The ABCs of Chemistry

Transcription

1 Hands-On Science The ABCs of Chemistry Michael Margolin illustrated by Lloyd Birmingham WALCH EDUCATION

2 Contents To the Teacher... v... vii... viii... xvi Glossary

3 7. Identifying Unknowns INSTRUCTIONAL OBJECTIVES Students will be able to: q observe and record data define the terms cation and precipitate NATIONAL SCIENCE EDUCATION STANDARDS CORRELATIONS GRADES 5 8 TEACHER RESOURCE PAGE Content standard Bullet number Content description Bullet number(s A 1 Abilities necessary to do scientific inquiry 1 7 A 2 Understandings about scientific inquiry 1, 4 B 1 Properties and changes of properties in matter 1 3 G 2 Nature of science 1, 3 GRADES 9 12 use solubility to identify cations in known and unknown solutions Content standard Bullet number Content description Bullet number(s A 1 Abilities necessary to do scientific inquiry 1, 2, 4 6 B 1 Structure of atoms 1 B 2 Structure and properties of matter 1, 3, 5 B 3 Chemical reactions 3 E 1 Abilities of technological design 2, 5 G 2 Nature of scientific knowledge 1, 2 VOCABULARY analytic chemistry: branch of chemistry that studies the chemical composition of natural and artificial materials anion: negatively charged ion cation: a positively charged ion, such as Na + or Ca 2+ decant: to pour off the liquid part of a mixture while leaving the solid behind insoluble: unable to dissolve in a particular solvent; for example, nail polish is insoluble in water, but soluble in acetone. See soluble. ion: a charged atom or group of atoms that has a charge because it has an unequal number of protons and electrons; examples include Na +, Cl 2, and SO 4. precipitate: a substance that is insoluble and drops out of a solution soluble: capable of being dissolved in a solvent solution: a homogeneous mixture consisting of a solute dissolved in a solvent 2010 Walch Education 52

4 7. Identifying Unknowns TEACHER RESOURCE PAGE q Part 1 MATERIALS five test tubes test-tube rack test-tube holder 10-ml graduated cylinder 100-ml graduated cylinder 300-ml beaker medicine droppers marking pencil or permanent marker stirring rod ring stand and ring safety goggles 0.1 M silver nitrate (AgNO M mercury (I nitrate (Hg 2 (NO M lead (II nitrate (Pb(NO M sodium nitrate (NaNO M calcium nitrate (Ca(NO M hydrochloric acid (HCl 1 M potassium chromate (K 2 CrO 4 5 M ammonia (NH M ammonium carbonate (NH 4 CO 3 wire gauze Bunsen burner matches distilled water rubber stoppers Part 2 five test tubes unknown samples 1 5 safety goggles HELPFUL HINTS AND DISCUSSION Time frame: two class periods Structure: individuals or cooperative learning groups of two to four students Location: classroom or lab In this activity, students will determine the precipitates that form when different anions are added to five cations: Na +, Ca 2+, Ag +, Hg 2+, and Pb 2+. They will learn to distinguish between these cations by observing their reactions with specific anions. They will use this information to determine whether or not these cations are present in unknown solutions. The first four unknowns will contain only one cation, and your students will determine which of two possible cations is present. You may choose to give some students an unknown containing one of these cations and some students an unknown containing the other. This will prevent students from copying results. If you do this, code the unknowns with a two-digit code for example, 1a and 1b. The fifth unknown will contain two or more cations, and students will be asked to plan a method for determining which cations are present. The unknown samples should be distributed in labeled vials, each containing 5 ml of the unknown. It is important to stress the necessity of cleaning glassware before using it, since contamination with any solution would alter the results. Explain (continued on next page 2010 Walch Education 53

5 7. Identifying Unknowns TEACHER RESOURCE PAGE that the word cation is pronounced cat-ion. You may wish to have students perform this activity over a two-day period. It will take some preparation time to prepare unknowns. Read the student instructions so that you will know which ions to place in each vial. You can begin this lesson by asking students to explain why many people use filtered or bottled water for drinking. What type of things do they want to remove from water? Safety note: Students must wear goggles while performing this activity. Hydrochloric acid can cause severe burns and damage skin. Students should handle all chemicals with caution, avoiding contact with skin and eyes. Students must be supervised by you or another adult while using the Bunsen burner. MEETING THE NEEDS OF DIVERSE LEARNERS For advanced students, you could make the identification of cations in the fifth unknown more difficult by telling students that the unknown might contain one through four cations. Explain to these students that a negative test result is as important as a positive test result. Encourage these students to perform the Follow-Up Activities, particularly numbers 1 and 4. You might also ask that students use chemical formulas and equations when answering questions. Some students might need extra help performing these activities and planning their scheme for identifying unknowns. You can provide a graphic organizer to help students more easily picture the tests for cations. You might wish to limit the list of possible cations in the fifth unknown to three rather than five. SCORING RUBRIC Students meet the standard for this activity by: correctly identifying unknowns defining the terms cation and precipitate identifying cations in known and unknown observing and recording data solutions 2010 Walch Education 54

