![]() The net effect is to shift the dynamic equilibrium between water in the vapor and the liquid phases, decreasing the vapor pressure of the solution compared with the vapor pressure of the pure solvent.įigure 13.13 A Model Depicting Why the Vapor Pressure of a Solution of Glucose Is Less Than the Vapor Pressure of Pure Water At the same time, the rate at which water molecules in the vapor phase collide with the surface and reenter the solution is unaffected. As a result, fewer water molecules can enter the vapor phase per unit time, even though the surface water molecules have the same kinetic energy distribution as they would in pure water. In an aqueous solution of glucose, a portion of the surface area is occupied by nonvolatile glucose molecules rather than by volatile water molecules. We can understand this phenomenon qualitatively by examining Figure 13.13 "A Model Depicting Why the Vapor Pressure of a Solution of Glucose Is Less Than the Vapor Pressure of Pure Water", which is a schematic diagram of the surface of a solution of glucose in water. Vapor Pressure of Solutions and Raoult’s LawĪdding a nonvolatile solute, one whose vapor pressure is too low to measure readily, to a volatile solvent decreases the vapor pressure of the solvent. In the following discussion, we must therefore keep the chemical nature of the solute firmly in mind. The sum of the concentrations of the dissolved solute particles dictates the physical properties of a solution. At higher concentrations (typically >1 M), especially with salts of small, highly charged ions (such as Mg 2+ or Al 3+), or in solutions with less polar solvents, dissociation to give separate ions is often incomplete (see Figure 13.21 "Ion Pairs"). These values are correct for dilute solutions, where the dissociation of the compounds to form separately solvated ions is complete. Similarly, the CaCl 2 solution contains 0.01 M Ca 2+ ions and 0.02 M Cl − ions, for a total particle concentration of 0.03 M. As a result, a 0.01 M aqueous solution of NaCl contains 0.01 M Na + ions and 0.01 M Cl − ions, for a total particle concentration of 0.02 M. ![]() In contrast, both NaCl and CaCl 2 are ionic compounds that dissociate in water to yield solvated ions. Because sucrose dissolves to give a solution of neutral molecules, the concentration of solute particles in a 0.01 M sucrose solution is 0.01 M. Consider, for example, 0.01 M aqueous solutions of sucrose, NaCl, and CaCl 2. When we determine the number of particles in a solution, it is important to remember that not all solutions with the same molarity contain the same concentration of solute particles. As we will see, the vapor pressure and osmotic pressure of solutions are also colligative properties. (from the Latin colligatus, meaning “bound together” as in a quantity). ![]() Such properties of solutions are called colligative properties A property of a solution that depends primarily on the number of solute particles rather than the kind of solute particles. ![]() Antifreeze also enables the cooling system to operate at temperatures greater than 100☌ without generating enough pressure to explode.Ĭhanges in the freezing point and boiling point of a solution depend primarily on the number of solute particles present rather than the kind of particles. This solute lowers the freezing point of the water, preventing the engine from cracking in very cold weather from the expansion of pure water on freezing. Probably one of the most familiar applications of this phenomenon is the addition of ethylene glycol (“antifreeze”) to the water in an automobile radiator. Aqueous solutions have both a lower freezing point and a higher boiling point than pure water. For example, the limited temperature range of liquid water (0☌–100☌) severely limits its use. Many of the physical properties of solutions differ significantly from those of the pure substances discussed in earlier chapters, and these differences have important consequences. To describe the relationship between solute concentration and the physical properties of a solution. ![]()
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