Materials Included In Kit

Additional Materials Required

Prelab Preparation

1. Gather 15 clean beakers (150- or 250-mL). Use waterproof ink or a pencil and the labels provided in the kit to label the 15 beakers as follows:
   {11816_Preparation_Table_1}

Part I. Anion Testing

2. Fill the eight beakers for Part I with approximately 15–20 mL of the appropriate solution as listed. Place 3–4 labeled pipets in each beaker for dispensing. Labels are provided so the pipets can be labeled. This will help to avoid accidental contamination. To label the pipets, use waterproof ink and write on only one-half of the label. Fold the label in half around the pipet barrel just below the bulb as shown in Figure 1.
   {11816_Preparation_Figure_1}

   Chloride Cl^– testing solution, 0.1 M LiCl
   Iodide I^– testing solution, 0.1 M NaI
   Sulfate SO₄^2– testing solution, 0.1 M K₂SO₄ 
   Carbonate CO₃^2– testing solution, 0.5 M Na₂CO₃ 
   Silver nitrate, 0.1 M AgNO₃ 
   Nitric acid, 3 M HNO₃ 
   Ammonium hydroxide, 3 M NH₄OH Note: Ammonium hydroxide should be dispensed in a fume hood and should be covered due to irritating fumes. This can be accomplished by using either a dropper bottle or by using a watchglass or Parafilm M^® over the beaker.
   Barium nitrate, 0.1 M Ba(NO₃)₂

Part II. Cation Testing

3. Fill the seven beakers for Part II with the appropriate solution as listed. The beakers for the four cation-testing solutions for flame tests will need approximately 50 mL of solution (Li⁺, Na⁺, K⁺, Ba²⁺) for soaking the wooden splints. The remaining three beakers will only need 15–20 mL of solution. Place 3–4 labeled pipets in each beaker for dispensing.

   Lithium Li⁺ testing solution, 0.1 M LiCl
   Sodium Na⁺ testing solution, 0.1 M NaI
   Potassium K⁺ testing solution, 0.1 M K₂SO₄
   Barium Ba²⁺ testing solution, 0.1 M Ba(NO₃)₂
   Silver Ag⁺ testing solution, 0.1 M AgNO₃
   Iron(III) Fe³⁺ testing solution, 0.1 M FeCl₃
   Potassium thiocyanate, 0.1 M KSCN

4. Place wooden splints into each of the four cation-testing solutions for flame tests (one wooden splint for each group). Allow the splints to soak for at least 15–20 minutes before students perform flame tests.

Part III. Unknowns

5. Prepare an unknown sample for each student or student group. Partially fill a Beral-type pipet with the desired unknown solution and record which sample was assigned to each group. Use the provided labels to label the pipet with the unknown code. Alternately, a small amount of solution may be placed in a vial for ease of distribution to students. In addition, provide each student with a pre-soaked wooden splint of the unknown. Simply write the unknown letter or code on the top of the wooden splint.

   Possible unknowns could be any soluble salt that contains one of the cations and one of the anions that were tested in Parts I and II.

   Suggested unknowns, included in the kit:

   LiCl, NaI, K₂SO₄, Na₂CO₃, FeCl₃

   Alternate suggested unknowns, not included in the kit, that you may have in your chemical storeroom:
   Solutions concentration is not crucial, although solutions in the range of 0.1 M to 1.0 M work well.

   NaCl, Na₂SO₄, Li₂SO₄, KCl, KI, K₂CO₃, BaCl₂, Fe₂(SO₄)₃

   Assign letters or a code to each unknown. More than one letter can be assigned for each unknown. For example, if only five different unknowns are available, assign three letters to each unknown (so that LiCl might be unknown A, F and K) to get 15 unknowns.

Safety Precautions

Ammonium hydroxide, both the solution and the vapor, is extremely irritating to the eyes and is a serious respiratory hazard; dispense in a fume hood and be sure an eyewash is accessible. Ammonium hydroxide is moderately toxic by ingestion and inhalation. Nitric acid solution is corrosive and is a strong oxidizer. Barium nitrate solution is an oxidizer and is toxic by ingestion. Potassium thiocyanate solution is moderately toxic by ingestion. Silver nitrate solution is toxic and stains skin and clothing. Avoid all eye and body tissue contact with all solutions. Instruct students to handle their unknowns as if the material is corrosive and toxic. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulation that may apply, before proceeding. The well plates can be rinsed out in the sink with plenty of tap water. Remaining amounts of the reagents included in the kit can be saved for later use or disposed of according to the appropriate Flinn Suggested Disposal Methods.

