7.2 Ionic Bonds and Ionic Compounds

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7.2 7.2 Ionic Bonds and Ionic Compounds

1 FOCUS You have heard of harvesting Objectives Guide for Reading crops such as wheat or rice—but salt? In many coastal countries that have 7.2.1 Explain the electrical charge of warm, relatively dry climates, salt is produced by the Key Concepts evaporation of seawater. The salty water is chan- an ionic compound. • What is the electrical charge of neled into a series of shallow ponds, where it an ionic compound? 7.2.2 Describe three properties of • What are three properties of becomes more concentrated as the water ionic compounds. ionic compounds? evaporates by exposure to the sun. When the Vocabulary saltwater is concentrated enough, it is Guide for Reading ionic compounds diverted into a pan, on which the sodium ionic bonds chloride crystals deposit. Salt farmers then chemical formula drain the pans and collect the salt into piles to L2 Build Vocabulary formula unit dry. In this section, you will learn how cations Paraphase Have students skim coordination number and anions combine to form stable compounds through the section to locate the mean- Reading Strategy such as sodium chloride. Previewing Before you read ings of the vocabulary terms. Then have this section, rewrite the headings them paraphrase each definition. as how, why, and what questions about ionic compounds. As Formation of Ionic Compounds Reading Strategy L2 you read, write answers to Compounds composed of cations and anions are called ionic compounds. the questions. Ionic compounds are usually composed of metal cations and nonmetal Identify Main Ideas/Details Have anions. For example, sodium chloride, or table salt, is composed of sodium students identify the main idea of each cations and chloride anions. Although they are composed of ions, ionic paragraph of this section. Have them compounds are electrically neutral. The total positive charge of the cations include these ideas in an outline of the equals the total negative charge of the anions. section. Ionic Bonds Anions and cations have opposite charges and attract one another by means of electrostatic forces. The electrostatic forces that hold ions together in ionic compounds are called ionic bonds. 2 INSTRUCT Sodium chloride provides a simple example of how ionic bonds are formed. Consider the reaction between a sodium atom and a chlorine atom. Sodium has a single valence electron that it can easily lose. (If the sodium atom loses its valence electron, it achieves the stable electron con- figuration of neon.) Chlorine has seven valence electrons and can easily Have students examine the section- gain one. (If the chlorine atom gains a valence electron, it achieves the sta- opening photograph and Figure 7.8. Dis- ble electron configuration of argon.) When sodium and chlorine react to cuss how the reactive (and poisonous) form a compound, the sodium atom gives its one valence electron to a elements sodium metal and chlorine gas chlorine atom. Thus sodium and chlorine atoms combine in a one-to-one can combine to form harmless table salt. ratio and both ions have stable octets...... Ask, What characteristics of sodium Na Cl.. . Na .Cl.. . s2 s2 p6 s1 sp2 s2 6 s2 p5 s22s p6 sp2 s2 6 s26p and chlorine atoms allow them to 1 2 2 3 1 2 2 3 3 1 (12 )21* 1 2 2 (13 )31* form the stable compound sodium octet octet Animation 8 Take an chloride, also known as table salt? atomic-level look at the Ne Ar (Sodium atoms can lose an electron easily, formation of KCl. 22 6 sp2 s2 6 s26p 1s (12s)21*p 1 2 2 (13 )31* and chlorine atoms can accept an electron withChemASAP octet octet easily. The resulting ions can combine with the other oppositely charged ions.) 194 Chapter 7 Remind students that NaCl is an example of an ionic compound. Formation of Ionic Section Resources Compounds Print Technology Discuss L2 •Guided Reading and Study Workbook, •Interactive Textbook with ChemASAP, Section 7.2 Animation 8, Simulation 5, Problem- Explain that the formation of positive •Core Teaching Resources, Section 7.2 Solving 7.12, Assessment 7.2 ions and of negative ions are simulta- Review, Interpreting Graphics •Go Online, Section 7.2 neous and interdependent processes. •Transparencies, T79–T81 •Virtual Chemistry Lab, Lab 2 An ionic compound is the result of the •Probeware Laboratory Manual, transfer of electrons from one set of Section 7.2 atoms to another set of atoms. An ionic compound consists entirely of ions.