6 7. Identifying Unknowns q q? RECOMMENDED INTERNET SITES About.com Chemistry: Flame Tests Launceston College Cations NASA s Langley Research Center Simple Spectroscope asd- YouTube Flame Test 07 ANSWER KEY TEACHER RESOURCE PAGE 1. The compounds formed are silver chloride (AgCl, lead chloride (PbCl 2, and mercury (I chloride (Hg 2 Cl The chromate ion (CrO 4 combines with the lead ion (Pb 2+ to form lead chromate (PbCrO It is important to wash the glassware to prevent contamination from another ion. 4. Answers will vary. Sample answer: Different cations combine with different anions to produce precipitates, each with a particular color. Knowing which ions combine and which don t helps one identify which ion is present. For example, if a chloride is added to a test tube with silver ions, it will form a black precipitate. 5. Add a sodium chloride solution to it. If lead ions are present, a white precipitate will form Walch Education 55

7 Name 7. Identifying Unknowns OBJECTIVE To identify different cations using their properties Date STUDENT ACTIVITY PAGE BEFORE YOU BEGIN Homeowners might want to know if minerals are present in their drinking water. A scientist might want to know what minerals are in lake water or what gases are in the air we breathe. A detective might want to know whether a person has taken drugs or been given poison. All of these people would turn to an analytic chemist for answers to these questions. Analytic chemistry is a branch of chemistry that deals with identifying the presence and quantities of chemicals. In this activity, you will become an analytic chemist. Solutions may contain positively charged ions that are called cations. Cations may combine with negative ions (anions to form insoluble compounds. When these insoluble compounds form, they drop out of solution as solids, known as precipitates. A precipitate is a solid compound that forms when two ions combine in a solution. Different cations form precipitates with different anions. In this activity, you will learn some of the precipitates that form. You will use this information to identify the presence of different cations in a solution. q Part 1 MATERIALS five test tubes test-tube rack test-tube holder 10-ml graduated cylinder 100-ml graduated cylinder 300-ml beaker medicine droppers marking pencil or permanent marker stirring rod ring stand and ring wire gauze Bunsen burner matches distilled water rubber stoppers safety goggles 0.1 M silver nitrate (AgNO M mercury (I nitrate (Hg 2 (NO M lead (II nitrate (Pb(NO M sodium nitrate (NaNO M calcium nitrate (Ca(NO M hydrochloric acid (HCl 1 M potassium chromate (K 2 CrO 4 5 M ammonia (NH M ammonium carbonate (NH 4 CO 3 Part 2 five test tubes unknown samples 1 5 safety goggles 2010 Walch Education 56

8 Name Date 7. Identifying Unknowns PROCEDURE STUDENT ACTIVITY PAGE Safety note: You must wear goggles while performing this activity. Hydrochloric acid can cause severe burns and damage skin. Handle all chemicals with caution, avoiding contact with skin and eyes. Part 1 1. Using the marking pencil or permanent marker, label five test tubes as follows: Na +, Ca 2+, Ag +, 2+ Hg 2, and Pb 2+. Place these test tubes in the test-tube rack. 2. Label the remaining five test tubes U1 through U5 and place them aside for use in Part Set up the Bunsen burner, ring stand, wire gauze, and ring for a water bath. Do not turn on the burner or add water to the beaker yet. You will use the water bath in step 9. See the diagram. 4. Use the 10-ml graduated cylinder to add 5 ml of each of the following to the appropriate test ring stand tube. Carefully and thoroughly wash and dry the graduated cylinder after each use. beaker sodium nitrate to the test tube labeled Na + calcium nitrate to the test tube labeled Ca 2+ silver nitrate to the test tube labeled Ag + mercury (I nitrate to the test tube labeled Hg 2 2+ lead (II nitrate to the test tube labeled Pb Use a clean medicine dropper to add 10 drops of hydrochloric acid to each of the five test tubes. Bunsen burner 6. Observe the test tubes. In which ones did precipitates form? Any precipitates that form are called chlorides. 7. Indicate the test tubes where precipitates formed and the color of each precipitate in Data Table 1 in the Data Collection and Analysis section. 8. Separate the test tubes containing precipitates from those that did not form precipitates. In Data Table 2, under Contents of heated tubes, write the name of each test tube that formed precipitates on the lines provided. (Some of the information in Data Table 2 is filled out for you. Do not write in the gray spaces. 9. For the water bath, fill the 100-ml beaker with enough water so that most of the test tube is submerged in water, but not so deep that the water spills into the tubes. ring 2010 Walch Education 57

