EXPERIMENT
Identification of Unknown Solutions
PURPOSE
As you work through this exercise you will learn how to:
 Make careful observations of chemical reactions.
 Develop logical testing procedures for performing qualitative analyses.
 Describe solubility rules and fundamental acid/base chemistry.
MATERIALS AND EQUIPMENT
In addition to the solutions themselves, you may also use the following materials:
• Universal pH indicator paper
• Spot-plate
• Pasteur pipets + bulb
DISCUSSION
Note: Review solubility rules and net ionic equations in your textbook
One of the central roles of a chemist involves the analysis of materials. In some instances the analysis is
quantitative, that is, a determination is made of how much of a particular substance is present. In other
cases the analysis is qualitative and a determination of what components are present is made. In this
experiment you will perform a qualitative analysis of a set of unknown solutions. Your team (four
students maximum) must first determine which set of unknowns you have, and then identify what is
present in each testtube or bottle. The identification process will involve observations of solution
properties such as pH, color, odor, energy change, and the results of mixing two solutions, such as the
formation of an insoluble solid (precipitate). The only reagents you will be allowed to use are the
unknown solutions themselves.
Page 1
Unknown Solutions
A little pre-lab preparation is in order. As an example, suppose you have a set of unknowns consisting
of the following solutions (in no particular order):
Fe (NO3)2(aq), KOH(aq), Cu(NO3)2(aq), Na3PO4(aq), HCl(aq)
By this time you should have covered strong and weak electrolytes in lecture, which include strong and
weak acids. Using a universal pH indicator paper you would carry out preliminary tests looking for
strong/weak acids and bases. The pH of a solution is a measure of the acidity or basicity of a substance.
A neutral solution has a pH of about 7. A solution is acidic if it has a pH below 7 and a solution is
considered basic if it has a pH above 7 (see chart below). You may also find a solution weakly acidic or
basic respectively if you have a salt that contains a cation which is acidic or an anion which is basic (the
conjugate acid of a weak base, or the conjugate base of a weak acid). Additionally, small metal ions
(like Al3+) with large charge have acidic properties (Lewis acid/base concept). Having given you the
foregoing information let’s proceed with how we make use of the information in the example solution
set above and their acid-base properties. An estimate of a solution pH can be made the following way.
Place a small drop of the solution from your spot-depression plate in contact with universal pH indicator
paper and note the color that the wetted paper becomes. Now look for the matching color and associated
pH value.
We want to note that in this example Cu2+ ion in solution has a distinctive blue color so the identification
of the Cu(NO3)2 solution can be made on the basis of color; the other four solutions are colorless. The
unknown solution sets you will be given are all colorless.
This example set contains a strong acid, HCl, and a strong base, KOH. A drop of one of the solutions on
the pH paper that gives a dark red color, would indicates a pH much less than 7 and a strongly acidic
solution, identifying HCl. A testing another solution may reveal a deep blue or purple color and suggest
a strongly basic solution that would identify KOH. Testing the remaining two solutions using universal
pH indicator paper would suggest which solution is Fe(NO3)2(aq) and which solution is the Na3PO4(aq).
As ordered below the first solution contains a Fe2+ salt and would be neutral. The other solution contains
the conjugate base of a Bronsted-Lowry ionic acid, and hence would give a weakly basic solution and
must be Na3PO4(aq). [Your instructor will demonstrate this property during the lab lecture.]
So in this example you would have been able to identify all of the five solutions on the basis of color
and their acidic and basic properties. This will not be the case with some of your unknown solution sets
and in order to identify them completely you may want to make observations as to what happens when
pairs of solutions are mixed. This requires that you set up a reaction grid as shown below.
Page 2
Unknown Solutions
Sol’ns
Fe(NO3)2(aq)
KOH(aq)
Cu(NO3)2(aq)
ppt.
Fe(NO3)2(aq)
KOH(aq)
Na3PO4(aq)
HCl(aq)
N.R.
ppt.
N.R.
ppt.