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Figure 7.7 shows aluminum and bromine reacting to form the com- Discuss L2 pound aluminum bromide. Each aluminum atom has three valence elec- Ask, Why are crystalline ionic com- trons to lose. Each bromine atom has seven valence electrons and readily gains one additional electron. Therefore, when aluminum and bromine pounds generally so rigid and brit- react, three bromine atoms combine with each aluminum atom. tle? (The crystal is rigid because it is held together by a specific three-dimensional Formula Units The ionic compound sodium chloride is composed of equal numbers of sodium cations (Na) and chloride anions (Cl). As you array of relatively strong attractive forces can see in Figure 7.8, the ions in solid sodium chloride are arranged in between anions and cations, which is an orderly pattern. There are no single discrete units, only a continuous accompanied by minimal charge repul- array of ions. sion of like ions. The crystal is brittle Chemists represent the composition of substances by writing chemical Bromine (Br ) Aluminum (Al) because the attractive interactions are formulas. A chemical formula shows the kinds and numbers of atoms in the 2 smallest representative unit of a substance. NaCl, for example, is the chem- specifically arranged within the crystal ical formula for sodium chloride. Note, however, that the formula NaCl structure. If this arrangement is dis- does not represent a single discrete unit. Because an ionic compound exists turbed, as it would be if the crystal were Aluminum bromide (AlBr ) as a collection of positively and negatively charged ions arranged in repeat- 3 hit with a hammer, charge repulsion ing patterns, its chemical formula refers to a ratio known as a formula unit. Figure 7.7 Aluminum metal and between ions of the same charge can formula unit A is the lowest whole-number ratio of ions in an ionic com- the nonmetal bromine react to force the crystal to fragment.) pound. For sodium chloride, the lowest whole-number ratio of the ions is form an ionic solid, aluminum bromide. 1:1 (one Na to each Cl ). Thus the formula unit for sodium chloride is Discuss L2 NaCl. Although ionic charges are used to derive the correct formula, they are not shown when you write the formula unit of the compound. To assess students’ prior knowledge The ionic compound magnesium chloride contains magnesium cat- about ionic bonds and crystals, ask, ions (Mg2 ) and chloride anions (Cl ). In magnesium chloride, the ratio of What is an ionic bond? (an electical 2 magnesium cations to chloride anions is 1:2 (one Mg to two Cl ). So its attraction between ions of opposite formula unit is MgCl2. Because there are twice as many chloride anions (each with a 1 charge) as magnesium cations (each with a 2 charge), the charge) How do a polyatomic ion and compound is electrically neutral. In aluminum bromide, described earlier, Figure 7.8 Sodium cations and a monatomic ion differ? (A mona- the ratio of aluminum cations to bromide ions is 1:3 (one Al3 to three Br chloride anions form a repeating tomic ion is an ion formed from a single ions), so the formula unit is AlBr . three-dimensional array in 3 sodium chloride (NaCl). atom; a polyatomic ion is a stable unit of Checkpoint What is the formula unit for magnesium chloride? Inferring How does the two or more tightly bound atoms that arrangement of ions in a sodium carries a charge.) Why are crystals of chloride crystal help explain why the compound is so stable? different ionic compounds different shapes? (The shapes reflect different Chloride ion (Cl ) geometric arrangements of anions and 18e cations with different sizes and charges.)

