41 Experiment
2
Identification of Compounds through
Physical and Chemical Changes
OBJECTIVE
To identify compounds on the basis of physical and chemical changes and design a procedure for
determining the presence of a particular compound.
EQUIPMENT
Fifteen 12 x 75-mm test tubes, test tube rack, test tube holder, permanent marker (to label test
tubes), three medicine droppers. If test tubes are not available, a 24-well plate may be used.
REAGENTS
Part A: Approximately 1 mL (~5-10 drops) each of aqueous known solutions of NaCl, Na2CO3,
MgSO4, NH4Cl, and H2O (this step is repeated for the three test reagents); 5-10 drops each of
aqueous test reagents of AgNO3, NaOH, and HCl.
Part B:
Approximately 5-10 drops each of solutions labeled 1 – 5; 5-10 drops of test reagents labeled
A, B, and C.
SAFETY AND DISPOSAL
Refer to the MSDS information available online when working with NaCl,1 Na2CO3,2 MgSO4,3
NH4Cl,4 AgNO3,5 NaOH,6 and HCl.7 Contact with silver nitrate solutions can lead to staining of the skin,
dermatitis, and painful burns.5
Disposal should be in accordance with local, state and federal regulations. Disposal for NaCl,
Na2CO3, MgSO4, NaOH, and HCl should be into a labeled laboratory waste container for inorganic
chemicals. All waste silver salts, especially those that result from the AgNO3 reagent observations found
in this experiment, should be recovered in a specially labeled waste container for silver salts.
INTRODUCTION
A goal of a research chemist is, for example, to separate the substances of a reaction mixture and
attempt to identify each substance through a systematic study of their chemical and physical properties.1
Any change in the appearance of a substance can be classified as either a physical or chemical change.
In a physical change, the same substance remains after the change (melting or boiling a liquid,
cutting a piece of wood, tearing paper, dissolving sugar in water, and pouring a liquid from one container
to another are some examples). These changes do not change the chemical character of the substance.
In a chemical change, the substance undergoes a change so that one or more new substances with
different characteristics are formed (burning, digestion, and fermenting are some examples). Let us take a
specific example. Sodium is a silvery, soft metal that reacts vigorously with water. Chlorine is a yellowgreen gas that is highly corrosive and poisonous. However, if these two elements are combined, they
produce a different substance, a white crystalline solid. This new substance is common table salt, sodium
chloride, which is neither reactive with water nor poisonous!
Experiment 2 Physical and Chemical Changes 41 42 As a rule of thumb, if there is gas formation, color change, precipitation, a reaction that generates
or removes energy, or odor change occurs, a chemical change has taken place.
A gas forms. This evolution may be quite rapid, or it may be a “fizzing” sound. For example, the
bubbles of gas observed when antacid is dropped into water.
A color change occurs. A substance added to the system may cause a color change. In describing
a colored liquid, name the color. In describing a liquid that has no
color, call it colorless. For example, milk would be described as a
cloudy white liquid. If it is a transparent, colorless liquid, describe the
liquid as clear and colorless.
A precipitate appears (or disappears). The nature of the
precipitate is important. It may be crystalline, it may have color or it
may merely cloud the solution. An example is shown in (Fig. 2-1).
An energy change (heat or absence of heat) occurs. Heat may
be evolved or absorbed. The reaction vessel becomes warm if it
releases heat (exothermic) or cools if it absorbs heat (endothermic).
An odor change is detected. The odor of a substance may
appear, disappear, or become more intense.
Part A of this experiment asks you to observe chemical
reactions of various compounds and identify these compounds based
on their chemical properties.
Fig. 2-1. Precipitate formation
In Part B you will be given an unknown compound (one that you have previously investigated in
Part A) to identify. The interpretations of the collected data (from Part A) will help you in identifying
your unknown.
