LESSON 15.3 Key Objectives 15.3.1 DISTINGUISH between a suspension and a solution. 15.3.2 IDENTIFY how to distinguish a colloid from a suspension and a solution. Additional Resources Reading and Study Workbook, Lesson 15.3 Core Teaching Resources, Lesson 15.3 Review Small-Scale Chemistry Laboratory Manual, Lab 24 Engage Have students study the photograph and read the text. Comment that the atmosphere is referred to as a homogeneous mixture called a colloid. Ask What does that tell you about the composition of the atmosphere? (Its composition is not uniform throughout.) Ask If a colloid such as the atmosphere differs from a solution with respect to the size of its particles, would you expect a colloid to have smaller or larger particles than a solution? (larger) Activate Prior Knowledge Review solutions and the solution process. Ask students to explain why the particles in a homogeneous aqueous system can pass through a filter, and have them predict whether the same can happen when filtering heterogeneous systems. National Science Education Standards E-2 Foundations for Reading BUILD VOCABULARY Have students make a Venn diagram using the following vocabulary terms: solution, suspension, colloid, Tyndall effect, Brownian motion. 15.3 Heterogeneous Aqueous Systems Key Questions What is the difference between a suspension and a solution? What distinguishes a colloid from a suspension and a solution? Vocabulary Figure 15.16 Solutions and Suspensions A solution is a homogeneous mixture. A suspension is a heterogeneous mixture. a. The small size of the solute particles in a solution allows them to pass through filter paper. b. The suspended particles of a suspension can be removed by filtration. 504 Focus on ELL Q: Why are some sunsets red? Have you ever wondered what causes the red glow of the evening sky? The atmosphere contains particles of water and dust. As sunlight passes through the particles, it is scattered. However, not all wavelengths are scattered to the same extent. The shorter wavelengths of visible light (blue and green) are scattered more than the longer wavelengths (red and orange). At sunrise and sunset, the longer wavelengths are more visible because the sun s light travels through more of Earth s atmosphere. Suspensions What is the difference between a suspension and a solution? So far in this chapter, you have learned about aqueous solutions, which are homogeneous mixtures. In contrast, heterogeneous mixtures are not solutions. If you shake a container containing a piece of clay with water, the clay breaks into fine particles. The water becomes cloudy because the clay particles are suspended in the water. If you stop shaking, the particles settle out. A suspension is a mixture from which particles settle out upon standing. A suspension differs from a solution because the particles of a suspension are much larger and do not stay suspended indefinitely. The particles in a typical suspension have an average diameter greater than 1000 nm. By contrast, the particle size in a solution is usually about 1 nm. Suspensions are heterogeneous because at least two substances can be clearly identified. In the example of clay particles mixed with water, you can clearly see the dispersed phase (clay) in the dispersion medium (water). Figure 15.16 shows how the difference between a solution and suspension is easily seen when each type of mixture is filtered. a b 1 CONTENT AND LANGUAGE Have students use a dictionary to look up the various uses of the word disperse or dispersion. Ask students to explain how the seeds of a plant being dispersed by the wind are related to the dispersed phase and dispersion medium of a colloid. 2 FRONTLOAD THE LESSON Provide students with a partially completed comparecontrast table for heterogeneous solutions, and ask them to complete the table as they read the lesson. Students should then use the self-questioning strategy to note the similarities and differences between the solutions. 504 Chapter 15 Lesson 3 3 COMPREHENSIBLE INPUT Provide small groups of students samples of saltwater, oil-and-vinegar salad dressing, mayonnaise, and grape jelly to compare macroscopically and microscopically. Assign students the online Concepts in Action activity on emulsions.