17p 18n0

Sodium ion (Na) 10e

11p 12n0

Structures of sodium Arrangement of Na ions Crystals of sodium chloride ion and chloride ion and Cl ions in a crystal of sodium chloride

Section 7.2 Ionic Bonds and Ionic Compounds 195

Differentiated Instruction English Learners L1 Encourage students to look up and define terms used to describe ionic compounds. Answers to... Students should define terms such as crystal Figure 7.8 Each ion in the arrange- and formula unit in English and their native ment is strongly attracted to its language. Emphasize how their strong neighbors and repulsions are bonding arrangement accounts for crystals minimized.

having unique properties. Checkpoint MgCl2

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Section 7.2 (continued) CONCEPTUAL PROBLEM 7.2

CONCEPTUAL PROBLEM 7.2 Predicting Formulas of Ionic Compounds The ionic compound formed from potassium and oxygen is used in ceramic glazes. Use electron dot structures to predict Answers the formulas of the ionic compounds formed from the 12. a. KI following elements. a. potassium and oxygen b. magnesium and nitrogen b. Al2O3 13. CaCl2 Analyze Identify the relevant concepts. Electrons lost now equals electrons gained. L2 Practice Problems Plus Atoms of metals lose their valence electrons The formula of the compound formed (potas- Use electron dot structures to deter- when forming an ionic compound. Atoms of sium oxide) is K2O. mine chemical formulas of the ionic nonmetals gain electrons. Enough atoms of b. Start with the atoms. . . each element must be used in the formula so . . compounds formed when the follow- Mg and N. . that electrons lost equals electrons gained. . ing elements combine: Each nitrogen needs three electrons to have Solve Apply concepts to this situation. a. magnesium and chlorine (MgCl2) an octet, but each magnesium can lose only (Al S ) a. Start with the atoms. two electrons. Thus three magnesium atoms b. aluminum and sulfur 2 3 .. K. and .O. are needed for every two nitrogen atoms. . . 2 In order to have a completely filled valence Mg . Mg .. Properties of Ionic . . . . .3 . N. .N.. . shell, oxygen must gain two electrons. These 2 Mg . Mg .. Compounds . . . . .3 electrons come from two potassium atoms, . N. .N.. . Mg 2 each of which loses one electron. . Mg . The formula of the compound formed (mag- K .. K .. CLASS Activity . . . .2 O. O.. nesium nitride) is Mg3N2. K. K “Hardness” of Water L2 Purpose Students detect the presence Practice Problems of ions in water samples. 12. Use electron dot structures to determine formu- Materials tap water samples, 10-mL las of the ionic compounds formed when a. graduated cylinder, potassium thiocy- potassium reacts with iodine. Problem-Solving 7.12 Solve b. aluminum reacts with oxygen. Problem 12 with the help of an anate solution, dilute ethanoic acid, 13. What is the formula of the ionic compound com- interactive guided tutorial. sodium oxalate solution, dropper posed of calcium cations and chloride anions? withChemASAP Procedure Water “hardness” is based on ions present in the water. Have stu- dents bring water samples from home Properties of Ionic Compounds to test for hardness. Test 2-mL samples Figure 7.9 shows the striking beauty of the crystals of some ionic as follows. Add three drops of potas- compounds. Most ionic compounds are crystalline solids at room sium thiocyanate (KSCN) to the first temperature. The component ions in such crystals are arranged in repeat- ing three-dimensional patterns. The composition of a crystal of sodium sample. Add three drops of dilute etha- chloride is typical. In solid NaCl, each sodium ion is surrounded by six noic acid, CH3COOH, and three drops chloride ions, and each chloride ion is surrounded by six sodium ions. In of sodium oxalate, Na C O , to the sec- For: Links on Ionic this arrangement, each ion is attracted strongly to each of its neighbors and 2 2 4 Compounds repulsions are minimized. The large attractive forces result in a very stable ond sample. Mix well. Visit: www.SciLinks.org Web Code: cdn-1072 structure. This is reflected in the fact that NaCl has a melting point of Expected Outcome A red color from about 800°C. Ionic compounds generally have high melting points. the iron(III) thiocyanate ion, Fe(SCN)2+, indicates the presence of Fe3+ ions. A white precipitate of calcium oxalate, 196 Chapter 7 CaC2O4, indicates the presence of Ca2+ ions. Facts and Figures Do You Have “Soft” Water? Explain that water hardness varies with loca- western states have predominantly soft tion and source. Generally, water from water. Generally, hard water of varying Download a worksheet on Ionic groundwater sources is harder than water degrees is found in the southwestern and Compounds to complete, and find from surface sources. In the United States, midwestern states. additional teacher support on NSTA most northeastern, southern, and north SciLinks.