The chemical properties of the following known compounds, dissolved in water, will be
investigated in Part A:
sodium chloride
sodium carbonate
magnesium sulfate
NaCl (aq)
Na2CO3 (aq)
MgSO4 (aq)
ammonium chloride
water
NH4Cl (aq)
H2O (l)
The following reagents will be used to identify and characterize the above compounds:
silver nitrate
AgNO3 (aq)
sodium hydroxide
NaOH (aq)
hydrochloric acid
HCl (aq)
Note: You should discuss your findings and interpretations with a partner, but each of you should analyze
your own unknown. Remember, you will be conducting a test on each known compound and your
unknown with a single test reagent. To organize your work, there is a “reaction matrix” provided for you
to describe your observations. Because the space is limited, you may want to use the following codes:
pc – precipitate + color
g – gas, no odor
cc – cloudy + color
go – gas, odor
nr – no reaction
Experiment 2 Physical and Chemical Changes
45 PROCEDURE
Wear your safety goggles at all times in the laboratory.
Part A
1. Silver nitrate (AgNO3) reagent observations.
Label five small, clean test tubes with a permanent marker (Fig. 2-2). Alternatively, you can
use a clean 24-well plate (Fig. 2-3). You should ask your lab instructor which setup to use.
Place 5-10 drops of each of the five “known” solutions into
your labeled test tubes (or wells A1-A5, Fig. 2-3).
Use a dropper bottle (or a dropper pipet) to deliver 3-5
drops of AgNO3 solution. (Caution: AgNO3 forms black
stains on skin that can be painful. Thoroughly rinse your
hands after working with silver salts.) If you observe a
chemical change, add 5-10 more drops to see if there are
any additional changes. Record your observations in the
“reaction matrix”.
SAVE YOUR TEST SOLUTIONS for part A. 4 of your
Fig. 2-2. Arrangement of test tubes for
report. You will need to write the formula for each
AgNO3 test
precipitate that forms. For example, as shown in Fig. 2-1,
a precipitate of AgCl(s) will form when you mix solutions of NaCl(aq) and AgNO3(aq). You
may ask your lab instructor for assistance. You may also refer to a solubility table if needed.
2. Sodium hydroxide (NaOH) reagent observations.
Follow the same procedure
as in part A.1.a. above for
your second set of test
tubes (or wells B1-B5, Fig.
2-3).
Fig. 2-3. Arrangement
of 24-well plate for all
To each test tube, slowly
reagents
add 5-10 drops of NaOH;
make observations as you add the NaOH. In particular, observe if any gas evolves in any of
the tests. Check for odor.
SAVE YOUR TEST SOLUTIONS for part A. 4 of your report. You will need to write the
formula for each precipitate that forms.
3. Hydrochloric acid (HCl) reagent observations.
Follow the same procedure as above for your third set of test tubes (or wells C1-C5, Fig. 2-3).
To each test tube, slowly add 5-10 drops of HCl; make observations as you did for the NaOH.
In particular, observe if any gas evolves in any of the tests. Check for odor.
SAVE YOUR TEST SOLUTIONS for part A. 4 of your report. You will need to write the
formula for each precipitate that forms.
4. Identification of unknown.
Obtain an unknown for Part A from your instructor. Perform the three reagent tests on your
unknown. Identify the unknown compound based on the data from the ‘known’ solutions
collected in your “reaction matrix”.
Experiment 2 Physical and Chemical Changes 45 46 Part B
The Part B experiment design is similar to that of Part A. Thus, you will need either 15 clean test
tubes or a clean 24-well plate.
1. Prepare solutions
Place ~1 mL of each test solution (there are five solutions labeled 1 through 5) in the
clean test tubes or in the well plate.
2. Test solutions
Test each of the five solutions with reagent A. If, after adding several drops, you observe a
chemical change, add 5-10 drops more to see if there are any additional changes. Record
your observations in the “reaction matrix”.
With a fresh set of solutions 1-5 in clean test tubes (or wells), test each with reagent B.
Repeat with reagent C.
3.
Identification of unknown
Obtain an unknown for Part B from your instructor. Perform the three reagent tests on your
unknown. Identify the unknown as one of the five solutions from Part B. 1 above. Record
your observations.
