A Self Directed Inquiry into the Identification of Unknowns

Developers:

Donna K. Reinhart

Cheltenham H.S., Wyncote, PA

Michael J. Gavaghan, Faith Lipford

Catalyst Technical Service Lab, Rohm and Haas Company

Grade Level:

High School

Discipline:

Chemistry

Goals:

Upon completion of this inquiry experiment, the student will:

1. Have experience in designing an experiment.
2. Be able to use logic in selecting specific tests for unknowns.
3. Be skilled in comparing specific properties of substances and evaluating their importance.
4. Recognize the use of spreadsheets in helping one to make clear comparisons.

Specific Objectives:

Upon completion of this inquiry experiment, the student will:

1. Be able to test for the presence of sulfate, carbonate, phosphate and halide anions.
2. Be able to test for magnesium and calcium cations.
3. Be able to differentiate between two salts that contain similar anions by performing a titration.
4. Be able to conduct a flame test.
5. Be able to assess the pH of a salt when it has undergone hydrolysis.
6. Be able to differentiate between hygroscopic and deliquescent materials.
7. Be able to differentiate between several crystal systems.
8. Be able to create a spreadsheet for use in comparing physical and chemical properties.
9. Be able to make up molar solutions accurately.

Outline of the inquiry:

This lab was developed for chemistry students in the last quarter of their academic year. They will have had experience testing for various substances, as well as an understanding of pH, solubility, and other properties of chemical compounds, and should be well equipped to delve in to this mystery lab. On day 1, the introduction should be handed out which introduces students to the problem. A soggy box of chemicals arrives at the high school. The labels have all fallen off the jars and a packing slip is enclosed that is barely readable. Some items are backordered, but they cannot be determined from the slip. The students know what has been ordered, but not what has arrived. With the knowledge that 19 chemicals have been ordered and only 16 have been received, they are challenged to identify those that have arrived in the box. They are instructed to work in teams of four, to plan a strategy and to set it into motion. The teacher should circulate to make sure that students are on task and are working on a reasonable plan. Other than that, very little intervention should take place. At the end of the experiment, a sheet of paper should be handed in that details the measures that each group has taken to identify the jars of chemicals along with their final identification.

Background:

When attempting to identify different compounds, one may use observations of physical and chemical properties. Examples of physical properties are crystalline structure, color, smell, taste (never used in a lab), hardness, melting point and boiling point, and deliquescence or efflorescence. Flame tests are used to identify certain cations as their electrons are energized and emit light of a definite color when returning to their ground state.

Chemical properties are always determined when assessing how a certain compound behaves during a chemical reaction. Various chemical procedures are used to test for ions in solution. Positive tests are indicated by color changes, the formation of precipitates and the evolution of gases. Many of the tests you will become familiar with in this inquiry experiment are used routinely by forensic scientists.

Examples of each test that you may decide to use will be demonstrated by your teacher. It is up to you to choose those that will provide you with needed information about your unknowns.

HARDNESS: A TEST FOR CALCIUM AND MAGNESIUM IONS

Early in the 20th century the hardness of water was considered a measure of the ability of water to precipitate soap. The scum that forms in your bathtub is precipitated chiefly by calcium and magnesium ions that are commonly present in water. The test that will be described is called the EDTA Titrimetric Method. In this test a chelated soluble complex is formed by the combination of certain metal cations and EDTA and its sodium salts. A small amount of Eriochrome Black T dye is added at a pH of about 10.0 and the solution becomes wine red in the presence of magnesium or calcium ions.

Materials:

1% solutions of each unknown
Eriochrome Black T indicator
Buffer solution: To 16.9 g ammonium chloride in 143 ml concentrated ammonium hydroxide add 1.25 g of the magnesium salt of EDTA and dilute to 250 ml with distilled water.

Procedure:

1. Prepare a 1% solution of your unknown compound.
2. Add enough of your buffer solution to attain a pH of 10 to 12 (about 1ml).
3. Add a few grains of Eriochrome Black T indicator.
4. A red color is a positive test for calcium or magnesium ions.

SULFATE DETERMINATION

The sulfate ion is present in nature in a variety of concentrations ranging from a few to several thousand mg/l. Sulfates are precipitated in a hydrochloric acid medium with the addition of barium chloride. Interference is sometimes caused by the precipitation of carbonate or phosphate ions; however, this can be avoided by maintaining an acidic reaction medium.

Materials:

10% barium chloride solution
1% solutions of unknowns
phenolphthalein indicator
.5 N HCl

Procedure:

1. Prepare 1% solutions of all unknowns.
2. Add about 4 drops of phenolphthalein indicator.
3. Add 0.5 N HCl until the solution becomes clear.
4. Add several drops of the barium chloride solution.
5. A positive test is indicated by the presence of a precipitate.

HALIDE DETERMINATION

Almost every element in the Periodic Table will form halides, several of them in a variety of oxidation states. Halides are some of our most important and most common compounds. They range in form from simple molecules to complicated polymers and beautiful crystals. Halides react easily with silver nitrate solution to give white or slightly colored precipitates.

