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Identification of Unknowns
Introduction
A variety of chemical tests can be used to identify an unknown substance. In this experiment
you will carry out some simple chemical tests to determine the identities of compounds which
are unknown to you. You will perform qualitative analyses, that is, tests that are done simply to
identify the material, but not to determine the amount of that material present. Quantitative
analysis, on the other hand, involves the determination of the amounts of materials.
In the lab you will be given three compounds (your “unknowns”) that will be named only by
letters (for example, unknowns A, B, and C). The flow chart on page 2 outlines the chemical
tests you will perform in order to identify your unknowns. Follow along on the flow chart as you
read the following explanation.
How to use the flow chart (page 2):
The box at the top lists all the possible identities for the unknowns.
Some of these compounds are soluble in water, and some are not. Therefore, you will first
perform a water-solubility test by attempting to dissolve the unknown in water. If the unknown is
not soluble in water, you know that it must be one of the 3 non-water-soluble compounds listed
in the box on the left. If the unknown is soluble in water, then it must be one of the 8 watersoluble compounds listed in the box on the right.
Suppose you determined that your unknown was not soluble in water. You know that it must be
CaSO4, CaCO3, or cornstarch (compounds in box on left). Following the flow chart, you would
next perform the iodine test. As the flow chart shows, cornstarch will turn purple when iodine is
added, but the other two compounds turn a shade of brown or yellow. If you add iodine to your
unknown and it turns purple, you know that the unknown is cornstarch. At that point you can
stop the analysis since cornstarch is the only unknown that will turn purple with iodine—you
have unambiguously identified the unknown.
However, if you add iodine and your unknown turns brown or yellow, it could be CaSO4 OR
CaCO3. In that case, you would need to proceed to the next test to determine which of the two it
is. The flow chart shows that you should use the vinegar test to distinguish between the two. If
you add vinegar to the unknown and see bubbles of gas rising from the compound, your
unknown is CaCO3. If no gas bubbles are produced, your unknown is CaSO4. You can then
stop the analysis since you will have unambiguously determined that your unknown was CaCO 3
or CaSO4, based on the vinegar test.
The remainder of the flow chart (on the right side), would be followed in a similar manner for any
of the water-soluble unknowns.
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Identification of Unknowns
Prelab
Name________________
1. In this experiment you will do qualitative analyses to determine the identity of unknown
compounds.
What does qualitative analysis mean?
What does quantitative analysis mean?
2. Use the flow chart to answer the following questions (A-E).
A. Which non-water-soluble compound produces bubbles of gas when vinegar is added?
B. Which two water-soluble compounds produce bubbles of gas when vinegar is added?
C. Carefully examine the chemical formulas of the three compounds you just named above.
What ion do all three have in common? Be sure to include the correct charge on the ion.
D. Suppose that you are working with a water-soluble unknown which does not produce a bright
pink colored solution when phenolphthalein is added. Based on this result, which of the
unknowns might you have? (list all possible identities)
E. After getting the results of this phenolphthalein test (no pink color; part D), which test should
you perform next to narrow down the list of possible identities for your unknown?
3. It is very important not to use too much of the unknown compound when carrying out the
water-solubility test. Suppose you are working with an unknown that is actually soluble in water:
you take a sample of the unknown that is too large (larger than the amount specified in the
experimental procedure) and add some water. What do you think will happen?
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Identification of Unknowns
Procedure
Part 1: Practice Using Known Compounds
In order to learn how to recognize a specific outcome of a chemical test (for example, how to
recognize when a compound is soluble versus non-soluble in water), you will perform a few of
the chemical tests on known compounds. Since the identities of these compounds will be known
to you, you will know what outcome to expect for each chemical test.
A. Determining whether or not a compound is soluble in water.
General instructions to follow when determining water-solubility:
 Don’t use too much of the compound being tested. A small sample about ½ the size of a
pea is appropriate.
 Stir/shake the solution since the compound may be composed of large particles that need
to be broken up in order to dissolve.