FURTHER READING
1. http://fscimage.fishersci.com/msds/21105.htm (NaCl, last accessed 08/2013)
2. http://www.flinnsci.com/Documents/MSDS/S/SodiumCarbonate.pdf (Na2CO3, last accessed 08/2013)
3. http://www.sciencelab.com/msds.php?msdsId=9927218 (MgSO4, last accessed 08/2013)
4. http://www.sciencelab.com/msds.php?msdsId=9927431 (NH4Cl, last accessed 08/2013)
5. http://www.sciencelab.com/msds.php?msdsId=9927411 (AgNO3, last accessed 08/2013)
6. http://www.sciencelab.com/msds.php?msdsId=9924997 (NaOH, last accessed 08/2013)
7. http://www.sciencelab.com/msds.php?msdsId=9924285 (HCl, last accessed 08/2013)
8. Selco, J., M. Bruno, and S. Chan, “Discovering Periodicity: Hands-On, Minds-On Organization of the
Periodic Table by Visualizing the Unseen.” J. Chem. Educ., 90 (2013) 995-1002.
9. DeMeo, S., “Synthesis and Decomposition of Zinc Iodide: Model Reactions for Investigating Chemical
Change in the Introductory Laboratory.” J. Chem. Educ., 72 (1995) 836.
Experiment 2 Physical and Chemical Changes
47 Name _____________________________________ Lab Section_______________Date__________
REPORT ON EXPERIMENT 2
Identification of Compounds through Physical and Chemical Changes
Part A Reaction matrix
Test
NaCl (aq)
Na2CO3 (aq)
MgSO4 (aq)
NH4Cl (aq)
H2O (l)
Unknown
H2O (l)
Unknown
5
Unknown
AgNO3 (aq)
NaOH (aq)
HCl (aq)
Write formulas for the precipitates that formed in the reaction matrix above.
Test
NaCl (aq)
Na2CO3 (aq)
MgSO4 (aq)
NH4Cl (aq)
AgNO3 (aq)
NaOH (aq)
HCl (aq)
Sample number of unknown for Part A: ____________________
Compound in the unknown solution: ______________________
Part B Reaction matrix
Solution
1
2
3
4
Reagent A
Reagent B
Reagent C
Sample number of unknown for Part B:
____________________
Compound in the unknown is the same as solution number: ____________________
Experiment 2 Physical and Chemical Changes 47 48 QUESTIONS (Submit your answers on a separate sheet if necessary)
1. Predict what would be observed (and explain why) when aqueous solutions of the following are
mixed:
a. K2CO3 and HCl
_____________________________
b. KCl and AgNO3
_____________________________
c. MgCl2 and NaOH
_____________________________
d. NH4NO3 and NaOH _____________________________
2. Physical changes are often used to separate two or more substances in a mixture. What physical
change and what property do you think could be used to separate each of the following pairs?
a. Salt and iron filings _____________________________
b. Iron filings and aluminum filings _____________________________
c. Sand and water _____________________________
d. Rubies and emeralds _____________________________
3. Classify each of the following as a physical change or a chemical change.
a. Liquid water freezes at 0 ˚C
_____________________________
b. A sheet of paper caught on fire
_____________________________
c. A copper penny is oxidized (turns black)
_____________________________
d. Ice melts to form water
_____________________________
4. Imagine a lovely beach scene –something majestic like sand, surf, and some salt crusting the
sheer black cliffs that rise up from the water. Which of the substances in that image are water
soluble?
a. Sand _____________________
c. Salt ______________________
b. Rocks ____________________
d. Shells- ____________________
5. The Wicked Witch of the West in "The Wizard of Oz." is about to do something very, very
wicked to Dorothy and her friends. As any well-meaning Kansas girl might do, Dorothy dumps a
big ol’ bucket of water on the Wicked Witch. “I'm melting! Meeeeeellllttiiiinnggg!” cries the
witch as she turns into a puddle of goo. Is the Wicked Witch of the West water-soluble? Is she
really melting?
6. Calcium carbonate is an extremely common substance. You can find it in the following:
minerals calcite and aragonite, the shell of an egg or an oyster, a stately marble floor, or that nasty
Experiment 2 Physical and Chemical Changes
49 whitish crud around your faucet. Is calcium carbonate (CaCO3) easily water-soluble? Explain
your answer.
Experiment 2 Physical and Chemical Changes 49