N.R
heat evolved
ppt.
N.R
Cu(NO3)2(aq)
heat evolved
Na3PO4(aq)
HCl(aq)
By mixing each solution with each of the others and using the solubility rules in Table 1., you can
predict the outcome of the reaction and verify what is in each solution. Note that if Cu(NO3)2 solution is
mixed with each of the other four solutions, a precipitate of an insoluble copper salt is expected with
KOH and Na3PO4, but not with Fe(NO3)2 (see Table 1.). Thus, the identity of the Fe(NO3)2 solution can
be made.
The possible sets of unknown solutions in this experiment are:
Set A: AgNO3(aq), Mn(NO3)2(aq), Ba(NO3)2(aq), HCl(aq), NaOH(aq)
Set B: Zn(NO3)2(aq), Al(NO3)3(aq), AgNO3(aq), NaOH(aq), NH3(aq)
Set C: AgNO3(aq), Ba(NO3)2(aq), HCl(aq), H2SO4(aq), NaOH(aq)
Set D: AgNO3(aq), Pb(NO3)2(aq), HCl(aq), NH3(aq), H2O(aq)
As part of your pre-laboratory assignment set up four reaction grids and using Table 1. Predict the
results when a small amount of one solution is mixed with each of the others. Also make use of the vast
amount of information available to you over the internet and find out what colors the predicted
precipitates would be.
Table 1. Solubility Guidelines for Salts and Bases in Water
Soluble Compounds
Compounds containing:
Important Exceptions
• None
NO3–
C2H3O2–
–
–
Cl , Br , or I
Insoluble Compounds
Compounds containing:
• None
–
• Compounds of Ag+, Hg22+, and Pb2+
SO42–
• Compounds of Sr2+, Ba2+, Hg22+, and Pb2+
S2–
Important Exceptions
• Compounds of NH4+, the alkali metal
cations, and Ca2+, Sr2+, and Ba2+
CO32–
• Compounds of NH4+ and the alkali metal
cations (IA)
PO43–
• Compounds of NH4+ and the alkali metal
cations (IA)
OH–
• Compounds of the alkali metal cations (IA),
Sr2+, and Ba2+, Ca2+ (slightly soluble)
Page 3
Unknown Solutions
PROCEDURE
1. Each team should obtain a set of unknown solutions. Record the unknown number for your set.
2. Carry out preliminary tests to decide which of the four possible sets of unknown solutions you have.
Be sure to record detailed observations as you perform these tests (preferrably in your lab notebook).
3. Once you have determined which group of solutions you have, make additional tests (by mixing
solutions) so that you can identify what is specifically in each of the five solutions.
Caution: When taking samples for testing on the spot-plate, be careful not to contaminate
any of the solutions. Use a separate Pasteur or plastic pipet for each solution.
When finished turn in your report form (below) and pre-laboratory worksheet for this experiment.
Cut along the solid lines lines
Chemistry 101 Unknown Solutions Report
Unknown #: __________
Members of Group 1. ___________________ 2. ___________________
Set (circle): A, B, C, D
Members of Group 3. ___________________ 4. ___________________
Contents of Tube: A __________ B __________ C __________ D __________ E __________
Comments (optional):
Page 4
Pre-Lab Questions
1. a. Write a balanced equation for the reaction that occurs when an aqueous solution of iron(II)
chloride is mixed with an aqueous solution of potassium hydroxide.
b. Write a net ionic equation for the above reaction.
2. What simple test would you use to identify the possible presence of HCl(aq)? What result do you
expect if the solution is HCl(aq)?
3. What preliminary test results are you looking for to identify conclusively your unknown solutions as
Set C? Explain.
4. When an aqueous solution suspected of containing Ba2+ or Pb2+ or both is mixed with sulfuric acid
a precipitate forms. In another test, when the original solution is mixed with an aqueous solution of
sodium sulfide no precipitate forms. What do these two tests indicate about the likely presence of
Ba2+ and Pb 2+ in the original solution? Explain your answer.
Page 5