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Figure 7.9 The beauty of crystalline solids, such as these, CLASS comes from the orderly CLASS Activity arrangement of their component ions. Types of Ionic Compounds L2 Purpose Students investigate proper- ties of different classes of ionic com- pounds. Simulation 5 Simulate the formation of ionic compounds Materials library or Internet access at the atomic level. Procedure Divide the class into Grossularite (Ca Al (SiO ) ) Chem Fluorite (CaF2) 3 2 4 3 with ASAP groups. Have each group choose a dif- ferent class of ionic compounds to research and write about. For example, one group could work with oxides while another group worked with sul- fides. Initially, each student should work alone to discover information such as where the compounds occur in nature, how they are produced, their physical and chemical properties, and any important uses. Finally, students in

Aragonite (CaCO3) Barite (BaSO4) and calcite (CaCO3) Wulfenite (PbMoO4) each group can pool their information to prepare a class display or report. Expected Outcome Students will discover that different classes of ionic compounds share some properties with other ionic compounds and have some unique properties.

CLASS Activity

2 2 3 3 Crystal Structures L2 Beryl (BeAl (SiO ) ) Franklinite ((Zn, Mn , Fe )(Fe , Mn )) Pyrite (FeS2) 2 3 6 Purpose Students observe different types of crystals. Materials crystals of ionic compounds, watch glasses, magnifying glasses Safety Use only nontoxic crystals. Remind students to not touch the crystals. Procedure Pass around crystals of ionic compounds of various types in watch glasses. Have the students

Hematite (Fe2O3) Rutile (TiO2) Cinnabar (HgS) examine the crystals with magnifying glasses and write down their observa- Section 7.2 Ionic Bonds and Ionic Compounds 197 tions. Make a list of these observations and then discuss them in terms of the underlying ionic lattice structures. Expected Outcome Students should Differentiated Instruction observe the different geometries of Gifted and Talented L3 different crystal structures. Have students research how minerals are minerals and how these properties are used in categorized according to their ionic nature. mineral identification. Encourage students to Suggest that their written report include infor- include drawings, photos, or examples of mation concerning the physical properties of minerals from each category.

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Section 7.2 (continued) The coordination number of an ion is the number of ions of opposite charge that surround the ion in a crystal. Figure 7.10a shows the three- dimensional arrangement of ions in NaCl. Because each Na ion is sur- rounded by six Cl ions, Na has a coordination number of 6. Each Cl ion Quick LAB is surrounded by six Na ions and also has a coordination number of 6. Cesium chloride (CsCl) has a formula unit that is similar to that of NaCl. As Figure 7.10b illustrates, both compounds have cubic crystals, but their L2 Solutions Containing Ions internal crystal structures are different. Each Cs ion is surrounded by Objective After completing this activ- eight Cl ions, and each Cl ion is surrounded by eight Cs ions. The anion ity, students will be able to: and cation in cesium chloride each have a coordination number of 8. Cl a Sodium chloride Figure 7.10c shows the crystalline form of titanium dioxide (TiO2), also show that ions in solution conduct an • (NaCl) Na known as rutile. In this compound, the coordination number for the cation electric current. (Ti4) is 6. Each Ti4 ion is surrounded by six O2 ions. The coordination number of the anion (O2) is 3. Each O2 ion is surrounded by three Ti4 ions. Another characteristic property of ionic compounds has to do with conductivity. Ionic compounds can conduct an electric current when melted or dissolved in water. As Figure 7.11 shows, when sodium chloride is melted, the orderly crystal structure breaks down. If a voltage is applied Students may think that the solutions across this molten mass, cations migrate freely to one electrode and anions contain only one ion. Clarify that each migrate to the other. This ion movement allows electricity to flow between solution contains both cations and the electrodes through an external wire. For a similar reason, ionic com- pounds also conduct electricity if they are dissolved in water. When dis- anions. solved, the ions are free to move about in the aqueous solution. b Cesium chloride Cl Skills Focus Observing, experimenting, (CsCl) Cs Checkpoint What is the coordination number of Ti4+ in TiO ? concluding 2

