16.3 Colligative Properties of Solutions 16.3 1 FOCUS Connecting to Your World The wood frog is a remarkable Guide for Reading creature because it can survive being frozen. Scientists believe that a Objectives substance in the cells of this frog acts as a natural antifreeze, which Key Concepts What are three colligative 16.3.1 Identify three colligative prop- prevents the cells from freezing. Although fluids • properties of solutions? erties of solutions. surrounding the frog's cells may freeze, the • What factor determines the cells themselves do not. In this section, amount by which a solution’s 16.3.2 Explain why the vapor pres- you will discover how a solute can vapor pressure, freezing point, and boiling point differ from sure, freezing point, and boiling change the freezing point of a solution. those properties of the solvent? point of a solution differ from Vocabulary those properties of the pure colligative property solvent. freezing-point depression boiling-point elevation Reading Strategy Guide for Reading Vapor-Pressure Lowering Relating Text and Visuals As you read, look carefully at L2 You probably won’t be surprised to learn that the physical properties of a Figure 16.14. In your notebook, Build Vocabulary solution differ from those of the pure solvent used to make the solution. explain how this visual explains Word Parts The adjective colligative After all, tea is not the same as pure water. But it might surprise you to learn why equal molar solutions of dif- comes from the Latin colligare mean- that some of these differences in properties have little to do with the spe- ferent substances can have differ- ent freezing point depressions cific identity of the solute. Instead, they depend upon the number of solute ing “to tie.” A colligative property is and boiling point elevations. particles in the solution. A property that depends only upon the number of one in which particles are seemingly solute particles, and not upon their identity is called a colligative property. tied together in their actions and act as Three important colligative properties of solutions are vapor-pressure a group rather than as individuals. lowering, boiling-point elevation, and freezing-point depression. Recall that vapor pressure is the pressure exerted by a vapor that is in Reading Strategy L2 dynamic equilibrium with its liquid in a closed system. A solution that con- tains a solute that is nonvolatile (not easily vaporized) always has a lower Use Prior Knowledge Relate the sec- Figure 16.13 The vapor tion to prior knowledge by having stu- vapor pressure than the pure solvent, as shown in Figure 16.13. Glucose, a pressure of a solution of a molecular compound, and sodium chloride, an ionic compound, are nonvolatile solute is less than the dents write the chemical equations for examples of nonvolatile solutes. When glucose or sodium chloride is dis- vapor pressure of a pure solvent. the following examples: solved in a solvent, the vapor pressure of the solution is lower than the a In a pure solvent, equilibrium •hydrochloric acid in water vapor pressure of the pure solvent. Why is this true? is established between the liquid + − and the vapor. b In a solution, (HCl(g) → H (aq) + Cl (aq)) solute particles reduce the •magnesium chloride in water number of free solvent particles (MgCl (s) → Mg2+(aq) + 2Cl−(aq)) Solvent particle Solute particle able to escape the liquid. 2 a b Equilibrium is established at a •Glucose in water → lower vapor pressure. (C6H12O6(s) C6H12O6(aq)) Interpreting Diagrams How is decreased vapor pressure represented in the diagram? 2 INSTRUCT Pure solvent Higher vapor Solution containing Lower vapor pressure nonvolatile solute pressure Ask, What is a possible explanation for the wood frog’s ability to survive Section 16.3 Colligative Properties of Solutions 487 being frozen? (The frog’s cells contain a natural antifreeze.) Mention to students that some fish and reindeer also bene- Section Resources fit from so-called “natural antifreeze.” When water freezes, the water mole- Print Technology cules form a crystalline lattice work. •Guided Reading and Study Workbook, •Interactive Textbook with ChemASAP, How might a solute change the Section 16.3 Assessment 16.3 freezing point of a solid? (It might •Core Teaching Resources, Section 16.3 •Go Online, Section 16.3 slow down the formation of the crystal Review lattice.) •Transparencies, T175–T176 Answers to... Figure 16.13 by fewer solvent parti- cles in the space above the liquid Solutions 487 chem_TE_ch16.fm Page 488 Friday, April 15, 2005 12:12 PM a Glucose in solution b Sodium chloride in solution c Calcium chloride in solution Section 16.3 (continued) Glucose Vapor-Pressure Lowering Discuss L2 ϩ Na 2ϩ Remind students that ionic com- Ca ClϪ pounds and certain molecular com- ClϪ pounds, such as HCl, produce two or more particles when they dissolve in