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15.3 Heterogeneous Aqueous Systems 15.3 chem_TE_ch15_PPL.fm Page 459 Wednesday, August 4, 2004 5:53 AM 15.3 Heterogeneous Aqueous Systems 15.3 1 FOCUS Connecting to Your World It wiggles and jiggles. It comes Guide for Reading in many colors and flavors. When you pop it in your mouth, it dissolves. It Key Concepts Objectives is gelatin, one of the most popular desserts in the United States. In fact, • What is the difference between 15.3.1 Distinguish between a suspen- more than a million packages of gelatin are purchased a suspension and a solution? or eaten every day. Gelatin has even traveled into • What distinguishes a colloid sion and a solution. space. In 1996, American astronaut Shannon from a suspension and a 15.3.2 Identify the distinguishing solution? Lucid shared a gelatin dessert with her characteristic of a colloid. Russian crewmates. Gelatin is a heteroge- Vocabulary neous mixture called a colloid. In this section, suspension colloid Guide for Reading you will learn more about the characteristics Tyndall effect of colloids and a related mixture called a Brownian motion Build Vocabulary L2 suspension. emulsion Reading Strategy Venn Diagram Have students make a Previewing Before you read, Venn diagram using the following rewrite the headings of this sec- vocabulary terms: solution, suspension, tion as questions. As you read, colloid, Tyndall effect, Brownian motion. Suspensions write answers to your questions. So far in this chapter, you have learned about homogeneous mixtures that Reading Strategy L2 are formed when compounds such as inorganic acids, bases, and ionic salts Predict Before students begin to read mix with water. These mixtures are classified as solutions. In contrast, het- this chapter, have them preview the erogeneous mixtures are not solutions. If you shake a piece of clay with water, the clay breaks into fine parti- section and predict how the properties cles. The water becomes cloudy because the clay particles are suspended in of suspensions and colloids differ from the water. But if you stop shaking, the particles begin to settle out. A those of solutions. Then, as they read, suspension is a mixture from which particles settle out upon standing. have them correct any misconceptions A suspension differs from a solution because the particles of a suspen- they may have. sion 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. The larger size 2 INSTRUCT of suspended particles means that gravity plays a larger role in causing them to settle out of the mixture. Cooks use suspensions of flour or corn- starch in water to thicken sauces and gravies. These mixtures must be shaken or stirred immediately before use because the suspended particles quickly settle out. Have students study the photograph Suspensions are heterogeneous because at least two substances can be and read the text. Comment that gela- clearly identified. In the example of clay particles mixed with water, you tin is referred to as a heterogeneous can clearly see the dispersed phase (clay) in the dispersion medium mixture called a colloid. Ask, What (water). Figure 15.14 shows that if muddy water is filtered, the filter traps does that tell you about the compo- the suspended clay particles and clear water passes through. sition of gelatin? (Its composition is Figure 15.14 A suspension is a heterogeneous mixture. not uniform throughout.) If a colloid Suspended particles can be removed by filtration. Comparing such as gelatin differs from a solu- and Contrasting How does the filtration of a suspension compare with the filtration of a solution? tion with respect to the size of its particles, would you expect a colloid Section 15.3 Heterogeneous Aqueous Systems 459 to have smaller or larger particles than a solution? (larger) Section Resources Print Technology • Guided Reading and Study Workbook, • Interactive Textbook with ChemASAP, Section 15.3 Assessment 15.3 • Core Teaching Resources, Section 15.3 • Go Online, Section 15.3 Review • Transparencies, T166–T168 Answers to... • Small-Scale Chemistry Laboratory Figure 15.14 When a solution is fil- Manual, Lab 24 tered, nothing is removed. When a suspension is filtered, suspended particles are removed. Water and Aqueous Systems 459 chem_TE_ch15_PPL.fm Page 460 Wednesday, August 4, 2004 5:53 AM Section 15.3 (continued) Word Origins Colloids You read in Connecting to Your World that gelatin is a type of mixture called Colloid comes from the a colloid. A colloid is a heterogeneous mixture containing particles that Suspensions Greek word kolla meaning range in size from 1 nm to 1000 nm. The particles are spread throughout Discuss L1 “glue.” The word colloid is the dispersion medium, which can be a solid, liquid, or