Honors Chemistry Per: 2
Name: Brandon,
Date 2/25/25
Partners: Jack, Kai
Identifying Unknown Solutions
The guiding question of this investigation is, What are the identities of the unknown chemicals?
Oh no, the labels have fallen off of four of the bottles in Mr. Stine’s stockroom and he needs your help
identifying the substances. The labels found on the floor are
NaOH
Sr(NO3)2
KI
AgNO3
Mr Stine has prepared aqueous solutions of each of the chemicals and labeled them A, B, C, and D.
Your task is to figure out which letter goes with which solution!
Preparation: Working with your group, develop a procedure that will enable you to determine the
identity of the unknown solutions. To answer the guiding question, you will need some background
information.
The first piece of information is the solubility rules. These solubility rules can help you determine what
negative ions are present in the solution and thus identify the solutions.
Solubility Rules
Anion Name
Anion Formula Soluble with...
Commonly Precipitates with...
nitrate
NO31-
ALL Cations
No Cations
acetate
C2H3O21-
Nearly all Cations
Ag1+
chloride
Cl1-
Most Cations
Ag1+, Pb2+, Pb4+
bromide
Br1-
Most Cations
Ag1+, Pb2+, Pb4+
iodide
I1-
Most Cations
Ag1+, Pb2+, Pb4+
sulfate
SO42-
Most Cations
Ag1+, Pb2+, Pb4+, Ba2+, Sr2+, Ca2+
chromate
CrO42-
Only NH41+, Group 1
Most Cations
carbonate
CO32-
Only NH41+, Group 1*
Most Cations *including Li1+
phosphate
PO43-
Only NH41+, Group 1*
Most Cations *including Li1+
sulfide
S2-
NH41+, Group 1, some Group 2 ions are slightly soluble
Most Cations
hydroxide
OH1-
NH41+, Group 1, some Group 2 ions are slightly soluble
Most Cations
The second piece of information you may need to know is flame test data. Flame test data can help you
identify what positive ions are present in the solution and thus identify the solution.
Flame Test Data
The final piece of information you may need is pH data. In the synthesis and decomposition lab, we saw
that metal hydroxides are strong bases, and thus solutions of metal hydroxides are basic, pH >>7.
Knowing that metal hydroxides are basic may help you determine the identity of the solutions.
To solve this puzzle you have access to the following
●
●
●
●
●
●
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Acetate paper (plastic film we used in the solubility lab)
Solutions to the unknowns
Solutions of known ions from the solubility lab
Wooden splints
Propane Gas Canisters
pH paper
pipette for the solutions
Given all this information you now have to figure out how to identify the solutions. Working with your
group, come up with a procedure. You must perform at least two different experiments to identify the
unknowns. Think about what data you need to collect and how you are going to present your data in
the data section. Write your procedure on the next page and create a data table on the page after the
procedure. You must get both your procedure and data table approved by Mr. Stine before beginning
your experiment.
Record Your Procedure Here:
Solubility Test
1. Place a piece of paper under an acetate sheet to create a 1x4 grid, labeled A, B, C, D
2. Use a pipette to get one drop of each of the original 4 solutions in their own respective space
on the grid (each solution should have its own pipette)
3. Use a pipette to add 1 drop of Pb2+ to each solution
4. Observe and record your results in data table 1 (precipitate formed?)
5. In the blanks on data table 2, input the two solutions that formed a precipitate with Pb2+
6. Clean your acetate sheet
7. Create a new grid, this time being 1x2, using the solutions that formed a precipitate (make
sure to keep consistent labeling)
8. Use a pipette to add one drop of each solution to their own respective space on the grid
9. Use a pipette to add 1 drop of Ca2+ to each solution
10. Observe and record your results in data table 2
Flame Test
11. Fill in the blanks on data table 3 with the two solutions that did not form a precipitate in Test
1 with Pb2+
12. Put safety goggles on
13. Take two wooden splints, each soaked in one of the two solutions that did not form a
precipitate in Test 1
14. Remove one wood splint from a solution to be tested. Place the soaked end of it into the
flame and observe the color of the light produced. You will see the color for only a few
seconds. Place the wood splint back into the solution and repeat the test as many times as
necessary. Once you are sure of the color, record it in your data table.
15. Do the same for the other solution.
Solubility Test (Cont´d)
16. Just to make sure of your results, Create a new grid, this time being 1x2, using the solutions
that did not form a precipitate (make sure to keep consistent labeling)
17. Use a pipette to add one drop of each solution to their own respective space on the grid
18. Use a pipette to add 1 drop of I1- to each solution
19. Observe and record your results in data table 3
Final Observations
20. Fill out the Final Observations table with your findings on which solutions are labeled A, B, C,
or D.