9 Name Date 7. Identifying Unknowns STUDENT ACTIVITY PAGE Safety note: The following steps must be performed under the supervision of your teacher or another adult. Use caution when working with the Bunsen burner. 10. Place the beaker on the ring of the ring stand. Light the Bunsen burner and heat the water. 11. Decant the liquid in each of the three test tubes that formed precipitates by pouring off the liquid that remains after each precipitate forms. Use the stirring rod to keep the chloride precipitate in the test tubes as you pour out the liquid. 12. Wash the graduated cylinder carefully and use it to add 20 ml of distilled water to each of the test tubes containing precipitates. 13. Carefully place these three test tubes into the hot-water bath. Leave them there for 5 minutes. 14. While the test tubes are in the hot-water bath, work with the two test tubes in which no precipitates formed. Use a clean medicine dropper to add 5 drops of ammonium carbonate to each of these tubes. 15. Observe the test tubes. Do precipitates form this time? If so, write down which tube(s and the color of any precipitates in Data Table 1. If no precipitates form, record that also. 16. Turn off the Bunsen burner after 5 minutes have passed. Use the test-tube holder to carefully remove the test tubes from the hot-water bath and place them in the test-tube rack. Let the test tubes cool. 17. After the test tubes have cooled, place a rubber stopper into each one and shake it. Record which precipitates dissolve in hot water in Data Table Use a clean medicine dropper to add 5 drops of 0.1 M potassium chromate to the test tube labeled Pb 2+. Describe the precipitate that forms. 19. Use a clean medicine dropper to add 10 drops of ammonia to each of the remaining two test tubes that you listed in Data Table 2. If a precipitate forms, describe it in Data Table 2. If no precipitates form, record that also. Part 2 For each unknown tested, identify the cation(s present using the methods you learned in Part 1. Record which cations are present and explain how you determined this in Data Table 3 in the Data Collection and Analysis section. Safety note: Always be very careful when handling unknown chemicals. Do not let them touch your skin or eyes. Do not smell unknown chemicals. 1. Obtain the unknown samples from your teacher. Add them to the following test tubes: Pour Unknown 1 into the test tube labeled U1. Pour Unknown 2 into the test tube labeled U2. Pour Unknown 3 into the test tube labeled U3. Pour Unknown 4 into the test tube labeled U4. Pour Unknown 5 into the test tube labeled U Walch Education 58

10 Name Date 7. Identifying Unknowns STUDENT ACTIVITY PAGE 2. Tube U1 contains either Ca 2+ or Na +. Identify which cation is present in the tube and explain your reasoning in Data Table Tube U2 contains either Ag + or Na +. Identify which cation is present in the tube and explain your reasoning. 4. Tube U3 contains either Ag + or Pb 2+. Identify which cation is present in the tube and explain your reasoning. 5. Tube U4 contains either Ag + 2+ or Hg 2. Identify which cation is present in the tube and explain your reasoning. 6. Tube U5 contains two cations. Formulate and explain a plan for identifying the cations present. Identify the cations present. Record their identities and your reasoning in Data Table 3. DATA COLLECTION AND ANALYSIS Part 1 DATA TABLE 1 Cation tested Na + Ca 2+ Ag + 2+ Hg 2 Pb 2+ Does a precipitate form after adding hydrochloric acid (HCl? (Y/N Precipitate color Does a precipitate form after adding ammonium carbonate (NH 4 CO 3? (Y, N, or N/A Precipitate color Key: Y = Yes, N = No, N/A = Not applicable DATA TABLE 2 Contents of heated tubes Pb 2+ Does the precipitate dissolve in hot water? (Y, N, or N/A Describe precipitate after adding potassium chromate (K 2 CrO 4 Result after adding ammonia (NH Walch Education 59

11 Name 7. Identifying Unknowns Date STUDENT ACTIVITY PAGE Part 2 Unknowns U1 Cation(s present DATA TABLE 3 How cation was identified U2 U3 U4 U5 The plan I would use to identify the cations in Unknown 5: q? CONCLUDING QUESTIONS 1. Ag +, Pb 2+ 2+, and Hg 2 ions form precipitates when chloride ions (Cl are added to them. What compounds form? 2010 Walch Education 60

12 Name 7. Identifying Unknowns Date STUDENT ACTIVITY PAGE 2. When you added potassium chromate to the Pb 2+ ions, which ion combined with the Pb 2+ ions? 3. Why was it important to wash the glassware before each use? 4. Write a brief article explaining how you can identify the presence or absence of cations in a solution. 5. How would you verify that a solution does not contain lead ions? FOLLOW-UP ACTIVITIES 1. Devise a plan for identifying the cations in an unknown solution that contains four cations. Write a description of your plan and submit it to your teacher for approval before implementing it. 2. Research and write a report on the use of a spectroscope in identifying unknowns. Present your report to the class. 3. Research and write a report on the use of flame tests to identify cations. Give the report to your teacher. 4. Plan an experiment to test water samples for the presence of cations Walch Education 61