Materials Included In Kit

Additional Materials Required

Safety Precautions

Sodium hydroxide is corrosive both as the solid and in solution; skin burns are possible; avoid all body tissue contact. Iodine tincture, phenolphthalein solution and isopropyl alcohol solution are flammable liquids and are toxic by ingestion and inhalation. All other reagents and unknowns are considered nonhazardous; however, all may cause slight irritation to the skin, eyes or respiratory tract; avoid all body tissue contact. Inspect all test tubes for cracks or chips and replace, if necessary. Wear chemical splash goggles, a chemical-resistant apron and chemical-resistant gloves. 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. Remaining amounts of the eleven white solids can be saved for later use or disposed of in the solid waste according to Flinn Suggested Disposal Method #26a. The solutions in the test tubes can be rinsed down the drain with water according to Flinn Suggested Disposal Method #26b. Excess test reagents can be saved for later use or disposed of according to the appropriate Flinn Suggested Disposal Methods.

Teacher Tips

• Note that the 11 substances to be identified by the student have not been assigned unknown letters. We have left this up to the individual instructor who may want to vary the identity of the unknowns for different groups in one class or for different classes throughout the day.
• This kit is a “super-kit” and will easily serve five sections of 30 students working in pairs, providing enough for 75–100 qualitative analysis tests.
• Eleven polypropylene jars are provided so the correct amount of each of the 11 unknowns needed for each class can be set out. These jars can be labeled with unknown letters A–K.
• The actual lab experiment is designed to be completed in one 50-minute lab period. However, it is suggested that two days be allotted for this lab. On day one, provide an explanation of qualitative analysis, give a pre-lab instruction on following a flow chart, and discuss/demonstrate the physical and chemical tests. The students can perform the lab on day two.
• This is a qualitative lab; therefore, exact amounts used in the tests are not crucial.
• Stress to students that a small scoop is only about 0.25 g, or about the size of a small pea. Using too much of the solids may create unclear test results, especially for the solubility test. It may be helpful to weigh 0.25 g out once before beginning the lab to show the students how little the amount of a small scoop is and to stress that more is not always better.
• The distinction between sucrose and sodium chloride is their difference in melting points. Sucrose melts at 185 °C while sodium chloride melts at 801 °C.
• This experiment can be adapted for various levels of instruction. The lab can be performed as written for general chemistry classes. For more advanced classes, consider not providing the step-by-step procedure or the flow chart. Allow the students to design their own identification scheme given only the names of the 11 white solids. Known samples of each of the solids can be provided to students for testing; then the unknowns can be tested and their identity determined.
• All of the compounds used as the unknowns are white solids and are common household substances found in drugstores or supermarkets. You may consider sharing the information listed below with your students.
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Sample Data

Notice that the unknowns are listed alphabetically. We suggest varying the unknown order for the actual lab; perhaps use a different order for each class. Also notice that, for ease of grading, students have been asked to write not only descriptive observations but also the corresponding result numbers. The numbers make it easier to follow the student’s pathway through the flow chart and to see how the unknown was identified.

References

Solomon, S.; Fulep-Poszmik, A.; Lee, A. J. Chem Ed; 1991, 68, 328–329.

Introduction

Identify 11 unknown substances by following a qualitative analysis flow chart. Learn to use a variety of chemical and physical tests to identify 11 common household substances, all of which are white solids.

Concepts

• Qualitative analysis
• Physical and chemical properties
• Flow diagrams

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. A qualitative analysis scheme using simple chemical and physical tests is designed, in this laboratory experiment, for the identification of 11 common household white solids: boric acid, calcium carbonate, calcium sulfate, cornstarch, levulose, magnesium sulfate, sodium bicarbonate, sodium carbonate, sodium chloride, sodium hydroxide and sucrose. Qualitative analysis schemes are generally summarized by a flow diagram, like the one shown on page four. A flow diagram is designed with the procedural steps on the vertical lines, the possible test results on the horizontal lines and the resulting identifications in the boxes.

Qualitative analysis procedures include physical tests as well as chemical tests. The physical tests in this lab are melting point determination and solubility in water or in alcohol. The chemical reactions or tests in this lab are with iodine, vinegar, sodium hydroxide, phenolphthalein, and Benedict’s solution. All of these tests involve either formation of a precipitate, a color change, or evolution of gas bubbles. On the basis of observations, each of the eleven white household substances can be positively identified.

Experiment Overview

The goal of this lab is to learn to use physical and chemical tests to determine the identities of unknowns A–K.

Materials

Prelab Questions

1. The steps of the procedure correspond to the qualitative analysis flow chart provided in the Procedure section. As each step is followed, record detailed observations of your results on the provided data sheet.
2. Examine the flow chart. The numbers 1–20 are provided next to each of the possible results. Write the corresponding number onto your data sheet as you go through the flow chart. For example, if unknowns A, B and C are water-insoluble (which is result 1) and unknowns D–K are water-soluble (which is result 2), then write a “1” by A, B and C and record “Insoluble in H₂O”; and write a “2” by D–K and record “Soluble in H₂O” on your data sheet. To further explain, if unknown “X” is found to be 1, 4 and 6, then the observations would read “Insoluble in H₂O; orange-brown (NR) with I₂; NR with vinegar” on your data sheet. Following 1 to 4 to 6 on the flow chart, the unknown “X” is then clearly identified as CaSO₄.