Prep Time 20 minutes Power source Materials D-cell batteries, masking Flow of Current meter electrons Flow of tape, 30-cm lengths of bell wire with electrons ends scraped bare, clear plastic cups, Molten salt distilled water, tap water, vinegar, Inert metal (801°C —1412°C) electrode Inert metal sucrose, sodium chloride, baking soda, (cathode) electrode conductivity probe (optional) 2 c Rutile (TiO2) O (anode) Class Time 30 minutes Ti4 Safety Students should handle wires Figure 7.10 Sodium chloride Cl with caution. The wires may become and cesium chloride form cubic hot during the activity. crystals. a In NaCl, each ion has Na a coordination number of 6. Expected Outcome When ions are b In CsCl, each ion has a present in solution, the solution con- coordination number of 8. ducts an electric current. c Titanium dioxide forms tetragonal crystals. In TiO2, each Ti4 ion has a coordination Analyze and Conclude number of 6, while each O2 ion Figure 7.11 When sodium chloride melts, the sodium and chloride 1. Solutions of vinegar, sodium chloride, has a coordination number of 3. ions are free to move throughout the molten salt. If a voltage is and baking soda (and maybe tap water) applied, positive sodium ions move to the negative electrode (the contain ions and therefore conduct elec- cathode), and negative chloride ions move to the positive electrode (the anode). Predicting What would happen if the voltage was tric current and produce bubbles. applied across a solution of NaCl dissolved in water? 2. Distilled water and sugar solution (and maybe tap water) do not contain 198 Chapter 7 ions and therefore do not conduct an electric current or produce bubbles. 3. Answers will vary but should indi- cate that a larger number of batteries Differentiated Instruction will increase the current, which will, in produced and check them for the pres- L3 turn, cause the rate at which the bub- ence of hydrogen and oxygen. Hydro- Gifted and Talented bles appear to increase. gen will burn when a lit splint is placed Have students write the formulas for ionic in it, and a glowing split will begin to compounds formed from selected pairs of For Enrichment L3 flame when placed in oxygen. cations and anions. Include various poly- Ask, What gases form the bubbles atomic cations and anions as well. you observe? (hydrogen and oxygen) Examples: + 2− What is the source of these gases? 1) K and S (K2S) 2+ 3− (water) Have students collect the gases 2) Ca and HCO (Ca(HCO3)2)