Colloids What distinguishes a colloid from a suspension and a solution? Gelatin is an example of a type of mixture called a colloid. A colloid is a heterogeneous mixture containing particles that range in size from 1 nm to 1000 nm. The particles are spread, or dispersed, throughout the dispersion medium, which can be a solid, liquid, or a gas. The first substances to be identified as colloids were glues. Other colloids include such mixtures as paint, aerosol sprays, and smoke. Table 15.3 lists some common colloidal systems and gives examples of familiar colloids. How do the properties of colloids differ from those of suspensions and solutions? Many colloids are cloudy or milky in appearance, like suspensions, when they are concentrated. Colloids may look clear or almost clear, like solutions, when they are dilute. The important difference between colloids and solutions and suspensions is in the size of the particles. Colloids have particles smaller than those in suspensions and larger than those in solutions. These intermediate-sized particles cannot be retained by filter paper as are the larger particles of a suspension, and they do not settle out with time. Colloids can be distinguished by a phenomenon called the Tyndall effect and by the observation of Brownian motion. They are also subject to coagulation or clumping together, and they can be emulsified or made stable. Table 15.3 Dispersed phase System Some Colloidal Systems Dispersion medium Type Example Gas Liquid Foam Whipped cream Gas Solid Foam Marshmallow Liquid Liquid Emulsion Milk, mayonnaise Liquid Gas Aerosol Fog, aerosol Solid Gas Smoke Dust in air Solid Liquid Sols, gels Egg white, jelly, paint, blood, starch in water, gelatin Understanding by Design Water and Aqueous Systems 505 What is the difference between a suspension and a solution? Assess students knowledge about heterogeneous aqueous systems by asking them to explain how to tell if a sample of an aqueous system is homogeneous or heterogeneous. ADJUST INSTRUCTION If students are having difficulty distinguishing between the two systems, have them review the Solutions section in Lesson 15.2 and the Suspensions section in Lesson 15.3. Then have students revise their answers. READING STRATEGY Before students begin to read this lesson, have them preview the pages and predict how the properties of suspensions and colloids differ from those of solutions. Then, as they read, have them correct any misconceptions they may have. Explain Suspensions APPLY CONCEPTS Explain to students that municipal wastewater treatment plants take advantage of the properties of suspensions during the water treatment process. Point out that sewage is a suspension rather than a solution. This allows treatment facilities to use sedimentation to remove larger particles. Then, lime and alum are added to help coagulate smaller particles before the remaining liquid is filtered through sandy loams (diatomaceous earth). This process removes the remaining suspended particles. The diatomaceous earth may also absorb colloidal material. Colloids USE VISUALS Display Table 15.3 on an overhead projector. Explain that colloids are characterized by the physical state of the dispersed phase and of the continuous phase also called the dispersion medium. Ask What is the dispersion medium in fog? (air) Ask What is dispersed in fog? (water droplets) Help students understand this two-part system by comparing it to a solution, which, instead of a dispersed phase and a dispersion medium, consists of a solute and a solvent. Extend Explain that homogenized milk is a colloid of protein and fat. Curdling occurs when bacteria produce enough lactic acid to cause dispersed protein and fat particles to coagulate into larger particles, which then separate from the rest of the mixture. Have students write an analogy to explain how this process is comparable to the clotting of blood. LESSON 15.3 Water and Aqueous Systems 505