Materials:

1% solution of unknowns
0.10 M AgNO3
1 M nitric acid solution (90 grams of 70% nitric acid added to enough water to make up 1 liter)

Procedure:

1. Take a 5 ml sample of your unknown
2. Add a few drops of 0.1N silver nitrate.
3. If a precipitate has formed, you may have a positive test for a halide.
4. Other ions may also give a positive test. Make sure that you have a halide by acidifying your reaction medium with 1 M nitric acid. If the precipitate disappears, a halide is not present.

CARBONATE DETERMINATION

Many compounds contain the carbonate polyatomic ion. Carbon dioxide is evolved when these compounds are heated and also when they are treated with an acid such as hydrochloric acid. In the latter instance, a salt is formed and carbonic acid is formed which then decomposes to form water and carbon dioxide. Tests may be done on compounds in either the solid form or in solution.

Materials:

1 Molar HCl
1% solutions of unknowns

Procedure:

1. Take a 5 ml sample of your unknown.
2. Add 5 drops of HCl.
3. Immediately observe any evidence of a reaction such as the formation of bubbles.
4. Formation of a gas is a positive test for the carbonate ion.

PHOSPHATE DETERMINATION

Phosphorus occurs in many forms in nature such as orthophosphates, condensed phosphates and organically bound phosphates. The analysis of phosphates is made up of two general procedural steps: the conversion to a soluble orthophosphate and the colorimetric determination of soluble orthophosphate.

Materials:

1 % solutions of unknowns
Molybdate reagent (Hach Company #2236)
Amino Acid reagent (Hach Company #1934)

Procedure:

1. Take about 4 drops of your unknown and dilute with 10 ml of water.
2. Add about 1 ml of molybdate reagent.
3. Add a pillow of amino acid reagent.
4. Development of a dark blue color indicates the presence of the phosphate ion.

FLAME TESTS

Flame tests are often used in analytical labs as a preliminary test for cations. Traditionally, salts are dissolved in solutions of water or HCl and then held in the flame of a bunsen burner on a platinum loop until the electrons are sufficiently excited to jump to a higher energy state. When they return to their ground state, they give off colored light. Different elements have characteristic colors. The July issue of the Journal of Chemical Education described a procedure in which salts were dissolved in either methanol or isopropanol in small vials. Wicks were made of rolled paper towels, which were inserted, into the vials. When the solution had crept up the wick, it was lighted and the colors observed. We repeated this procedure with mixed results. Another, more reliable procedure is listed below, although much attention should be given to safety procedures, since this procedure involves open dishes of alcohol and fire.

Materials:

glass petri dishes
salts of unknowns

Procedure:

1. Pour about 10 ml of methanol into each petri dish.
2. Add about 2 to 3 grams of your unknown salt.
3. Carefully light with a match.
4. Observe the colors formed.
5. Na = yellow, Ca = orange, Li = red, B = green, K = violet

HYDROLYSIS OF SALTS

Salts are ionic compounds that are made up the cations of acids and the anions of bases. Those made up of strong acids and weak bases will exhibit a pH lower than 7 while those made up of strong bases and weak acids will show a pH above 7.

Materials:

1 % solutions of your unknown salts
pH meter or pH paper

Procedure:

1. Prepare 1 % solutions of your unknowns.
2. Take the pH of each solution with a meter or pH paper and record.

HYGROSCOPIC OR DELIQUESCENT?

Compounds that give off water have vapor pressures that are higher than that of the surrounding air. They are classified as deliquescent. Those compounds that take water into themselves have vapor pressures lower than the atmosphere and are called hygroscopic.

Materials:

unknown solid compounds
glass watch glasses
balance

Procedure:

1. Weigh out a quantity of at least one gram of your unknown to .001 g on a watch glass.
2. Let the compounds remain in the lab for at least 48 hours.
3. Reweigh your watch glasses to see if they have either gained or lost weight.

TITRATION

A titration of several compounds with an acid or a base standard (1N sulfuric acid or 1N sodium hydroxide) may be carried out in the following manner. This is a good means for differentiating between salts that contain the same anions in different proportions.

Materials:

burets
standard solutions of 1N NaOH and 1N H2SO4
.2 grams of your unknown, weighed to .001 gram
pH meter

Procedure:

1. Weigh out your unknown to .001 grams
2. Add to the titration container, an Erlenmeyer flask or beaker
3. Add about 30 ml of distilled water.
4. Add 3 drops of phenolphthalein and 3 drops of methyl orange.
5. Titrate each substance with sulfuric acid and note the endpoints with a pH meter in each case.
6. Repeat #5 but titrate with sodium hydroxide.

TEACHER NOTES

This experiment is designed to be an "inquiry exercise" in which students are presented with a number of unknowns and asked to develop a plan for identifying these unknowns. A scenario is presented at the beginning of the lesson to engage the students in the mystery quality of this investigation. They should be allowed part of the period to read and discuss how they will approach the problem within their lab groups. It may also be helpful to brainstorm with the entire class.