 Allow enough time for the compound to dissolve. Many soluble compounds won’t
dissolve immediately upon adding water—they may require stirring for several minutes
before they dissolve fully.
NaOH—water soluble
Put 5 mL of purified water into a test tube. Add one pellet of solid NaOH. Stir or shake the test
tube until the pellet dissolves completely (no particles visible). This may take a few minutes.
When the NaOH has dissolved completely, examine the solution carefully—this is what a
solution containing a water-soluble compound looks like. On the report sheet, describe the
appearance of the solution. Save the test tube containing the NaOH solution for later.
MgSO4—water soluble
Put 5 mL of purified water into a test tube. Add a small amount (1/2 the size of a pea) of MgSO4.
Stir or shake the test tube until the solid dissolves completely (no particles visible). This may
take a few minutes. When the MgSO4 has dissolved completely, examine the solution
carefully—this is another example of what a solution containing a water-soluble compound looks
like. On the report sheet, describe the appearance of the solution.
CaSO4—non-water soluble
Put 5 mL of purified water into a test tube. Add a small amount of CaSO4. Stir or shake the test
tube for a few minutes. Then examine the solution carefully. Are there solid particles of CaSO4
sitting at the bottom of the tube? Is the solution very cloudy/milky? If so, it is due to undissolved
particles that are suspended in the liquid. This is an example of what a solution containing a
non-water-soluble compound looks like. On the report sheet, describe the appearance of the
solution.
B. Determining whether a compound is basic.
Phenolphthalein is a chemical which reacts with bases to form a very bright pink (fuschia)
product. If a phenolphthalein test produces only a pale pink or a colorless solution, this indicates
that the compound being tested is either an acid, or a very weak base.
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NaOH—a strong base
Take the test tube containing the NaOH solution from above and add one drop of
phenolphthalein solution. Take careful note of the bright pink (fuschia) color. This is the color
that is produced when phenolphthalein reacts with a base. You may now discard the contents of
all test tubes down the drain. IMPORTANT: Any test tubes containing phenolphthalein must be
washed very thoroughly to remove all traces of this chemical and prevent it from interfering with
other tests.
Part 2: Identification of Unknown Compounds
The instructor will assign three unknowns to your lab group. Record the letters of the unknowns
on the report sheet.
Choose one of your three unknowns to begin with. Use the flow chart to determine which
chemical tests to carry out on this unknown: begin at the top of the chart with the water-solubility
test, and work down the right or left side of the chart, depending on the outcome of the solubility
test. The instructions for carrying out each test are given below. On the report sheet, record in
order all the tests you carried out, along with the outcome of each test. For example, use this
format:
Unknown __X__
____Chemical Test Carried Out
1. Water solubility test
2. Phenolphthalein test
3. Vinegar test
Results of Test___
compound dissolved
solution turned bright pink
bubbles of gas were produced
Unknown must be _Na2CO3__
Instructions for Chemical Tests
Water solubility test
Put 5 mL of purified water into a test tube. Add a small amount of the unknown (1/2 the size of a
pea). Stir or shake the test tube for a few minutes. Observe the resulting solution, using what
you learned in Part 1 for the known compounds. When finished, save the solution in the test
tube and use it for the next test.
Iodine test
Take the test tube containing your unknown that you saved after the water-solubility test, and
add one or two drops of the iodine solution. The solution will turn either a shade of brown
(ranging from yellow to dark brown), or a shade of purple (may be bluish, or very dark, nearly
black). When finished, discard the solution down the drain.
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Phenolphthalein test
Take the test tube containing your unknown that you saved after the water-solubility test, and
add one drop of phenolphthalein solution. If the solution turns very bright pink (fuschia), the
unknown is basic. If the solution is pale pink or colorless, the unknown is acidic or only very
weakly basic. When finished, discard the solution down the drain. IMPORTANT: Any test
tubes containing phenolphthalein must be washed very thoroughly to remove all traces of this
chemical and prevent it from interfering with other tests.