LESSON 15.3 Explain START A CONVERSATION Point out that the main difference between solutions, suspensions, and colloids is particle size. Solution particles are typically less than 1 nm in diameter. Colloid particles are between 1 nm and 1000 nm in diameter. Suspension particles are typically larger than 1000 nm. Smaller particles are less susceptible to the effects of gravity and are influenced more by the effects of Brownian motion. The collisions of molecules with extremely small colloidal particles are sufficiently energetic to move colloidal particles in a random fashion that prevents their settling to the bottom. Explore The Tyndall Effect Teacher Demo PURPOSE Students will observe the Tyndall effect. MATERIALS whole milk, water, beaker, stirring rod, projector or laser pointer PROCEDURE Add a small amount of whole milk to some water in a beaker, and stir to mix. In a darkened room, shine a light from a projector or a laser pointer through the beaker. SAFETY If a laser pointer is used, warn students not to look directly at it. EXPECTED OUTCOME When viewed from the side, the path of the light beam is observed. This is the Tyndall effect. Students also can observe Brownian motion of airborne dust particles in the light beam. You will have a better chance of seeing a red sunset if the air contains a lot of water and dust particles. That way more of the shorter wavelengths of light will be scattered from the sunlight leaving the the sky with a reddish appearance. a b Flashlight Solution Colloid Suspension Figure 15.17 Scattering of Light The path of light is visible only when the light is scattered by particles. a. Fog or mist is a colloid and thus exhibits the Tyndall effect. b. Solutions do not scatter light. Particles in colloids and suspensions reflect or scatter light in all directions. Explain Why is it easier to see the light beam of an automobile on a foggy night than on a clear night? Q: What would be the ideal conditions to see a red sunset? 506 The Tyndall Effect Ordinarily you cannot see a beam of sunlight unless the light passes through particles of water (mist) or dust in the air. These particles scatter the sunlight. Similarly, a beam of light is visible as it passes through a colloid. The scattering of visible light by colloidal particles is called the Tyndall effect. Suspensions also exhibit the Tyndall effect. Solutions do not exhibit the Tyndall effect. The particles in solutions are too small to scatter light. Figure 15.17 shows how the Tyndall effect can differentiate solutions from colloids and suspensions. Brownian Motion Flashes of light, or scintillations, are seen when colloids are studied under a microscope. Colloids scintillate because the particles reflecting and scattering the light move erratically. The chaotic movement of colloidal particles, which was first observed by the Scottish botanist Robert Brown (1773 1858), is called Brownian motion. Brownian motion is caused by collisions of the molecules of the dispersion medium with the small, dispersed colloidal particles. These collisions help prevent the colloidal particles from settling. Coagulation Colloidal particles also tend to stay suspended because they become charged by adsorbing ions from the dispersing medium onto their surface. Adsorption means to adhere to a surface. Some colloidal particles become positively charged by adsorbing positively charged ions. Other colloidal particles become negatively charged by adsorbing negatively charged ions. All the colloidal particles in a particular colloidal system will have the same charge, although the colloidal system is neutral. The repulsion between the like-charged particles prevents the particles from forming heavier aggregates that would have a greater tendency to settle out. Thus, a colloidal system can be destroyed or coagulated by the addition of electrolytes. The added ions neutralize the charged colloidal particles. The particles can clump together to form heavier aggregates and settle out from the dispersion. Paint Students may be interested in knowing why paint must be stirred before it is used. Explain that latex paint is a complex mixture of binders, pigments, and drying agents. Some ingredients are water soluble; others are present as dispersed particles. For uniform color and composition, the paint must be stirred well before being used. 506 Chapter 15 Lesson 3
