Chemistry Everyday for Everyone
A Chemical Puzzle Harry E. Spencer and Leonard Kusdra Department of Chemistry, Oberlin College, Oberlin, OH 44074
Prospective and new college students have repeatedly performed a brief qualitative analysis experiment that requires only a high-school background in chemistry and serves as an introduction to college-level chemistry. It has proven to be a popular chemical puzzle and example of chemical reasoning. Among its arresting features are the science-fiction setting originated by one of us (LK) as a first-year student, colorful liquid solutions and precipitates, and puzzle-solving challenges. Students determine qualitatively four cations (Fe3+, Co2+, 2+ Ni , and Cu 2+) and two anions (Cl᎑ and SO42᎑) using only a few solutions without fixed procedures. Each of the four unknown acidic solutions contains a different combination of two cations and one anion. Two charts, one for cations and one for anions, outline the needed chemistry. The cation chart lists the colors of solutions and precipitates produced by adding one of the following to an acidic solution of each of the four cations: a combination of ammonia and ammonium nitrate, the same reagents followed by dimethyl glyoxime solution, and potassium thiocyanate solution. Definitive colors, such as that of the cupric–ammonia complex, are highlighted. The anion chart lists precipitates formed when either silver nitrate or barium nitrate solution is added to an acidic solution containing either of the anions. A centrifuge is required for separating solid ferric hydroxide from the remaining solution. We display test-tube examples of the various additions. To pique student interest, the laboratory instructions are given in the form of messages intercepted from the forces of E.V.I.L. These messages reveal the unusual names of captured unknown solutions and the cations and anions contained therein. Elixir of Life contains the knight, queen, and rook; Socrates’ Hemlock—the pawn, king, and bishop; Primal Ooze—the queen, king, and rook; and Noah’s Floodwater— the pawn, knight, and bishop. The king, queen, knight, and pawn are cations, whereas the rook and bishop are anions. In order to foil the forces of E.V.I.L. students are challenged to identify which ion corresponds to which chess piece in the captured unknown solutions. Most students solve the puzzle within one hour; some, in less time. Often failure to recognize the importance of what does not occur slows analysis: Some do not recognize, for example, that the lack of brown precipitate after addition of ammonia and ammonium nitrate solutions means that Fe3+ is missing. Whether solved rapidly or slowly, however, the puzzle has been widely enjoyed, thus fulfilling our major purpose of demonstrating that chemical experimentation can be fun. Related References Ricketts, John A. A Laboratory Exercise Emphasizing Deductive Chemical Reasoning; J. Chem. Educ. 1960, 37, 311–312. Thompson, M. L.; Bixler, J. W. Abridged Qualitative Analysis with Limited Reagents; J. Chem. Educ. 1971, 48, 113–114. Kilner, C. Qualitative Analysis of Some Transition Metals; J. Chem. Educ. 1985, 62, 80.
Identification of Royal Court Ions MISSION: Red Alert! The forces of E.V.I.L. are at it again, and we need your help! For years, we, the organization of S.A.F.E., have made it our sole purpose to stop the actions of E.V.I.L. in their attempts to conquer the world. Many times they have tried their diabolical schemes, and many times we have thwarted their efforts. Now, they have built a secret weapon, a space station, which, when fully operational, will be able to blast to pieces any site on planet Earth! The space station is protected by a force shield, but with your help, we will be able to thwart E.V.I.L.’s plans. You have been chosen especially for this mission because of your invaluable capability for critical thinking—a qualification essential to our success. Time is of the essence. Your mission: Find the identities of the Royal Court members! BRIEFING: Our spies have been able to steal secret files from the space station. Many lives were lost to obtain this valuable information. What the secret files have revealed is that a self-destruct mechanism is located in the central core of the space station. Unfortunately, the self-destruct mechanism is controlled by the central computer, access to which requires a certain code. This code consists of the names of six “Royal Court” members: King, Queen, Knight, Rook, Bishop, and Pawn, each of which is assigned to a particular cation or anion. From the space station, our clever spies also stole four solutions, labeled Elixir of Life, Socrates’ Hemlock, Primal Ooze, and Noah’s Floodwater, containing these cations and anions. We do not know the composition of each solution or the identity of each member. Fortunately we intercepted a message from earth to the space station that said: Elixir of Life contains the Knight, Queen, and Rook Socrates’ Hemlock contains the Pawn, King, and Bishop Primal Ooze contains the Queen, King, and Rook Noah’s Floodwater contains the Pawn, Knight, and Bishop With this knowledge, your goal becomes clear. You must perform suitable experiments so that you can decide which anions and cations are present in each solution. Once you know that, you can match each chess piece with the appropriate cation or anion—thus breaking the code! Our spies also have narrowed the possibilities. Possible Identities King, Queen, Knight, Pawn Rook, Bishop Ferric ion (Fe3+) Cobaltous ion (Co2+) Nickel ion (Ni2+) Copper ion (Cu 2+)
Chloride ion (Cl᎑) Sulfate ion (SO42᎑ )
JChemEd.chem.wisc.edu • Vol. 75 No. 4 April 1998 • Journal of Chemical Education
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Chemistry Everyday for Everyone
We need your help in discovering the ionic identities of the six Royal Court members. Tables 1 and 2 describe tests for identifying ions. All you need to do is transfer about 1 mL of an unknown to a test tube. Then add the reagents to the test tube and stir the mixture. Observe the color, particularly any distinctive color, and whether a precipitate forms. Consider the significance of nonformation of precipitates and solution colors. You might need to repeat the process one or more times, using a different reagent each time. With the use of the accompanying charts, clever choices of reagents, and a minimum number of tests, you can determine the identities of the Royal Court members! For some tests, you may need to separate a precipitate from its surrounding liquid by centrifugation. Good luck! The future of the earth is in your hands!!! Experimental Procedure MIXING: After each addition of a test solution, mix the resulting liquid; otherwise the results may be misleading. A convenient method for mixing a liquid in a test tube is to hold the tube at its lip with a finger and thumb of one hand while applying sharp but glancing downward taps near the bottom of the tube with a finger of the other hand.
