|
|
 |
 |
|
|
|
|
 |
|
|
 |
|
|
|
|
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:
Specific Objectives: Upon completion of this inquiry experiment, the student will:
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 Procedure:
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 Procedure:
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 Procedure:
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 Procedure:
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 Procedure:
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 Procedure:
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 Procedure:
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 Procedure:
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 Procedure:
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:
Expected Results of Ion Tests FLAME TEST: violet - potassium chloride, potassium sulfate, potassium iodide, potassium chloride 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. |