water. Most molecular compounds, Figure 16.14 Particle b Three moles of sodium c Three moles of calcium such as glucose, do not dissociate concentrations differ for chloride dissolved in water chloride dissolved in water when they dissolve in water. For each dissolved covalent and ionic produce 6 mol of particles produce 9 mol of particles compounds in water. a Three because each formula unit of because each formula unit of formula unit of MgCl2 that dissolves, moles of glucose dissolved in NaCl dissociates into two ions. CaCl2 dissociates into three ions. three particles are formed in solution. water produce 3 mol of particles Ask, How many particles are formed because glucose does not when FeCl dissolves in water? (4) dissociate. In an aqueous solution of sodium chloride, sodium ions and chloride 3 ions are dispersed throughout the liquid water. Both within the liquid and at the surface, the ions are surrounded by layers of associated water mole- Freezing-Point cules, or shells of water of solvation. The formation of these shells of water Depression of solvation reduces the number of solvent molecules that have enough kinetic energy to escape as vapor. Thus, the solution has a lower vapor pres- Use Visuals L2 sure than the pure solvent (water) would have at the same temperature. Figure 16.14 Emphasize that colliga- Ionic solutes that dissociate, such as sodium chloride and calcium tive properties do not depend on the chloride, have greater effects on the vapor pressure than does a nondisso- ciating solute such as glucose. Recall that each formula unit of the ionic kind of particles, but on their concentra- compound sodium chloride (NaCl) produces two particles in solution, a tion. For colligative properties, a mole of sodium ion and a chloride ion. Each formula unit of calcium chloride one kind of particle has the same effect (CaCl2) produces three particles, a calcium ion and two chloride ions. as a mole of any other kind of particle. When glucose dissolves, the molecules do not dissociate. Figure 16.14 Ask, Which produces a greater compares the number of particles in three solutions of the same concen- tration. The vapor-pressure lowering caused by 0.1 mol of sodium chloride change in colligative properties—an in 1000 g of water is twice that caused by 0.1 mol of glucose in the same ionic solid or a molecular solid? (An quantity of water. In the same way, 0.1 mol CaCl2 in 1000 g of water pro- ionic solid produces a greater change duces three times the vapor-pressure lowering as 0.1 mol of glucose in the because it will produce two or more same quantity of water. The decrease in a solution’s vapor pressure is moles of ions for every mole of solid that proportional to the number of particles the solute makes in solution. dissolves.) Checkpoint Which compound affects the vapor pressure of a solution the least: glucose, sodium chloride, or calcium chloride? Freezing-Point Depression When a substance freezes, the particles of the solid take on an orderly pat- tern. The presence of a solute in water disrupts the formation of this pat- tern because of the shells of water of solvation. As a result, more kinetic Download a worksheeet on Solu- energy must be withdrawn from a solution than from the pure solvent to tions for students to complete, and cause the solution to solidify. The freezing point of a solution is lower than For: Links on Solutions find additional teacher support Visit: www.SciLinks.org the freezing point of the pure solvent. The difference in temperature from NSTA SciLinks. Web Code: cdn-1163 between the freezing point of a solution and the freezing point of the pure solvent is the freezing-point depression. 488 Chapter 16 Facts and Figures Osmotic Pressure Another colligative property, which is impor- maintains the correct osmotic balance tant in many biological processes, is osmotic between the inside and outside of the cell pressure: the tendency for solvent to flow membrane. Physicians make use of the through a semipermeable membrane from a osmotic effect to treat patients with kidney region of low solute concentration to a failure. Afflicted individuals undergo dialysis, region of higher solute concentration. a process in which the blood is circulated Osmotic pressure helps regulate the move- through a machine equipped with a semi- ment of fluids across cell membranes. Cells permeable membrane. In the machine, remain healthy as long as they are continu- osmotic pressure is used to separate waste ally bathed in an isotonic medium, which materials from the blood. 488 Chapter 16 chem_TE_ch16.fm Page 489 Tuesday, April 18, 2006 11:24 AM Quick LAB Quick LAB Solutions and Colloids Purpose Procedure L2 To classify mixtures as solu- 1. In a cup, make a paste by mixing Solutions and Colloids 1 tions or colloids using the 2 teaspoon cornstarch with Objective After completing this activ- Tyndall effect.