Materials:
-
Lab Worksheet
Acetate sheet
Safety goggles
Laminated Paper
Pipette
Flint
Wooden Splints
Propane torch
NaOH solution
Sr(NO3)2 solution
KI solution
AgNO3 solution
Beaker x4
Data: Record your data here:
Precipitate Formed - (Yes/No)
Data Table 1
A
B
C
D
Test 1 (Pb2+)
Yes
No
Yes
No
Precipitate Formed - (Yes/No)
Data Table 2
Test 2 (Ca2+)
A
C
No
Yes
Flame Test - Solution
Flame Color
__B__
Light Orange
__D__
Deep Red
Precipitate Formed - (Yes/No)
Data Table 3
Test 3 ( I1-)
B
D
Yes
No
Final Observations (A-B-C-D):
NaOH
Sr(NO3)2
KI
AgNO3
C
D
A
B
Conclusion
Once your group has finished collecting and analyzing your data, you will need to develop an initial
argument. Your argument must include a claim, which is your answer to the guiding question. What are
the identities of the unknown chemicals? You need to be able to support your claim with evidence from
the lab. The evidence is your analysis of the data and your interpretation of what the analysis means.
Finally, you must include a reason your argument is valid. You will therefore need to explain why the
evidence that you decided to use is relevant and important.
Claim:
-NaOH is solution C
-Sr(NO3)2 is solution D
-KI is solution A
-AgNO3 is solution B.
-The solubility test with Pb2+ in test 1 helps to separate our solutions into two groups, NaOH (C) and KI
(A), and Sr(NO3)2 (D) and AgNO3 (B).
-this is because any compound with a nitrate will never form a precipitate, but Pb2+ will
precipitate with NaOH (C) and KI (A).
-in test 2, the purpose is to identify the difference between NaOH and KI
-Ca2+ will only form a precipitate with NaOH (C) due to solubility rules
-this easily differentiates between NaOH (C) and KI (A)
-in the flame test, we took the two solutions that originally did not form a precipitate with Pb2+ being
Sr(NO3)2 (D) and AgNO3 (B)
- The strontium in Sr(NO3)2 (D) makes the flame burn bright red, so seeing that one of the flames
was very obviously bright red, and the other was not, we concluded that solution D was
Sr(NO3)2.
- this is because each element will produce different colors of light when burnt, due to the
emission spectra
-to confirm our results with B being AgNO3, one last solubility test was needed
-we mixed AgNO3 and Sr(NO3)2 with I1-, to easily distinguish the difference
-AgNO3 will form a precipitate, and Sr(NO3)2 will not, according to solubility rules
Lab Activity - Identifying Unknown Solutions - Grading Rubric
Criteria
4 - Exemplary
3 - Proficient
2 - Developing
1 - Beginning
Procedure
Procedure is clear,
detailed and logically
sequenced with all
steps. Includes at least 2
different test types
(flame test, pH,
precipitation). Safety
precautions noted.
Materials listed
completely.
Procedure is
mostly clear and
logical. Includes
2 test types.
Most steps
present but may
miss minor
details. Basic
safety noted.
Most materials
listed.
Procedure is
somewhat
unclear or out of
sequence. Only 1
test type
included.
Missing several
steps. Limited
safety notes.
Materials list
incomplete.
Procedure is
confusing or
illogical. Test
types unclear
or missing.
Many missing
steps. No
safety notes.
No materials
list.
Data Table
Data table is clearly
organized with labeled
columns/rows. All
observations recorded
with proper units.
Multiple trials included.
Data is thorough and
neat.
Data table has
basic
organization and
labels. Most
observations
recorded with
units. At least
one trial for
each test. Data
is mostly
complete.
Data table
poorly
organized. Some
labels missing.
Observations
lack detail or
units. Missing
trials. Data is
incomplete.
Data table
disorganized or
missing.
Few/no labels.
Minimal
observations.
No units. Very
incomplete
data.
Your Score
Conclusion
Claim clearly states
identity of all unknowns.
Evidence from multiple
tests supports claim
with detailed analysis.
Reasoning explains why
evidence proves
identities. All parts
thoroughly explained.
Claim states
most unknown
identities.
Evidence from
tests mostly
supports claim.
Basic reasoning
connects
evidence to
identities. Most
parts explained.
Claim unclear or
missing some
unknowns.
Limited evidence
provided. Weak
reasoning
between
evidence and
identities. Parts
poorly
explained.
Claim missing
or incorrect.
Little/no
evidence
provided. No
clear
reasoning.
Parts missing
or incorrect.