Emulsions Mayonnaise is an example of a colloidal system called an emulsion. An emulsion is a colloidal dispersion of a liquid in a liquid. An emulsifying agent is essential for the formation of an emulsion and for maintaining the emulsion s stability. For example, oils and greases are not soluble in water. However, oils and greases readily form a colloidal dispersion if soap or detergent is added to the water. Soaps and detergents are emulsifying agents. One end of a large soap or detergent molecule is polar and is attracted to water molecules. The other end of the soap or detergent molecule is nonpolar and is soluble in oil or grease. Soaps and other emulsifying agents thus allow the formation of colloidal dispersions between liquids that do not ordinarily mix. Mayonnaise is a heterogeneous mixture of oil and vinegar. Such a mixture would quickly separate without the presence of egg yolk, which is the emulsifying agent. Other foods such as milk, margarine, and butter are also emulsions. Cosmetics, shampoos, and lotions are formulated with emulsifiers to maintain consistent quality. Table 15.4 summarizes the properties of solutions, colloids, and suspensions. Table 15.4 Property Particle type Properties of Solutions, Colloids, and Suspensions System Solution Colloid Suspension Ions, atoms, small molecules Large molecules or particles Large particles or aggregates Particle size 0.1 1 nm 1 1000 nm 1000 nm and larger Effect of light No scattering Exhibits Tyndall effect Exhibits Tyndall effect Effect of gravity Filtration Stable, does not separate Particles not retained Stable, does not separate Particles not retained Go online to see why oil and water don t mix. Unstable, sediment forms Particles retained Uniformity Homogeneous Heterogeneous Heterogeneous I N CONCEPTS I O N A C T Evaluate Informal Assessment Ask In what way are colloids similar to solutions? (In both, dispersed particles are small enough to pass through standard filter paper and to withstand the pull of gravity.) Ask In what way are colloids similar to suspensions? (Both types of mixtures produce the Tyndall effect.) Then have students complete the 15.3 Lesson Check. Reteach On the board, draw the relative sizes of solute, colloid, and suspension particles by making an analogy to golf balls, baseballs, and basketballs. Ask students to identify which particles would get caught in a sieve with holes a bit smaller than a basketball hoop and which particles would pass through. Remind students that solution particles are typically less than 1 nm in diameter, whereas colloid particles are ten to one thousand times as large and those of suspensions exceed the largest colloid particles. LESSON 15.3 E ONLINE O P R O B L E M S R O M 15.3 LessonCheck 18. Describe How does a suspension differ from a solution? 19. Explain What distinguishes a colloid from a suspension and a solution? 20. Apply Concepts How can you determine through observation that a mixture is a suspension? 21. Explain Could you separate a colloid by filtering? Explain. 22. Infer How can the Tyndall effect be used to distinguish between a colloid and a solution? 23. Relate Cause and Effect Can the presence of Brownian motion distinguish between a solution and a colloid? Explain. Water and Aqueous Systems 507 Lesson Check Answers 18. The particles of a suspension are much larger and do not stay suspended indefinitely. 19. Colloids have particles smaller than those in suspensions and larger than those in solutions. 20. The particles in a suspension will settle out over time. 21. Particles in a colloid such as gelatin are smaller than the holes in filter paper and cannot be removed by filtering. 22. A beam of light is visible as it passes through a colloid; a beam of light is invisible as it passes through a solution. 23. Flashes of light, or scintillations, are seen when colloids are studied under a microscope. Colloids scintillate because the particles move erratically. The particles in a solution are too small to be seen under a microscope and do not cause scintillations. Answers FIGURE 5.17 Fog is a colloid that produces the Tyndall effect. Bright lights produce a higher degree of light scattering in all directions, including straight back into the driver s eyes. Water and Aqueous Systems 507
