Guided Inquiry Lab #3: Identification of an Unknown Cation and Anion by
Qualitative Analysis
Purpose:
To identify an unknown compound found in an unlabeled chemical container this
summer in the back of Mr. Thomas’ chemical cabinet using a series of chemical tests and
qualitative observations.
Introduction:
Qualitative analysis has long been a fundamental practice in research chemistry. Entire
books have been written to detail the various experiments and tests that can be used to identify
the presence of certain cations, anions, and even types of organic molecules. In this experiment,
you will be given the opportunity to develop your own procedures to identify an unknown cation
and anion in a series of aqueous mixtures. You will have access to pH paper, concentrated
sulfuric acid, conc. FeSO4(aq) and 1% hydrogen peroxide (H2O2) in water, along with any other
materials requested from the laboratory teacher.
The following is a suggested list of tests, but is not all inclusive. If identification cannot
be made after performing these tests, further identification procedures will need to be researched
and performed.

Brown-ring test: This test is used to identify the presence of nitrate, NO3–. Add a small
amount of the test solution to ~1 mL of conc. iron(II) sulfate in a small test tube. Then,
using a transfer pipet, slowly add concentrated sulfuric acid to the test tube. The sulfuric
acid will form a second layer (it is far more dense than the FeSO4 solution), and at the
interface of the two layers, the appearance of a brown ring signifies the presence of NO3–.
The brown ring is actually trapped NO(g), which is produced through an oxidationreduction reaction with the Fe2+:
NO3–(aq) + 4 H+(aq) + 3 Fe2+(aq)  NO(g) + 3 Fe3+(aq) + 2 H2O(l)
The solution should also turn slightly yellow, as Fe3+ complexes with water to form
yellow Fe(OH2)63+.

pH Test: You will have access to pH paper to test the pH of the various solutions.

Flame Test: When metal ions are heated in a flame, they give off a characteristic color due to
the excitement of certain electron transitions. By dipping a nichrome wire into a solution
and placing it in a Bunsen burner flame, you can observe the colors.

Ammonium Test: One can test for the presence of ammonia by adding sodium hydroxide to the
solution in question. The hydroxide will pull a hydrogen ion off of the ammonium to form
ammonia:
OH–(aq) + NH4+(aq)  NH3(g) + H2O(l).
The formation of ammonia can either be detected by its pungent smell, or by holding a
piece of damp acidic (red) pH paper above the solution.

Solubility Tests: One of the best ways to identify the presence of certain cations and
anions is to look for the formation of precipitates as certain combinations of cation and
anion are mixed. There are two ways for you to determine which tests to perform. One
is to randomly test all combinations of cations and anions, the other is to use a solubility
chart from either your book or the web to narrow down the tests to specific combinations
of ions that might assist in their identification.

Iodine Test: In acidic solution, hydrogen peroxide can oxidize I– to form I2, which then
reacts with another I– to form I3–, which has a yellow-brown color:
2 I–(aq) + H2O2(aq) + 2 H+(aq)  I2(aq) + 2 H2O(l)
I–(aq) + I2(aq) 
I3–(aq)
This test can be performed by adding a some HCl to the solution in question and then
adding some H2O2 dropwise with a transfer pipet.
Prelab:
1. Before you come to the lab on the first day, you should get together with your labmates and
develop a strategy for the tests you will do on the first day. Also, you will need to look up
the solubilities of various combinations of the cations and anions to determine which
combinations might be useful in identification. Remember, nitrate, ammonium, and
hydrogen ions will not form precipitates with anything.
2. Before coming to lab on the second day, you should prepare a procedure for identifying the
cation and anion based on the first day’s tests.
Materials:
well plate
dilute nitric acid solution
test tubes
sodium carbonate solution
transfer pipets
potassium hydroxide solution
pH paper
ammonium chloride solution
niochrome wire
1% hydrogen peroxide solution
Bunsen burner
concentrated sulfuric acid solution
iron(II) sulfate solution
concentrated iron(II) sulfate solution
Procedure:
In this experiment, you will be performing and recording your own procedures. Be sure
that whoever is reading your report can actually perform each step recorded as you have it
written. Be brief, but thorough…short, sweet and potent!
Pre-lab Questions:
1. Hydrogen ions will never register any color in a flame test. Why is this?
2. If you look at the list of cations and anions that form precipitates, two striking features are
clear. First, it is very rare to find a precipitate formed by a +1 cation and a –1 anion (Ag+ and
Hg22+ being the exceptions). Second, as you move up the periodic table in a group (say the
alkaline earth metals, Ba through Mg) the precipitates they form become less and less
soluble. What is the explanation for this?
3. Even though sulfate is the conjugate base of a weak acid (HSO4–), it does not register as basic
in a pH test. There are two very good explanations for this (one mathematical, the other
practical). What are both explanations?