Teacher Tips

• This kit is a Super Vale Kit and will easily serve 5 sections of 30 students working in pairs, providing more than enough for 75 tests. This lab experiment is designed to be completed in one 50-minute lab period.
• There are 160 labels included in the kit. Use 15 labels to label the beakers as outlined in the preparation section; use 75 labels to label the pipets for the unknowns (15 groups x 5 classes); use the remaining 60 labels to label the pipets for the 15 solutions (4 pipets per beaker). Labeled pipets can be rinsed out with water, saved, and reused with the same solution. Be sure to use waterproof ink or pencil on the labels.
• Saturate the wooden splints with the cation testing solutions (Li⁺, Na⁺, K⁺ and Ba²⁺) for at least 15–20 minutes at the beginning of the lab period before performing flame tests. For even better flame test results, soak the wooden splints overnight before the lab.
• Divide the class in half so that half of the class is working on Part I while the other half is working on Part II. This may help to relieve congestion around the test solution.
• Students will need to share the pipets and solutions from the reagent table. Caution students to read the labels on the pipets carefully and place the correct pipet back into the corresponding beaker to avoid contamination. Put out only what is needed for that class period in case contamination does occur.
• The procedure is written so that the unknowns are given to students in Part III after completing Parts I and II rather than simultaneously. The reason for this is that it forces the student to make and record detailed observations during Parts I and II. Students can then use their observations to determine the identity of the unknown salt. If time is a limiting factor, the unknowns can be given along with the knowns and tested side-by-side.

Sample Data

See Teacher PDF.

Answers to Questions

1. Write a net ionic equation for each reaction that occurred in Parts I and II.

   Part I. Anion Testing

   {11816_Answers_Table_1}
   Part II. Cation Testing
   {11816_Answers_Table_2}
2. Student answers will vary depending on the unknown.

Teacher Handouts

11816_Teacher1.pdf

References

Timberlake, K. Laboratory Manual for Chemistry; Harper & Row: New York, 1983; pp 91–99.

Wilbraham, A. C.; Staley, D. D.; Simpson, C. J.; Matta, M. S. Chemistry Laboratory Manual; Addison-Wesley: Menlo Park, CA,1990; pp 53–59.

Introduction

How are unknown chemicals analyzed? One method is by making comparisons to “known” chemicals. In this laboratory activity, ion tests will be performed and observations made for the reactions of four known anions and six known cations. Then an unknown salt will be identified by analyzing and comparing results to the knowns.

Concepts

• Qualitative analysis
• Precipitates
• Cations and anions

Background

The process of determining the identities of unknown substances is called qualitative analysis. This can be contrasted to quantitative analysis, which is the process of determining how much of a given component is present in a sample. Qualitative analysis procedures use physical tests as well as chemical tests. The physical tests in this lab involve observing colors of solutions and colors produced in flame tests. The chemical tests in this lab involve chemical reactions, as evidenced by formation of a precipitate, dissolving of a precipitate to form a complex ion, a color change or evolution of a gas.

Formation of a Precipitate
An ionic salt is a compound composed of two parts—cations (positively charged ions) and anions (negatively charged ions). When an ionic salt is dissolved in water, the salt crystal dissociates or separates into its corresponding cations and anions. For example, potassium iodide (KI) dissociates into potassium cations (K⁺) and iodide anions (I^–) according to Equation 1:

