Analysis of a Mixture:
Isolation of Lead Chromate (Chrome Yellow Pigment)
Purpose
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To separate and identify silver and lead compounds in a solution.
To develop skill in making careful observations.
To better understand how pigments are produced, and the chemistry of pigments.
Introduction
In this experiment, you will qualitatively analyze a solution for silver and lead by
separating the metal ions using a precipitation technique. The solution that you will analyze has
been made by dissolving a silver compound called silver nitrate and a lead compound called lead
nitrate in deionized water. The silver and lead can be separated by converting them into different
compounds which do not dissolve very much in water, and which have different properties of
their own. The following vocabulary words will be used throughout this experiment:
1. Precipitate—an insoluble solid formed when the particles of the compounds in two
different solutions react.
2. Filtrate—a solution which passes through the filter paper during filtration.
3. Residue—a solid which remains on the filter paper during filtration.
4. Washings—liquid used to wash a residue already on a filter paper.
Materials
Test tubes and test tube rack
Test solution (mixture of silver and lead nitrates)
Sodium chloride (NaCl) solution
Potassium chromate (K2CrO4) solution
Graduated cylinder
Deionized water
Test tube holder
Beakers (2)
Filter paper and funnel
Hot plate with hot water bath
Watch glass or petrie dish
Procedure
1. Rinse all glassware with deionized water before use.
2. Measure approximately 5 mL of the test solution into a test tube. Describe the physical
properties of this solution in your data table. CAUTION: handle this solution carefully, as it will
stain your hands.
3. Measure approximately 5 mL of the sodium chloride solution into a second test tube.
Describe the physical properties of this solution in your data table.
4. Place the test tube with the test solution into your test tube rack, and slowly add the sodium
chloride solution to it. Note the changes in the system, and describe the new properties in your
data table. The solid (or precipitate) is a mixture of lead chloride and silver chloride.
5. Warm the solution gently by placing the test tube into the warm water bath for approximately
2 minutes. Do not allow it to boil!
6. Set up a filtration apparatus using a funnel, filter paper and beaker. Using a test tube holder to
remove the test tube from the warm water bath, filter the solution.
7. Rinse the test tube with 10 mL of hot deionized water to remove any precipitate that is
clinging to the sides. Pour this hot water through the same filter paper from Step #6. Repeat with
another 10mL of hot deionized water. The filtrate and the washings should be collected in the
same beaker. Describe the properties of the residue and the filtrate and washings at this point.
8. Carefully remove the filter paper from the funnel. Keeping it folded, place it on a watch glass
or petrie dish. The solid is mainly silver chloride.
9. Rinse the funnel and graduated cylinder with deionized water. Set up another filtration
apparatus with a new piece of filter paper and a new beaker.
10. Measure out approximately 5 mL of potassium chromate in the graduated cylinder. Describe
the physical properties of this solution.
11. Add the potassium chromate to the beaker containing the filtrate and washings from step 7.
Describe the appearance of the system in the beaker. The precipitate is lead chromate.
12. Filter the solution. Discard the filtrate by rinsing it down the sink with running water.
Carefully place the filter paper and residue on a watch glass or petrie dish, and allow to dry until
the following day.
13. As your teacher instructs, take the filter paper and residue from Step 8 and expose it to
ultraviolet light. Describe any changes that you observe in the system on exposure to light.
Discard the filter paper and residue in the trash can. Wash your hands thoroughly.
14. Your teacher will grind the lead chromate residues with linseed oil to produce a paint. On the
following day, you will be able to paint a design using this paint. When this design is dry, attach
it to your lab report.
Analysis of a Mixture:
Pre-Lab Questions
1. What compounds does the test solution contain?
2. What lab technique will you use to separate the compounds in the test solution?
3. Define the following words: precipitate, filtrate, residue, washings.
4. What must you do to your glassware before use?
5. What is the first precipitate you will isolate? What will you do to this precipitate?
6. What is the second precipitate you will isolate? What will you do to this precipitate?
7. What safety equipment will you use in the lab?
8. How will you dispose of the first residue and the second filtrate?
Analysis of a Mixture: Data Table
Step Number
#2: Test
Solution
#3: Sodium
Chloride
Solution
#4: Test
Solution +
Sodium
Chloride
#7: Residue
and filtrate
#10:
Potassium
chlorate
solution
#11: Filtrate +
potassium
chlorate
#13: Silver
chloride
residue
exposed to
light
#14: Paint
Observations
Post-Lab Questions:
1. What differences did you observe in the systems between the separate solutions in Step 2 and
3, and the mixture in Step 4? What evidence of a chemical change did you see?
2. What differences did you observe in the systems between the filtrate in Step 7, the solution in
Step 10, and the mixture in Step 11? What evidence of a chemical change did you see?
3. Why do you think you needed to use hot water to rinse the silver chloride precipitate?
4. What evidence of a chemical change did you observe when the silver chloride residue was
exposed to sunlight? Does the change in this substance suggest any practical applications for
which it might be used?
5. Why do you think you needed to use deionized water in this experiment to rinse your
glassware and to wash the residue on your filter paper?