SMALL-SCALE LAB Small-Scale Lab OBJECTIVE After completing this activity, students will be able to classify compounds as electrolytes or nonelectrolytes by testing them for conductivity. ADDRESS MISCONCEPTIONS Students often think that a substance will conduct electricity if it dissolves in water. Tell students that some covalent compounds dissolve in water but do not conduct electricity. PREP TIME 45 minutes CLASS TIME 20 minutes ADVANCE PREPARATION Make up reagents as follows: Solution Preparation 1.0M HCl 82 ml of 12M in 1.0 L 1.0M HNO 3 63 ml of 15.8M in 1.0 L 1.0M H 2 SO 4 56 ml of 18M in 1.0 L CAUTION: Always add acid to water. 1.0M NH 3 67 ml of 15M in 1.0 L CAUTION: Using proper ventilation, add ammonia to water. CH 3 COOH Use white vinegar. 0.5M NaOH 20.0 g in 1.0 L Obtain various liquids such as rubbing alcohol, distilled water, soft drinks, orange juice, pickle juice, and coffee. SAFETY Be sure students wear goggles and aprons. TEACHING TIP Ask students to bring in small samples of orange juice and other liquid foods. EXPECTED OUTCOME None of the compounds conduct electricity when in the solid state. Aqueous solutions of NaCl, MgSO 4, NaH, KCl, and KI conduct electricity and are thus electrolytes. Cornstarch and table sugar do not conduct electricity and are nonelectrolytes. ANALYZE AND CONCLUDE 1. Electrolytes: NaCl, MgSO 4, NaH, KCl, KI; nonelectrolytes: table sugar, cornstarch 2. no, because the ions are locked in a crystal lattice and cannot move 3. Table sugar and cornstarch are covalent compounds. NaCl, MgSO 4, NaH, KCl, and KI are ionic compounds. An electrolyte is a compound that dissociates into ions in solution. 4. It must conduct electricity. YOU RE THE CHEMIST 1. NaH Na (aq) + H (aq) KCl K (aq) + Cl (aq) MgSO 4 Mg 2 (aq) + SO 4 2 (aq) KI K (aq) + I (aq) Diagrams should be similar to Figure 15.9. 2. Test each solution. Strong electrolytes: HCl, H 2 SO 4, HNO 3, NaOH; Weak electrolytes: CH 3 COOH, NH 3 ; Nonelectrolytes: rubbing alcohol, distilled water 3. Strong electrolytes: soft drinks, pickle juice Weak electrolytes: orange juice, coffee VIRTUAL L A B Small-Scale Lab Electrolytes Purpose To classify compounds as electrolytes by testing their conductivity in aqueous solution Materials reaction surface chemicals shown in the grid below conductivity tester Procedure 1. On separate sheets of paper, draw two grids similar to the one below. Make each square 2 cm on each side. 2. Place a reaction surface over one of the grids and place a few grains of each solid in the indicated places. 3. Test each solid for conductivity. 4. Add 1 drop of water to each solid and test the wet mixture for conductivity. Be sure to clean and dry the conductivity leads between each test. NaCl Na 2 NaH KCl MgSO 4 Table sugar C 12 H 22 O 11 Cornstarch (C 6 H 10 O 5 )n KI 508 Focus on ELL micropipet or dropper water Analyze and Conclude 1. Infer Which compounds in your table are electrolytes? Which are not electrolytes? 2. Observe Do any of these electrolytes conduct electric current in the solid form? Explain. 3. Classify Identify each compound in the grid as ionic or covalent. 4. Draw Conclusions For a compound to be an electrolyte, what must happen when it dissolves in water? You re the Chemist 1. Analyze Data When an ionic solid dissolves in water, water molecules attract the ions, causing them to come apart, or dissociate. The resulting dissolved ions are electrically charged particles that allow the solution to conduct electric current. The following chemical equations represent this phenomenon. H NaCl 2O Na (aq) Cl (aq) H Na 2 2O 2Na (aq) CO 2 3 (aq) Write a similar chemical equation for each electrolyte you tested. Draw diagrams to explain how the ions conduct electric current. 2. Design an Experiment Obtain the following aqueous solutions: HCl, H 2 SO 4, HNO 3, C 2 H 4 O 2, NH 3, NaOH, rubbing alcohol, and distilled water. Design and carry out an experiment to test their conductivities. Use your data to classify each substance as a strong electrolyte, weak electrolyte, or nonelectrolyte. 3. Design an Experiment Test various liquids for conductivity. Try soft drinks, orange juice, pickle juice, and coffee. Which liquids are electrolytes? 4 LANGUAGE PRODUCTION Have students work in groups of four to complete the lab. Make sure each group has ELLs of varied language proficiencies so that more proficient students can help less proficient ones. Have students work according to their proficiency level. BEGINNING: LOW/HIGH Provide students with a checklist for each solid. For example, if the solid is conductive, they should check yes. INTERMEDIATE: LOW/HIGH Break down each step in the procedure into smaller steps with fewer words. ADVANCED: LOW/HIGH Have students help classmates of lower English proficiencies with following the procedures and drawing conclusions. 508 Chapter 15 Small-Scale Lab