Similarly, the ionic salt lead nitrate [Pb(NO₃)₂] dissociates into lead cations (Pb²⁺) and nitrate anions (NO₃^–) according to Equation 2: When two ionic salts are mixed together in water, two new combinations of cations and anions are possible. In some cases, the cation from one salt and the anion from the other salt may combine to form an insoluble solid product, which is called a precipitate. For example, if solutions of potassium iodide and lead nitrate are mixed together, a solid precipitate of lead iodide (PbI₂) forms, as shown in Equation 3: Notice that the potassium cations (K⁺) and nitrate anions (NO₃^–) remain dissolved in solution. They do not combine to form a precipitate and thus do not participate in the reaction. These ions are referred to as spectator ions. Spectator ions do not participate in the overall reaction (hence, the term spectators) and are often omitted from the net ionic equation. A net ionic equation is one that includes only the ions participating in the reaction. Thus, Equation 3 can be reduced to Equation 4: Dissolving Precipitates Through Complex-Ion Formation
A complex ion is a water-soluble, charged species containing a central atom and other molecules bonded to it. The formation of a complex ion is commonly evidenced by the dissolution of a precipitate. For example, copper hydroxide [Cu(OH)₂] is insoluble in water but will dissolve when excess ammonia is added to it, forming a soluble copper amine complex ion [Cu(NH₃)₄²⁺] according to Equation 5: Evolution of a Gas
Certain anions, such as the carbonate ion (CO₃^2–) and sulfide ion (S^2–), evolve a gas when treated with a dilute strong acid. For example, the reaction of calcium carbonate (CaCO₃) with nitric acid (HNO₃) produces carbon dioxide gas (CO₂) according to Equation 6: Flame Colors
Some metallic salts will display a distinctive color of light when placed in a flame. When the colored light from any one of these flames is passed through a prism or viewed through a diffraction grating, a portion of the spectrum is visible, containing only a few colors at specific wavelengths, including the colors in the original flame. A partial spectrum that contains only discrete lines is called a line spectrum. When heated in a flame, electrons in the metal absorb energy from the flame and are promoted to excited energy levels. They emit light as they relax back down to the ground state. Each line in the line spectrum represents a different electronic transition. Since each element has a unique electronic configuration, an element’s line spectrum, and thus its flame color, is used for identification.

Experiment Overview

In Parts I and II of this lab, qualitative tests for four known anion solutions and six known cation solutions will be performed. Test results will be noted and recorded. In Part III, the same tests will be performed on an unknown ionic salt which contains one of the six possible cations and one of the four possible anions. The cation and the anion that make up the unknown ionic salt will then be identified.

Materials

Prelab Questions

• It is important to record detailed observations in the data table as each step of the procedure is performed. This way, when testing the unknown salt in Part III, these detailed observations can be used to help determine the identity of the unknown.
• At the beginning of the lab period, place a wooden splint into a beaker of each of the following cation-testing solutions to be used for flame testing in Part II: Li⁺, Na⁺, K⁺ and Ba²⁺. These splints should soak for at least 15–20 minutes before performing the flame tests. Note: Your teacher may already have the wooden splints soaking in beakers of these solutions at the chemical reagent table.
• In Part III, an unknown salt solution containing one of the cations and one of the anions will be assigned to your group to be tested. Use the results from the qualitative tests in Part I and Part II to identify which cation and anion make up the unknown salt. The formula for the unknown ionic salt will then be written.

Safety Precautions

Ammonium hydroxide, both the solution and the vapor, is extremely irritating to the eyes and is a serious respiratory hazard; dispense in a fume hood and be sure an eyewash is accessible. Ammonium hydroxide is moderately toxic by ingestion and inhalation. Nitric acid solution is corrosive and is a strong oxidizer. Barium nitrate solution is an oxidizer and is toxic by ingestion. Potassium thiocyanate solution is moderately toxic by ingestion. Silver nitrate solution is toxic and stains skin and clothing. Avoid eye and body tissue contact with all solutions. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Procedure

Part I. Anion Testing for Cl^–, I^–, SO₄^2– and CO₃^2–

Preparing the Well Plate

1. Obtain a 24-well reaction plate and set it on the lab bench as shown in Data Table 1. Place a sheet of notebook paper below the plate and label the paper on the top and side as shown in Data Table 1. Notice that the 24-well reaction plate is divided into 6 columns (1–6) and 4 rows (A–D).
2. Using a pipet, add 5 drops of the Cl^– anion-testing solution to wells A, B and C of column 1.
3. Using a pipet, add 5 drops of the I^– anion-testing solution to wells A, B and C of column
4. Using a pipet, add 5 drops of the SO₄^2– anion-testing solution to wells A, B and C of column 3.
5. Using a pipet, add 5 drops of the CO₃^2– anion-testing solution to wells A, B and C of column 4.

Performing the Tests

Silver Nitrate Test

6. 1. Add 3 drops of 0.1 M AgNO₃ to the first four wells across row A. Observe the formation of precipitates and/or color changes. Remove the white paper under the plate in order to clearly see the results. A dark background may be helpful. Record detailed observations in Data Table 1.
   2. Add 5 drops of 3 M HNO₃ to each of the precipitates from step 6a. Gently swirl the well plate to stir. Observe which precipitates dissolve and which do not. Record observations in Data Table 1.
   3. Add 10–12 drops of 3 M NH₄OH to each of the remaining precipitates from step 6b. Gently swirl to stir. Observe which precipitates dissolve and which do not. Record observations in Data Table 1.

