Unknowns in high-school chemistry

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  • BERNARD JURALE Meriden High School, Meriden, Connecticut

    THE most enjoyable and valuable unit in the Meriden 3. Alkaline earths: Ba++, Sr++, Ca++. High School chemistry course is the two weeks of lahora- 4. Cobalt nitrate test group: Mg++, Al+++, Zn++. tory work on "unknowns." Many former students now 5. Sulfide test group: Cd++, Sb+++, Pb++, As+++. taking college chemistry have said that it was the work 6. Borax-bead test group: Mn++, Cr+++, Co++, that helped them the most in their firsbyear course. Ni++, Fe++, Fe+++.

    This unit does not attempt to prepare for or to imi- 7. Replacement group: Cu++, Ag+, Pb++. - - - tate the procedure of conventional qualitative analysis. The imnortant obiective is to teach facts and nrinci~les The anions are divided into three groups:

    that are essential for the understanding of chemistry. It also offers an opportunity to teach laboratory tech- niques and to use facts determined experimentally. Laboratory work that does not give the student oppor- tunity to base his next steps on the facts that he has al- ready determined cannot be considered the best type of learning by doing.

    Tests for the various ions are sometimes made by the "hibor-miss" method. This lack of methodical pro- cedure is wasteful of chemicals and time. Every test should be made with the thought of what has to be de- termined. The method of testing described here is an attempt to unify the accepted test procedures of high- school chemistry into a workable laboratory exercise.

    We prefer to omit some laboratory work of doubtful value on the metals and to substitute a long period for topics that do not lend themselves to ordinary class- room procedures. We find that good laboratory tech- niques and use of the scientific method may be incul- cated by a longer laboratory unit better than by an equal time spread out in weekly laboratory periods. Also, the work on the "unknowns" is an excellent op- portunity to apply such topics as hydrolysis, electro- motive series, solubility rules, and ionic reactions.

    Hydrolysis is sometimes given inadequate considera- tion in high-school chemistry. This may be due to the lack of examples to show that hydrolysis plays an im- portant part in chemical reactions involving ions. It is, however, a valuable asset in determining the relative behavior of the ions in a salt. Universal indicator paper gives a much better check on pH value of a salt solution than does litmus.

    The preliminary work consists of reviewing the im- portant ions and their identification. These tests have either been taken up previously in laboratory work or shown as demonstrations in the classroom. In order to coordinate the procedure the cations are classified ac- cording to method of testing and the anions by the

    1. Strong acid ions: Sod--, NOS-, C1-. 2. Moderate acid ions: Br-, I-, CH,COO-. 3. Weak acid ions: SOS--, S--, GOS--.

    PROCEDURE

    Obtain a 1-g. sample of a simple salt or an alum from the instructor.

    (1) Record the color, appearance, and the form of the crystal if it is crystalline. Find out by a test whether or not it contains water of crystallization.

    (2) Dissolve one-half of the sample in 25 ml. of dis- tilled water. Test the solution with universal indicator paper and with litmus. Record the hydrolysis as A (strongly acid), a (weakly acid), N (neutral), b (weakly alkaline), B (strongly alkaline).

    If the solution is not clear, determine whether the precipitate is caused by hydrolysis or by insolubility. In the former case, in what group would you expect to find the metal?

    (3) Take a 3-ml. sample of the solution. Add sodium hydroxide solution carefully, a drop a t a time, shaking after each addition. Record the appearance, color, and type of the precipitate, if any. If no pre- cipitate, test for ammonium ion by warming and notic- ing the odor or the effect on moist red litmus held above the liquid in the tube.

    (4) From the results of the hydrolysis test and from the color, type, or lack of precipitate, determine to what group the metallic ion is likely to belong.

    Precipitate Ions (a) None NH4+, Li+, K+, Na+ (b) White cloudy Ca++, Sr++, Ba++ (c) White gelatinous Mg++, Al+++, Zn++ (d) White heavy curd Cd++, Sb+++, Pb++ (e) Colored Ag+, Fe++, Fe+++, Mn++,

    Cr+++, Co++, Ni++, Cu++

    strength of the related acids. Also, each student is assigned to a sample that is within his ability to iden-

    (5) Group (a). Ammonium ion test given. Flame test for other three.

    tify. The cation groups are as follows:

    Group (b). Flame tests. Check Ca++ by lack of a precipitate when Calgon (sodium

    1. Ammonium ion. hexametaphosphate) solution is added be- 2. Alkali metals: Li+, K+, Na+. fore ammonium oxalate solution, and the

  • FEBRUARY, 1951 103

    presence of a precipitate with ammonium oxalate solution in the absence of Calgon.

    Or, to distinguish between Ca++ and Ba++ if the flame test is not conclusive, acidify the sample of salt to be tested with dilute nitric acid. Add, a drop at a time, a solution of dilute sulfuric acid. A precipitate will be formed by the addition of one drop of the acid if the salt contains Ba++ ions. Calcium salt will require much more acid to obtain a precipitate. Group (c). Apply cobalt nitrate test. Group (d). Add hydrogen sulfide solution or

    ammonium sulfide and observe color of pre- cipitate.

    Group ( e ) . Mn++, Cr+++, Co++, Ni++ are identified by the borax bead test.

    Fe+++ gives a red color with KCSN solu- tion. Alternatively, a blue color is ob- tained with ICFe(CN)o solution.

    Fe++ is identified by a blue precipitate with K3Fe(CN)6 solution.

    Cu++ mill be replaced as a red metal by an iron nail, and Ag+ is replaced by a copper wire forming a black or a silvery deposit.

    Flame tests can be made without the use of platinum mire. Ignite a splint and produce a carbonized section by putting out the flame with water. A small amount of salt mill adhere to the charcoal. The flame test is made without interference from wood gases.

    The borax bead test can also he made without platinum. Heat the end of a glass rod about 2 mm. in diameter. Thrust into powdered borax and reheat. Repeat until a drop of melted borax adheres to the rod. Pick up a small amount of sample and heat. Rotate the rod in the flame and the sample will mix evenly with borax. These beads may be saved for reference and comparison.

    (6) When the cation has been identified, use the result of the hydrolysis test for an indication of the group to which the anion belongs. Make appropriate tests to identify the anion.

    (7) Write equations for all the reactions used in identifying your compound and point out clearly the steps you used to reach the identification.

    ACKNOWLEDGMENT

    The writer wishes to acknowledge his indebtedness to Elhert Weaver for his help in revising this paper.