Equipment and Chemicals Needed for the Experiment Identification of Royal Court Members •
•
Three sets of dropping bottles and three sets of unknown solutions to be placed on the center shelf of each bench Dropping bottles needed in each set: 7 M ammonia
4 M ammonium nitrate 0.2 M barium nitrate 0.1 M dimethyl glyoxime [in 95% ethanol] 6 M nitric acid 0.2 M potassium thiocyanate 0.1 M silver nitrate
•
Compositions of a set of unknown solutions. Concentration of each compound is 0.05 M. Elixir of Life: ferric nitrate, cupric nitrate, sulfuric acid Noah’s Floodwater: ferric nitrate, nickel nitrate, hydrochloric acid Primal Ooze: cupric nitrate, cobalt(II) nitrate, sulfuric acid Socrates’ Hemlock: cobalt(II) nitrate, nickel nitrate, hydrochloric acid
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Additional equipment needed: disposable test tubes, disposable plastic pipets (1 mL), test tube racks, centrifuges One display of sixteen solutions and liquid–solid mixtures for each bench
• 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
10 mL 0.05 M Fe 3+ Solution #1 plus 6 mL 7 M NH3 and 3 mL 4 M NH4 NO3 . Empty test tube Solution #1 plus 2 mL 0.2 M KSCN 10 mL 0.05 M Co2+ Solution #5 plus 6 mL 7 M NH 3 and 3 mL 4 M NH 4NO 3. Solution #6 plus 2 mL dimethyl glyoxime solution Solution #5 plus 2 mL 0.2 M KSCN 10 mL 0.05 M Ni 2+ Solution #9 plus 6 mL 7 M NH3 and 3 mL 4 M NH4 NO 3 Solution #10 plus 2 mL dimethyl glyoxime solution Solution #9 plus 2 mL 0.2 M KSCN 10 mL 0.05 M Cu2+ Solution #13 plus 6 mL 7 M NH 3 and 3 mL 4 M NH4NO 3 Solution #14 plus 2 mL dimethyl glyoxime solution Solution #13 plus 2 mL 0.2 M KSCN
Table 1. Four cations in different environments. Table entries describe what appears as a result of the addition listed in the column heading. The colors listed under “No addition” are for solutions containing only one of the four cations: Fe3+, Co2+, Ni2+, or Cu2+. Unknowns may have somewhat different colors. Any solution containing one or more cations must also contain at least one anion. Bold entries indicate results so distinctive in color or form or both that they are particularly useful for identification. Cation Solution
No addition
Addition of 14 drops of 7 M NH3 and 6 drops of 4 M NH4NO3 solutions
Addition of 14 drops of 7 M NH3 and 6 drops of 4 M NH4NO3 solutions [SEE NOTE] followed by 5 drops of 0.1 M dimethyl glyoxime solution*
Addition of 5 drops of 0.2 M KSCN solution
Fe3+
Yellow
Brown precipitate
SEE NOTE BELOW
Blood-red solution
Co2+
Pink
Pale red-brown solution
Brown solution
Pink solution
Ni2+
Faint green
Pale purple solution
Red-pink precipitate
Faint green solution
Cu2+
Light blue
Dark blue solution
Dark blue solution
Pale green solution with white precipitate
*The solvent for the dimethyl glyoxime solution is 95% ethanol. NOTE : You must add the NH3 and NH 4NO3 solutions before the dimethyl glyoxime solution. If a brown solid precipitates upon addition of NH3 and NH4 NO3 , then the unknown solution contained Fe3+ . In that case, you must remove the iron-containing precipitate before adding dimethyl glyoxime. To separate the brown solid, centrifuge the mixture (balance its test tube in the rotor with another one containing an equal volume of water), and then use a dropping pipet to transfer the liquid portion to another test tube. Only then should you add the dimethyl glyoxime solution to the separated liquid to test for Ni2+. (Discard the brown solid.)
Table 2. Two anions in different environments. Anion Solution
Addition of 2 drops of 6 M nitric acid and 5 drops of 0.1 M silver nitrate solution
Addition of 2 drops of 6 M nitric acid and 5 drops of 0.1 M barium nitrate solution
Cl –
White precipitate
No precipitate
SO42–
No precipitate
White precipitate
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Identities of the Royal Court Members King: Queen: Knight: Pawn: Rook: Bishop:
Journal of Chemical Education • Vol. 75 No. 4 April 1998 • JChemEd.chem.wisc.edu
Co2+ Cu2+ Fe3+ Ni2+ SO42᎑ Cl᎑
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