5.2-1
SCIENCE EXPERIMENTS ON FILE ™ Revised Edition
The Test-Tube Mystery
Valeria Alston
Topic
Observing chemical reactions; using logical deduction
Time
2 hours
!
Safety
Please click on the safety icon to view the safety precautions. The solutions must
be prepared by a teacher. All solutions should be treated as if they were poisonous
and corrosive. If you spill any solution on yourself, wash it off immediately with
large amounts of water. Avoid inhaling fumes. Wear safety goggles, lab apron, and
gloves. Clean up all residue, and containerize it for proper disposal. Dispose of all
chemicals in accordance with all local, state and federal laws.
Materials
spotting plate
red and blue litmus paper
box containing 10 dropping bottles of solutions numbered in sequence
from 1 to 10. In each bottle will be a .1 M solution of one of the
following compounds:
Name
ammonium chloride, NH4Cl
barium chloride, BaCl2
copper sulfate, CuSO4
hydrochloric acid, HCl
potassium iodide, KI
sodium bromide, NaBr
sodium hydroxide, NaOH
sodium sulfide, Na2S
sulfuric acid, H2SO4
silver nitrate, AgNO3
Compound Type
salt
salt
salt
acid
salt
salt
base
salt
acid
salt
Procedure
1. Your goal is to identify which solution is in each bottle. You will figure this out
by testing each solution with litmus paper and by combining all possible pairs
of the solutions (1 with 2, 1 with 3, etc.). After describing each reaction on the
data table and thinking carefully about the facts listed later, in “What’s Going
On,” you should be able to identify each solution.
2. First, put 2 drops of solution 1 in the depression of the spotting plate. Do not
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SCIENCE EXPERIMENTS ON FILE™ REVISED EDITION
5.2-2
poke the end of the dropping bottle into the pool on the spotting plate. Screw
the top back on the bottle tightly after you have used it.
3. Test solution 1 with red litmus paper and then with blue. Write the result in the
top box on the data table. If the solution turned red litmus paper blue, write R
to B. If it turned blue litmus paper red, write B to R. If there was no reaction,
write NLR.
D ATA T A B L E
(Litmus paper test results recorded in boxes on far right.)
Sol. #
Names of solutions
1
Keys:
Red litmus to blue – R to B
Blue litmus to red – B to R
No litmus reaction – NLR
Color of precipitate – color PPT
No reaction – NR
2
1
3
2
4
3
5
4
6
5
7
6
8
7
9
8
10
9
10
1
2
3
4
5
6
7
8
9
10
4. Wash and rinse the spotting plate. Then put 2 drops of solution 2 on the plate,
test it with red and blue litmus paper, and note the results on the data table in
the far right box of horizontal row 2.
5. Now combine 2 drops of solution 1 with 2 drops of solution 2 on the plate. Observe the reaction, and describe it in the box on the data table where vertical column 1 meets horizontal row 2. For example, if a white precipitate formed when
you mixed the two solutions, write “white PPT.” If nothing happened, write “NR.”
6. Continue in this way until you have filled in every box on the data table. This
means you will have tested each individual solution with litmus paper and will
have combined all possible pairs of the solutions. Remember not to touch the
top of any bottle to the plate, to wash and rinse the plate after each combination, and to screw the bottle caps on carefully after each use.
7. Compare the observations recorded on your data table with the following:
a. Acids will turn blue litmus paper red.
b. CuSO4 solutions are blue.
c. Bases will turn red litmus paper blue.
d. Acids will react with Na2S to release a gas, hydrogen sulfide, which smells
like rotten eggs. It is very poisonous—be very careful.
e. If a base is mixed with NH4Cl, the odor of ammonia (NH3) is given off.
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5.2-3
SCIENCE EXPERIMENTS ON FILE™ Revised Edition
Ammonia vapor will turn damp red litmus paper blue.
f. AgNO3 will react with compounds containing halogen (Br, Cl, or I) to form
white or yellowish precipitates.
g. BaCl2 will react with H2SO4 and CuSO4 to form a white precipitate,
BaSO4.
h. CuSO4 gives an orange-yellow precipitate when mixed with KI, but doesn’t
give a precipitate with NaBr.
i. A white cloudy precipitate, Ba(OH)2, forms when an excess of NaOH is
added to BaCl2.
j. HCl will react with AgNO3 to produce a white precipitate, AgCl.
By using logical reasoning and the process of elimination, you should be able to
identify each solution. Write its name next to its number on the data table.
8. After you have identified all the solutions, write balanced equations for each of
the reactions you performed.
What’s Going On
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SCIENCE EXPERIMENTS ON FILE™ REVISED EDITION
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
5.2-4
silver nitrate AgNO3
sulfuric acid H2SO4
sodium sulfide Na2S
sodium hydroxide NaOH
sodium bromide NaBr
potassium iodide KI
hydrochloric acid HCl
copper sulfate CuSO4
barium chloride BaCl2
ammonium chloride NH4Cl
FORMULAS
(3,1) Na2S + 2AgNO3 ➝ Ag2S + 2Na+ + 2NO3(4,1) 2NaOH + 2AgNO3 ➝ Ag20 ➝ 2NaNO3 + H2O
(5,1) NaBr + AgNO3 ➝ AgBr + Na+ + NO3(6,1) KI + AgNO3 ➝ AgI + K+ + NO3(7,1) HCI + AgNO3 ➝ AgCl + H+ + NO3(8,1) CuSO4 + 2AgNO3 ➝ Ag2SO4 + Cu++ + 2NO3(9,1) BaCl2 + 2AgNO3 ➝ 2AgCl + Ba++ + 2NO3(10,1) NH4CI + AgNO3 ➝ AgCI + NH4+ + NO3(3,2) Na2S + H2SO4 ➝ H2S + 2Na+ + SO4+ 2H2SO4 ➝ S + H2SO3 + H2O + 2Na+ + SO4(9,2) BaCl2 + H2SO4 ➝ BaSO4 + 2Cl- + 2H+
(7,3) 2HCl + Na2S ➝ H2S + 2Na+ + 2Cl(8,3) CuSO4 + Na2S ➝ CuS + SO4- + 2Na+
(9,3) BaCl2 + Na2S ➝ BaS + 2Cl- + 2Na+
(8,4) CuSO4 + 2NaOH ➝ Cu(OH)2 + SO4- + 2Na+
(9,4) BaCl2 +2NaOH ➝ Ba(OH)2 + 2Cl- + 2Na+
(10,4) NH4CI + NaOH ➝ NH3 + H2O + Na+ + Cl(8,6) 2CuSO4 + 4KI ➝ 2CuI + 4K+ + 2SO4- +I2
(9,8) BaCl2 + CuSO4 ➝ BaSO4 + Cu++ 2Cl-
Note: (3,2) and (8,6) give two results on table; (3,2) requires two equations to explain the results. (8,6) gives both results in one equation.
