL of unknown solution into a graduated cylinder.
2. Pour the solution into a test tube.
3. Clean the loop: Heat the wire loop in the outer edge of the Bunsen burners flame, keep
doing that till no color is in the flame.
4. Dip the clean wire loop into your unknown solution.
5. Move the loop up and down in the outer edge of the flame. Record the color of the
flame shown.
6. If you have extra time after identifying the complete formula, you may want to repeat
this test using one of the known compounds that has the same cation as you predicted.
You can then compare the color this produces to the color of your unknown.
Part III. Anion Tests
You will be conducting three different tests to identify your anion. You will be reacting a
solution of your unknown with Silver Nitrate, Barium Chloride, and Hydrochloric Acid.
A. Silver Nitrate with Ammonia
1. Add a few drops of dilute nitric acid (careful this is an irritant) followed by a few drops
of silver nitrate solution).
2. Does a precipitate form? If so, what color is it? Record your results.
3. If a precipitate formed, add dilute ammonia solution to the precipitate. Does the
precipitate dissolve? Record your results.
4. If dilute ammonia solution does not help your solid dissolve try adding more
concentrated ammonia solution. Does the precipitate dissolve? Record your results.
B. Barium Chloride
1. Go to your teacher and have them add a few drops of Barium Chloride to solution. (Be
careful barium chloride is very toxic).
2. Does a precipitate form? Record your observations and results on the data table.
C. Hydrochloric acid
1. Add 3-4 drops of dilute hydrochloric acid to solution and record your observations.
Data Table:
Test
Flame Test
Results and Observations
Silver Nitrate with
Ammonia Test
Barium Chloride
Hydrochloric acid
Data Interpretation
Cation test (Flame Test Colors)
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 excites the electrons of the metals ions, causing them to emit visible light. Every element
has a signature emission spectrum that can be used to differentiate between one element and
another.
Metal Ion
Color of flame
+2
Ca
Orange-red
Mg+2
No color
+
K
Violet
Na+
Yellow
+
NH4
No color
Anion Tests
Test
Anion
Chloride (Cl–)
Test and Observation
Add a few drops of dilute nitric acid (Irritant) followed
by a few drops of silver nitrate solution. A white
precipitate of silver chloride is formed. The precipitate
is soluble in ammonia solution.
Bromide (Br–)
Add a few drops of dilute nitric acid (Irritant) followed
a few drops of silver nitrate solution. A pale yellow
precipitate of silver bromide is formed. The precipitate
is slightly soluble in ammonia solution.
Iodide (I–)
Add a few drops of dilute nitric acid followed by a few
drops of silver nitrate solution. A yellow precipitate of
silver iodide is formed. It is insoluble in ammonia
solution.
Add a few drops of barium chloride solution (Toxic)
Barium chloride followed by a few drops of dilute
hydrochloric acid. A white precipitate of barium sulfate
is formed.
Add dilute hydrochloric acid to the solution (or add it
to Hydrochloric acid the solid). Bubbles of carbon
dioxide are given off.
Silver Nitrate with
Ammonia Test
Barium Chloride
Sulfate (SO42–)
Hydrochloric acid
Carbonate (CO32–)
Conclusion Questions
1. Which tests or reactions did you use to identify your unknown?
2. Based on the results of this test what is the cation? What evidence do you have to support
this?
3. Based on the results, what is the anion? What evidence do you have to support this?
4. How does each part of your proecedure (pH test, flame test, reactions, etc.) help to identify
your unknown? Please explicitly describe how each set of data you collected contributed to
finding your unknown.
5. Please write the name and molecular formula for your unknown compound:
6. How certain are you of your groups identification of the unknown? If you are not certain of
your groups identification what do you think are possible identities of your unknown?
7. What could you have done to be more certain in your identification of your unknown?
7. Can we ever be sure of the identity of the unknown?