Safety Precautions

Sodium hydroxide is corrosive both as the solid and in solution; skin burns are possible; avoid all body tissue contact. Iodine tincture, phenolphthalein solution, and isopropyl alcohol solution are flammable liquids and are toxic by ingestion and inhalation. All other reagents and unknowns are considered nonhazardous; however, all may cause slight irritation to the skin, eyes or respiratory tract; avoid all body tissue contact. Inspect all test tubes for cracks or chips and replace, if necessary. Since all of the samples are unknown to you, treat each and every one of the white solids as if it were a hazard. Follow all safety precautions; be sure to properly label all unknowns so the identity may be determined in the case of an emergency. Wear chemical splash goggles, a chemical-resistant apron and chemical-resistant gloves. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Procedure

1. Label 11 test tubes A–K with a marking pen and place the tubes in a test tube rack.
2. Place a small scoop of each of the 11 unknown substances, A–K, into the appropriate test tube. (Note: A small scoop is the pea-sized amount that will fit on the end of a spatula, approximately 0.25 g. Results may be affected by using more than that amount.)
3. Add approximately 5 mL of distilled or deionized water to each tube. (Note: This can be efficiently accomplished by measuring 5 mL once using a 10-mL graduated cylinder. Pour the 5 mL of water into test tube A and then add water to each of the 10 remaining tubes to the same height of the liquid in tube A.)
4. 1. Stir the contents of each tube gently with a separate wooden splint to attempt to dissolve the solids.
   2. Record observations of which substances are soluble and which are insoluble in water. Remember to record both the result # (1 or 2) as well as the written observation. (Note: Some soluble solids may take longer to dissolve than others.) Only three of the unknowns—cornstarch, calcium sulfate and calcium carbonate—will not readily dissolve in water and are considered insoluble.
5. Following the flow chart (see Figure 1), take the three tubes from step 4 containing the insoluble substances. Add 2 drops of iodine tincture to each of the three tubes. Two of the tubes will show no reaction with iodine and will be an orange-brown color. The contents of one of the tubes will turn a deep blue color. The deep blue color is a starch-iodine complex which positively indicates cornstarch.
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6. 1. Dispose of the contents of the two tubes that did not react with iodine. Rinse out the tubes. Prepare fresh tubes of these two unknowns by placing a small pea-sized scoop of the solid into the appropriate tube. Do not add water.
   2. Add approximately 10 drops of vinegar to these two tubes and note whether gas bubbles are produced. The evolution of carbon dioxide gas positively identifies calcium carbonate. The remaining solid must be calcium sulfate. Record observations.
7. The other eight solids are water soluble. To each of the eight tubes from step 4, add 3–4 drops of phenolphthalein solution. Two of the unknowns, sodium hydroxide and sodium carbonate, dissolve in water to produce alkaline solutions basic enough to give a bright pink color upon addition of phenolphthalein. Do not be concerned with precipitate formation at this point, or with faint pink coloration.
8. 1. Dispose of the contents of the two tubes that gave a positive test in step 7. Rinse out the tubes. Prepare fresh tubes of these two unknowns by placing a small scoop into the appropriate tube. Do not add water.
   2. Add approximately 10 drops of vinegar to each tube and note whether gas bubbles are produced. The evolution of carbon dioxide gas positively identifies sodium carbonate. The remaining solid must be sodium hydroxide.
9. 1. Dispose of the contents of the tubes containing the six solids that remain to be identified. Rinse out the tubes. Prepare fresh tubes of these six unknowns by placing a small scoop into the appropriate tube.
   2. Add 5 mL of distilled or deionized water to the six tubes and gently stir as in step 4 to dissolve the solids.
10. Add 3 drops of 0.2 M NaOH to each tube. All of the tubes should remain clear except one tube which gives a white precipitate. This white precipitate positively identifies magnesium sulfate, which forms an insoluble hydroxide upon addition of sodium hydroxide.
11. 1. Take the remaining five tubes from step 10 and add 10 drops of Benedict’s qualitative solution to each tube.
    2. Hold the tubes with a test tube holder and heat each tube gently over a Bunsen burner flame. In one tube, an orange precipitate will form while the remaining four tubes will stay blue in color. The color change to orange indicates that the copper ions in the Benedict’s solution are being reduced to copper by a reducing sugar group. Levulose (fructose) is a reducing sugar; thus, this test is a positive identification for levulose.
12. 1. Dispose of the contents of the tubes containing the four solids that remain to be identified. Rinse out the tubes. Prepare fresh tubes of these four unknowns. Do not add water.
    2. Add approximately 10 drops of vinegar to each tube and note whether gas bubbles are produced. The evolution of carbon dioxide gas positively identifies sodium bicarbonate.
13. 1. Dispose of the contents of the three remaining tubes. Rinse out the tubes and prepare fresh tubes of these three unknowns. Do not add water.
    2. Add approximately 5 mL of isopropyl alcohol to each tube. Gently stir the contents of each tube to attempt to dissolve the solids. Of the three solids, only boric acid does not dissolve in alcohol; thus, this test is a positive identification for boric acid.
14. 1. Dispose of the contents of the two remaining tubes. Rinse out the tubes. Prepare fresh tubes of these two unknowns. Do not add water.
    2. Hold the tubes with a test tube holder and heat each tube gently over a Bunsen burner flame. The solid in one tube will turn brown, smell sweet, and begin to melt in 1–2 minutes. This is an indication that the material has a low melting point and that it is sucrose. The other solid will not change as it is heated. This indicates that the solid has a high melting point and is sodium chloride.

Student Worksheet PDF

11802_Student1.pdf