gas. The first sub- based on the glue-like stances to be identified as colloids were glues. Other colloids include such Refer students to the Career feature on appearance of many col- mixtures as gelatin, paint, aerosol sprays, and smoke. Table 15.3 lists some wastewater treatment. Explain that loids and was coined by the common colloidal systems and gives examples of familiar colloids. sedimentation and filtration play English scientist Thomas How do the properties of colloids differ from those of suspensions and important roles in the treatment pro- Graham in 1861. As you solutions? Like suspensions, many colloids are cloudy or milky in appear- cess. First, large particles are removed read, identify examples of colloids that meet Gra- ance when they are concentrated. Like solutions, colloids may look clear from the water in large settling tanks ham’s original definition. or almost clear when they are dilute. The important difference between by sedimentation. Lime and alum are colloids and solutions and suspensions is in the size of the particles. added to help coagulate the particles. Colloids have particles smaller than those in suspensions and larger than Filtration through sandy loams (diato- 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 maceous earth) then removes the sus- out with time. Colloids can be distinguished by the Tyndall effect and by pended matter and may absorb the observation of Brownian motion. They are also subject to coagulation colloidal material as well. or clumping together, and they can be emulsified or made stable. Colloids Table 15.3 Some Colloidal Systems L2 Word Origins System Egg white, jellies, and other sols and Dispersed Dispersion gels often have a gluelike appearance. phase medium Type Example gas liquid foam whipped Use Visuals L1 cream Table 15.3 Display Table 15.3 on an overhead projector. Explain that col- gas solid foam marshmallow loids are characterized by the physical state of the dispersed phase and of the continuous phase—also called disper- liquid liquid emulsion milk, sion medium. Ask, What is the disper- mayonnaise sion medium in fog? (air) What is dispersed in fog? (water droplets) Help liquid gas aerosol fog, aerosols students understand this two-part sys- tem by comparing it to a solution, which, instead of a dispersed phase and a dispersion medium, consists of a solid gas smoke dust in air solute and a solvent. Discuss L2 solid liquid sols and egg white, jellies, Point out that the main difference gels paint, blood, colloidal gold, starch between solutions, suspensions, and in water, gelatin colloids is particle size. Solution parti- cles are typically less than 1 nm in 460 Chapter 15 diameter. Colloid particles are between 1 nm and 1000 nm in diameter. Sus- pension particles are typically larger than 1000 nm. Smaller particles are less Differentiated Instruction susceptible to the effects of gravity L2 and are influenced more by the effects English Learners of Brownian movement. The collisions Have students use a dictionary to look up of molecules with extremely small col- the various uses of the word disperse or dis- loidal particles are sufficiently ener- persion. Ask students to explain how the getic to move colloidal particles in a seeds of a plant being dispersed by the wind random fashion that prevents their set- are related to the dispersed phase and dis- tling to the bottom. persion medium of a colloid. 460 Chapter 15 chem_TE_ch15.fm Page 461 Monday, April 17, 2006 2:19 PM a Relate L2 Many foods and other familiar prod- ucts are colloids or suspensions. Bring to class examples of these products. For example, compare orange juice to orange soda. Ask students to apply the criteria they have learned to determine the proper classification for these items. (The soda is a solution; the pres- ence or absence of pulp affects the classi- fication of the juice.) Milk is a colloid of Light protein and fat. The curdling of milk source Figure 15.15 The path of light is occurs when bacteria produce enough b visible only when the light is scattered by particles. a Fog or lactic acid to cause dispersed protein Solution Colloid Suspension mist is a colloid and thus exhibits and fat particles to coagulate into the Tyndall effect. b Particles in larger particles, which separate from colloids and suspensions reflect The Tyndall Effect Ordinarily you can’t see a beam of sunlight unless or scatter light in all directions. the rest of the mixture. This process is the light passes through particles of water (mist) or dust in the air. These Solutions do not scatter light. comparable to the clotting of blood. 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, but TEACHER Demo solutions do not.
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