The Identity of an Unknown Compound from
Qualitative Observations
Purpose:
In this experiment you will use inferences drawn from qualitative observations to identify an
unknown compound.
Background:
This experiment emphasizes the importance and utility of qualitative observations and the
inferences that can be drawn from those observations. The only requirements for the successful
completion of this experiment are attention to the details of the procedure and accurate
observations.
Chemical reactions result in the production of a new substance(s). There are several phenomena
that indicate the occurrence of a chemical reaction. These include the production of a precipitate,
the production of a gas, the absorption or release of heat, and a change in color. Any or all of
these indications may occur during a given chemical reaction.
You will be given solid forms of NaCl (sodium chloride), NaI (sodium iodide), Na2CO3 (sodium
carbonate), Na2HPO4 (sodium hydrogen phosphate), and Na2SO4 (sodium sulfate) as well as
solutions of these chemicals in water. These compounds will be tested with solutions of HNO3
(nitric acid), Ba(NO3)2 (barium nitrate), NH3 (ammonia), and AgNO3 (silver nitrate) in order to
observe characteristic reactions (if any). An unknown sample, which will be either NaCl, NaI,
Na2CO3, Na2HPO4, or Na2SO4, will be tested simultaneously. The identity of the unknown will
be deduced by matching its characteristic reactions with those of the known compounds.
Procedure:
Obtain an unknown from Mr. Gensits. Use the small test tubes for each of the reactions. These
test tubes should be throughly washed and cleaned after each use. (Rinse them with distilled
water after they have been rinsed in the sink. They do not need to be completely dry.)
Tests for Gas Evolution and Other Effects
Mark the test tubes for recognition with a marking pencil. Use a clean spatula to place pea-sized
samples of NaCl, NaI, Na2CO3, Na2HPO4, Na2SO4, and your unknown in separate test tubes.
Add five drops of HNO3 to each substance and record your observations in your laboratory
notebook. Look for gas evolution and color changes.
Dissolving Your Unknown
Thoroughly wash a 10 mL graduated cylinder and a 100 mL beaker and rinse each with distilled
water. Place another pea-sized sample of your unknown in the beaker, add 10 mL of distilled
water, and swirl gently until all of the solid has dissolved. Save the remaining portion of your
unknown for use in the event of an unforeseen accident.
Test with Barium Nitrate
Place 20 drops of the solution of each known compound and your unknown substance in separate
test tubes. Add three drops of NH3 solution to each test tube. Insert a clean, dry stirring rod into
the solution, stir the solution, remove the stirring rod, and touch the adhering drop of solution to
a strip of red litmus paper. If the color does not turn to blue, add NH3 dropwise until it does.
Add five drops of Ba(NO3)2 to each test tube. Shake gently (do not use your finger as a stopper)
to obtain a homogeneous mixture and look for the formation of any insoluble precipitates. (Note:
It is always best to centrifuge a solution to determine whether any precipitate may be formed and
to note its color.) Record your observations.
Next, add ten drops of the HNO3 solution to each test tube that contains a precipitate. Using a
stirring rod, test each solution with blue litmus paper. If the color does not turn to pink, add
HNO3 dropwise until it does. Record your observations.
Tests with Silver Nitrate
Using fresh solutions, take the same quantities of the known samples and the unknown as in the
previous test. Add five drops of the AgNO3 solution to each test tube and record your
observations.
Add ten drops of HNO3 to each test tube that contains an insoluble precipitate and shake the test
tube gently. If each solution does not cause blue litmus paper to turn pink, add HNO3 dropwise
until that result is obtained. Record your observations.
Add 25 drops of NH3 solution to each of the test tubes that still contains an insoluble precipitate
and stir each thoroughly with a clean stirring rod for one or two minutes. Record your
observations.
Identity of the Unknown
Proper identification of the unknown requires that each of its characteristic reactions match those
of one of the known samples. If ambiguities occur, repeat as many tests as you desire.