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Quick LAB Discuss L2 Solutions Containing Ions Point out that the rusting of iron is the production of iron oxide from iron Purpose Procedure metal and oxygen gas. Point out that 3+ To show that ions in solu- Probe version available in the Fe is a stable cation of Fe. (Students − tion conduct an electric Probeware Lab Manual. should know that O2 is the stable current. 1. Tape the batteries together so the pos- anion of O.) The compound formed itive end of one touches the negative Materials end of another. Tape the bare end of from these ions is iron(III) oxide, or 3 D-cell batteries • one wire to the positive terminal of the Fe2O3. • masking tape battery assembly and the bare end of • 2 30-cm lengths of bell the other wire to the negative terminal. 3 wire with ends scraped CAUTION Bare wire ends can be sharp ASSESS bare and scratch skin. Handle with care. clear plastic cup • 2. Half fill the cup with distilled water. Evaluate Understanding L2 distilled water Analyze and Conclude • Hold the bare ends of the wires close Name ionic compounds and ask stu- tap water together in the water. Look for the pro- 1. Which solutions produced bubbles of • dents to identify the cation, anion, and vinegar duction of bubbles. They are a sign that gas? Explain. • ratio of cations to anions in each • sucrose the solution conducts electric current. 2. Which samples did not produce bub- bles of gas? Explain. compound. • sodium chloride 3. Repeat Step 2 with tap water, vinegar, baking soda and concentrated solutions of sucrose, 3. Would you expect the same results if • sodium chloride, and baking soda Reteach L1 • conductivity probe you used only one battery? If you used (optional) (sodium hydrogen carbonate). six batteries? Explain your answer. Emphasize that an ionic solid is a col- lection of independent ions. There is no joining of individual particles to form molecules. Each ion “belongs” as 7.2 Section Assessment much to one of its nearest neighbors as it belongs to any other. The arrange- 14. Key Concept How can you describe the elec- 21. Describe the arrangement of sodium ions and ment in an ionic crystal is such that trical charge of an ionic compound? chloride ions in a crystal of sodium chloride. each ion is surrounded by ions of 15. Key Concept What properties characterize 22. Why do ionic compounds conduct electric opposite charge, which produces a ionic compounds? current when they are melted or dissolved in strong bonding force. 16. Define an ionic bond. water? 17. How can you represent the composition of an ionic compound? Handbook Elements Handbook 18. Write the correct chemical formula for the com- Restoring Electrolytes Read about restoring elec- Na+ 1s22s22p6 pounds formed from each pair of ions. trolytes on page R8. Write electron configurations + 2 2 6 2 6 a. K, S2 b. Ca2, O2 for the two principal ions found in body fluids. K 1s 2s 2p 3s 3p c. Na, O2 d. Al3, N3 19. Write formulas for each compound. a. barium chloride b. magnesium oxide c. lithium oxide d. calcium fluoride 20. Which pairs of elements are likely to form If your class subscribes to the ionic compounds? Assessment 7.2 Test yourself on the concepts in Section 7.2. Interactive Textbook, use it to a. Cl, Br b. Li, Cl ChemASAP review key concepts in Section 7.2. c. K, He d. I, Na with with ChemASAP

Section 7.2 Ionic Bonds and Ionic Compounds 199

Section 7.2 Assessment Answers to... 14. electrically neutral 20. b and d Figure 7.11 The solution would + 15. usually solids at room temperature; have 21. Acceptable answers should describe a conduct an electric current; the Na high melting points; conduct an electric solid containing positive sodium ions ions in solution would migrate to – current when melted or dissolved in water and negative chloride ions in an the negative electrode, and the Cl 16. electrostatic forces that hold ions alternating, regular, and repeating ions in solution would migrate to together in an ionic compound three-dimensional pattern. the positive electrode.

17. by writing its chemical formula 22. The ions are free to move. Checkpoint 4+ 18. a. K2S b. CaO c. Na2O d. AlN Ti has a coor- 19. a. BaCl2 b. MgO c. Li2O d. CaF2 dination number of 6 in TiO2.