Connections
Chemists, like detectives, attempt to identify unknowns through a process of careful
and creative analysis. This often involves observing reactions of unknown substances
and comparing them with reactions of known substances. In this experiment, you
have tried to identify 10 different chemical compounds by reacting them with each
other, observing the results, and comparing the results with the known characteristics of some common chemicals.
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Safety Precautions
READ AND COPY BEFORE STARTING ANY EXPERIMENT
Experimental science can be dangerous. Events can happen very quickly while you are performing
an experiment. Things can spill, break, even catch fire. Basic safety procedures help prevent serious
accidents. Be sure to follow additional safety precautions and adult supervision requirements for
each experiment. If you are working in a lab or in the field, do not work alone.
This book assumes that you will read the safety precautions that follow, as well as those at the start
of each experiment you perform, and that you will remember them. These precautions will not always
be repeated in the instructions for the procedures. It is up to you to use good judgment and pay attention when performing potentially dangerous procedures. Just because the book does not always
say “be careful with hot liquids” or “don’t cut yourself with the knife” does not mean that you should
be careless when simmering water or stripping an electrical wire. It does mean that when you see a
special note to be careful, it is extremely important that you pay attention to it. If you ever have a
question about whether a procedure or material is dangerous, stop to find out for sure that it is safe
before continuing the experiment. To avoid accidents, always pay close attention to your work, take
your time, and practice the general safety procedures listed below.
PREPARE
• Clear all surfaces before beginning work.
• Read through the whole experiment before you start.
• Identify hazardous procedures and anticipate dangers.
PROTECT YOURSELF
• Follow all directions step by step; do only one procedure at a time.
• Locate exits, fire blanket and extinguisher, master gas and electricity shut-offs, eyewash, and
first-aid kit.
• Make sure that there is adequate ventilation.
• Do not horseplay.
• Wear an apron and goggles.
• Do not wear contact lenses, open shoes, and loose clothing; do not wear your hair loose.
• Keep floor and work space neat, clean, and dry.
• Clean up spills immediately.
• Never eat, drink, or smoke in the laboratory or near the work space.
• Do not taste any substances tested unless expressly permitted to do so by a science teacher in
charge.
USE EQUIPMENT WITH CARE
• Set up apparatus far from the edge of the desk.
• Use knives and other sharp or pointed instruments with caution; always cut away from yourself
and others.
• Pull plugs, not cords, when inserting and removing electrical plugs.
• Don’t use your mouth to pipette; use a suction bulb.
• Clean glassware before and after use.
• Check glassware for scratches, cracks, and sharp edges.
• Clean up broken glassware immediately.
v
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Safety
SCIENCE EXPERIMENTS ON FILE™ REVISED EDITION
• Do not use reflected sunlight to illuminate your microscope.
• Do not touch metal conductors.
• Use only low-voltage and low-current materials.
• Be careful when using stepstools, chairs, and ladders.
USING CHEMICALS
• Never taste or inhale chemicals.
• Label all bottles and apparatus containing chemicals.
• Read all labels carefully.
• Avoid chemical contact with skin and eyes (wear goggles, apron, and gloves).
• Do not touch chemical solutions.
• Wash hands before and after using solutions.
• Wipe up spills thoroughly.
HEATING INSTRUCTIONS
• Use goggles, apron, and gloves when boiling liquids.
• Keep your face away from test tubes and beakers.
• Never leave heating apparatus unattended.
• Use safety tongs and heat-resistant mittens.
• Turn off hot plates, bunsen burners, and gas when you are done.
• Keep flammable substances away from heat.
• Have a fire extinguisher on hand.
WORKING WITH MICROORGANISMS
• Assume that all microorganisms are infectious; handle them with care.
• Sterilize all equipment being used to handle microorganisms.
GOING ON FIELD TRIPS
• Do not go on a field trip by yourself.
• Tell a responsible adult where you are going, and maintain that route.
• Know the area and its potential hazards, such as poisonous plants, deep water, and rapids.
• Dress for terrain and weather conditions (prepare for exposure to sun as well as to cold).
• Bring along a first-aid kit.
• Do not drink water or eat plants found in the wild.
• Use the buddy system; do not experiment outdoors alone.
FINISHING UP
• Thoroughly clean your work area and glassware.
• Be careful not to return chemicals or contaminated reagents to the wrong containers.
• Don’t dispose of materials in the sink unless instructed to do so.
• Wash your hands thoroughly.
• Clean up all residue, and containerize it for proper disposal.
• Dispose of all chemicals according to local, state, and federal laws.
BE SAFETY-CONSCIOUS AT ALL TIMES
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