Barium Nitrate Test

7. 1. Add 3 drops of 0.1 M Ba(NO₃)₂ to the first four wells across row B. Observe the formation of precipitates and/or color changes. Remove the white paper under the plate in order to clearly see the results. A dark background may be helpful. Record detailed observations in Data Table 1.
   2. Add 5 drops of 3 M HNO3 to each of the precipitates from step 7a. Gently swirl the well plate to stir. Observe which precipitates dissolve and which do not. Record observations in Data Table 1.

Formation of a Gas Test

8. Add 3 drops of 3 M HNO3 to the first four wells across row C. Make observations, looking for the strong evolution of gas bubbles. Record observations in Data Table 1. 
9. Repeat any tests for which results are unclear in Row D of the well plate. Rinse the well plate in the sink with plenty of tap water; then rinse with distilled water to prepare the plate for Part II. 

Part II. Cation Testing for Li⁺, Na⁺, K⁺, Ag⁺, Ba²⁺ and Fe³⁺

Preparing the Well Plate

10. Obtain a 24-well reaction plate and set it on the lab bench as shown in Data Table 2. Place a sheet of notebook paper below the plate and label the paper on the top and side as shown in Data Table 2. Notice that the 24-well reaction plate is divided into 6 columns (1–6) and 4 rows (A–D).
11. Using a pipet, add 5 drops of the Li⁺ cation-testing solution to the top well (Row A) of column 1. 
12. Using a pipet, add 5 drops of the Na⁺ cation-testing solution to the top well (Row A) of column 2. 
13. Using a pipet, add 5 drops of the K⁺ cation-testing solution to the top well (Row A) of column 3. 
14. Using a pipet, place 5 drops of the Ag⁺ cation-testing solution to the top well (Row A) of column 4. 
15. Using a pipet, place 5 drops of the Ba²⁺ cation-testing solution to the top well (Row A) of column 5. 
16. Using a pipet, place 5 drops of the Fe³⁺ cation-testing solution to the top well (Row A) of column 6.

Performing the Tests

17. Observe each solution and record the color of each solution in Data Table 2 (Row A). 

Potassium Thiocyanate Test

18. Add 3 drops of 0.1 M KSCN to each of the six wells across row A. Gently swirl the well plate to stir. Observe the formation of precipitates and/or color changes. Record detailed observations in Data Table 2 (Row B).

Flame Tests
Note: Several of the cations may be identified using flame tests. The flame tests will be performed on the four cation solutions that did not show a reaction in step 17.

19. Set up a Bunsen burner. Adjust the air so that the flame color is blue (not yellow) and a distinct inner blue cone is apparent. 
20. Obtain a wooden splint which has been soaking in the Li⁺ cation testing solution for at least 15 minutes. 
21. Hold the wooden splint in the flame, flat side down. The top end of the splint should be placed directly into the inner blue cone. A distinct color should be apparent. Record the flame color in Data Table 2 (Row C). Note: Do not hold the splint in the flame for too long or the splint will begin to burn. 
22. Repeat steps 19 and 20 using the Na⁺ cation testing solution, the K⁺ cation testing solution, and the Ba²⁺ cation testing solution. Be careful not to gather all of the splints at once and perhaps touch them together. This may cause contamination. 
23. Repeat any tests for which results are unclear. Rinse the well plate in the sink with plenty of tap water; then rinse with distilled water to prepare the plate for Part III

Part III. Identification of an Unknown Salt

Note: The unknown salt is made up of one type of cation (Li⁺, Na⁺, K⁺, Ag⁺, Ba²⁺or Fe³⁺) and one type of anion (Cl^–, I^–,SO₄^2–and CO₃^2–).

24. Obtain from your teacher a pipet filled with an unknown salt solution and a presoaked wooden splint of the same unknown salt. Be sure to record the unknown letter in the data tables. 
25. Determine the identity of the cation and of the anion that make up the unknown salt. To do this, repeat the steps in Part I and in Part II. Record all observations for anion-testing in Data Table 1 and for cation-testing in Data Table 2.

Student Worksheet PDF

11816_Student1.pdf