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Small-Scale LAB Small-Scale LAB Analysis of Anions and Cations Analysis of Anions and Cations L2 Purpose To develop tests for various ions and use the tests to Objective analyze unknown substances. After completing this activity, students Materials will be able to: • pencil • paper •develop tests for various ions. • ruler • reaction surface •use the tests to analyze unknown • medicine droppers • pipet • chemicals shown in solutions. Figures A and B Skills Focus Observing, drawing conclusions, designing experiments Na2 4 HNO3 Na3 4 (؊) (NO ؊) ( ؊ ) 4 3 4 Procedure Prep Time 40 minutes On one sheet of paper, draw grids similar to Figure A and Figure B. Draw similar grids on a second sheet of paper. Materials pencil, paper, ruler, reaction AgNO3 surface, medicine droppers, pipet, Make each square 2 cm on each side. Place a reaction surface over the grids on one of the sheets of paper and staples or solid Fe, solutions from add one drop of each solution or one piece of each solid Figures A & B. HCl plus as shown in Figures A and B. Stir each solution by blowing 1 piece air through an empty pipet. Use the grids on the second of Fe(s) Solution Preparation sheet of paper as a data table to record your observations for each solution. 0.05M AgNO3 2.1 g in 250 mL Analyze Pb(NO ) 0.2M Pb(NO3)2 16.6 g in 250 mL 3 2 Using your experimental data, record the answers to the following questions below your data table. 0.2M Na2SO4 7.1 g in 250 mL 1. Carefully examine the reaction of Fe(s) and HCl in the 0.1M Na PO 9.5 g Na PO •12H O Figure A 3 4 3 4 2 Anion Analysis presence of HNO3. What is unique about this reaction? in 250 mL How can you use it to identify nitrate ion? 0.5M NaOH 20.0 g in 1.0 L 2. Which solutions from Figure A are the best for identify- ing each anion? Which solutions from Figure B are the 0.1M KSCN 2.4 g in 250 mL KI CaCl2 FeCl3 best for identifying each cation? Explain. ؉ ؉ ؉ (K ) (Ca2 ) (Fe3 ) 0.1M KI 4.2 g in 250 mL 3. Can your experiments identify K ions? Explain. 0.5M CaCl 13.9 g in 250 mL You’re the Chemist 2 NaOH The following small-scale activities allow you to develop 0.1M FeCl3 6.8 g FeCl3•6H2O in your own procedures and analyze the results. 25 mL of 1.0M NaCl; 1. Design It! Obtain a set of unknown anion solutions dilute to 250 mL from your teacher and design and carry out a series of KSCN tests that will identify each anion. 1.0M HCl 82 mL of 12M in 1.0 L 2. Design It! Obtain a set of unknown cation solutions 1.0M HNO 63 mL of 15.8M from your teacher and design and carry out a series of 3 tests that will identify each cation. in 1.0 L Figure B Cation Analysis 3. Design It! Obtain a set of unknown solid ionic com- pounds from your teacher. Design and carry out a CAUTION! Always add acid to water series of tests that will identify each ion present. carefully and slowly. Class Time 40 minutes + 3− 200 Chapter 7 Expected Outcome Ag and PO4 form a yellow precipitate; Fe in HCl − turns yellow with NO ; Pb2+ forms 3 − white precipitates with SO 2 and − − 4 3. No, neither of the solutions produced a L3 PO 3 ; OH and Ca2+ form a white For Enrichment 4 visible product. product; KSCN and Fe3+ form a Have students hypothesize about why none of red one. You’re the Chemist the tests in the lab identified the potassium ion. Ask, Why do you think potassium ions don’t Analyze 1. Mix a drop of each unknown with one drop of each solution from figure A. Compare the form precipitates with any of the anions 1. Nitrate ion is the only ion that pro- results to those with known solutions. used in the lab? (All potassium compounds are duces a yellow color with iron in the 2. Mix a drop of each unknown with one drop soluble in water.) Have students read through presence of acid. − of both solutions from Figure B. Compare the the Quick Lab on page 142. Have them use this 2. AgNO for PO 3 ; Fe and HCl for − 3 4 − results to those with known solutions. information to design an experiment that will NO ; Pb(NO ) for SO 2 ; NaOH 3 3 2 4 3. Mix an unknown with one drop of each solu- provide a positive test for the potassium ion. for Ca2+; and KSCN for Fe3+. tion from Figures A and B and compare The flame test for potassium produces a violet results. color. 200 Chapter 7