Vinegar test
Place a fresh sample of the unknown in a clean test tube (do not add water). Add approximately
1 mL of vinegar. Immediately look for the formation of gas bubbles. If gas is being formed, the
solution may appear to be fizzing as the bubbles rise to the surface. When finished, discard the
solution down the drain.
NaOH test
Place a fresh sample of the unknown in a clean test tube. Add approximately 4 mL of water and
stir until the compound is dissolved—the solution should be clear and colorless before
proceeding. Then add 3 drops of the 1.0 M NaOH solution (NOT the solid NaOH pellets). Look
for the appearance of a white precipitate—it may not settle to the bottom, but may just make the
solution very cloudy. When finished, discard the solution down the drain.
Benedict’s test
Prepare a boiling water bath by half-filling a small beaker with distilled water and placing it on a
hot plate; heat until the water boils. Place a fresh sample of the unknown in a clean test tube.
Add approximately 4 mL of water and stir until the compound is dissolved. Then add 1 mL of
Benedict’s reagent (which has a pale blue color). Place the test tube in the boiling water bath
and allow it to heat for about 3 minutes. The solution will either remain clear blue (due to the
Benedict’s reagent), or will form a chunky looking brick-red/orange precipitate. When finished,
discard the solution down the drain. Keep the water bath set up in case you need it later for the
hot water test.
Rubbing alcohol test
Place a fresh sample of the unknown in a clean test tube (do not add water). Add approximately
8 mL of rubbing alcohol (isopropyl alcohol). Stir/shake very vigorously for several minutes. If
after ~5 minutes the compound has not dissolved at all, you can conclude that it is not soluble in
alcohol. If the compound is completely dissolved or nearly dissolved after 5 minutes, you can
conclude that it is soluble in alcohol. When finished, discard the solution down the drain.
Hot water test
Prepare a hot water bath (not boiling) by half-filling a small beaker with distilled water and
placing it on a hot plate. Heat the water until it feels hot to the touch but is not boiling. Fill a
small test tube to the 1 mL level with the dry unknown solid to be tested (note: this is a lot more
of the unknown than you have used for any other tests). You can determine the 1 mL level by
comparing the test tube to your small graduated cylinder. Then add water to the test tube to the
5 mL level. Heat in the water bath for 2-3 minutes, stirring the contents of the test tube
occasionally. While heating, observe whether the solid dissolves readily or not.
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Identification of Unknowns
Report Sheet
Name __________________
Part 1: Practice Using Known Compounds
A. Determining whether or not a compound is soluble in water.
NaOH solution:
MgSO4 solution:
CaSO4 solution:
Part 2: Identification of Unknown Compounds
Unknown _____
____Chemical Test Carried Out
Unknown must be ________________
Results of Test___
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Unknown _____
____Chemical Test Carried Out
Results of Test___
Unknown must be ________________
_______________________________________________________________________
Unknown _____
____Chemical Test Carried Out
Unknown must be ________________
Results of Test___
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Identification of Unknowns
Postlab
NAME__________________
1. Circle the correct word in brackets and then complete the sentence below.
Today’s experiment was a [qualitative / quantitative] analysis because:
2. In the experiment titled “Separation of a Mixture” you determined how many grams of benzoic
acid were contained in a sample of a mixture. This was a [qualitative / quantitative] analysis.
(circle the correct word in brackets)
3. On the prelab you identified CaCO3, Na2CO3, and NaHCO3 as the three compounds that
produce bubbles of gas when vinegar is added. All three compounds are a source of the
carbonate ion, CO32-. When this ion comes in contact with vinegar, it reacts with the acid in the
vinegar and decomposes to form a gas. The gas is visible as bubbles travelling to the surface of
the liquid. What gas is being formed? If you’re not sure, ask the instructor for help.
4. When KOH (potassium hydroxide) is mixed with phenolphthalein, a bright pink color can be
observed. What can you conclude about KOH?