Each group will be given a list of the compounds that were ordered, some of which arrived in the damaged box. It is not known which compounds were backordered. Initially, students should gather as much information as possible about each compound. Sources include the Merck Index, the Handbook of Physics and Chemistry, MSDS papers as well as online references. It is suggested that a spreadsheet chart be developed and filled in with physical and chemical characteristics of the compounds. In this way, students can easily refer to the chart when testing their unknowns. Students should also keep a record of their own tests and observations in a blank spreadsheet.

HONORS CLASSES

For more advanced students, a minimum of direction should be give. Access to all tests for ions as well as reference books should be provided. Suitable glassware should be available. Conferencing with the teacher should occur at the beginning of the exercise when students are in their planning stage and again at the end when lab reports are handed in. Any other teacher contact should be restricted.

ACADEMIC CLASSES and GENERAL CLASSES

Tests for ions should be demonstrated in front of the class before individual groups begin their identification. A concise discussion of what occurs chemically in each test and its relevance in the identification process should be given. Students in these classes may also wish to set up controls for each test so that they can see visually what a positive result is. There are difficulties with false positives in some of the tests and the students are directed to increase the acidity of the solution. They should be reminded that not all initially positive tests are really positive for that ion.

ASSESSMENT

At the completion of this inquiry exercise, the following should be handed in:

1. A summary of the procedure that the lab group decided upon.
2. A clear flow chart showing how various compounds were identified.
3. A list of tests that were used, including materials and procedures, and the results obtained for each compound.
4. A spreadsheet that was developed during the testing.
5. A discussion of difficulties that arose and suggestions for improvement of the approach that the group took.
6. A conclusion which includes a complete identification of the unknowns given that lab group.

Expected Results of Ion Tests

FLAME TEST:

violet - potassium chloride, potassium sulfate, potassium iodide, potassium chloride
yellow - sodium sulfate, sodium lauryl sulfate, sodium chloride, sodium bicarbonate, sodium carbonate
orange - calcium chloride
green - boric acid

HALIDE TEST:

positive tests: potassium chloride, sodium chloride, potassium iodide, calcium chloride (the potassium iodide gives a creamier precipitate when compared with the potassium chloride.

SULFATE TEST:

positive tests: sodium sulfate, sodium lauryl sulfate, potassium sulfate, magnesium sulfate, sodium tripolysulfate.

CARBONATE TEST:

positive tests: sodium carbonate, sodium bicarbonate.

HARDNESS TEST:

positive tests: calcium chloride, magnesium sulfate.

TITRATION:

Either this procedure, or a simple pH test on the hydrolyzed salt must be done to distinguish the three phosphates from each other and the two carbonates.

Physical and Chemical Properties of Some White Crystalline Substances   Molecular weight Melting Pt. Odor Appearance Solublity with H20 g/100ml with EtOH PO4 SO4 Halide CO3 Hygroscopic Deliquescent pH of salt Sodium Sulfate Na2SO4 142.06 800 none white crystals 4.7 insol no yes no no     7.9 Sodium Lauryl Sulfate CH3(CH2)10CH2OOSO3Na 288.38   fat like white crystals 10   no yes no no     6.9 Magnesium Sulfate MgSO4*7H2O 246.48 200 none white crystals 71 sl.sol no yes no no no yes 7.3 Potassium Chloride KCl 74.55 773 none white crystals 34.7 sl.sol no no yes no     8.2 Sodium Citrate Na3C6H5O7 258.07 150 none white crystals 72 sl.sol no no no no   yes 9.1 Potassium Sulfate K2SO4 174.26 1067 none white crystals 12 insol no yes no no no no 8.57 Potassium Iodide KI 166.02 680 none white/ colorless crystals 127.5 1.88 no no yes no   slight 06-Jan Sodium Chloride NaCl 58.5 801 none white crystals 35.7 sl.sol no no yes no yes   7.1 Sodium Bicarbonate NaHCO3 84 270 none white crystals 6.9 sl.sol no no no yes     11 Sodium Tetraborate Na2B4O7*10H2O3 201.27 741 none colorless crystals 22.65 insol no no no no no yes 9.3 Sodium Carbonate NaCO3 106 851 none white crystals 7.1 sl.sol no no no yes no yes 11.3 Calcium Chloride CaCl2 110.99 772 none white crystals 74.5 sol no no yes no yes   7.1 Boric Acid H3BO3 61.84 169 none colorless crystals 6.35 sol no no no no no   3.9 Sodium Tripolyphosphate Na5O3P10 473 no data none white crystals 15 insol yes no no no slightly no 9 Sodium Phosphate(monobasic) NaH2PO4 137.99 100 none white crystals 59.5 insol yes no no no no slightly 4.6 Sodium Phosphate(dibasic) Na2HPO4 141.98 240 none white crystals   insol yes no no no very no 8.9 Calcium Sulfate CaSO4 136.14 >200 none white crystals 0.209 ? no yes no no very no ? Calcium Phosphate Ca(H2PO4)2*H2O 252.07 109 none white crystals 1.8 sol yes no no no no yes ? Magnesium Chloride MgCl2 95.23 712 none white crystals 54.25 7.4 no no yes no no yes 7 This experiment is courtesy of