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Solutions

Solutions. Homogeneous Mixtures. Key Definitions. Solute Dissolved particles in a solution The lesser component Solvent The dissolving medium in a solution The greater component Example: Saline Solution for contact lenses Solute = sodium chloride Solvent = water. More definitions.

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Solutions

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  1. Solutions Homogeneous Mixtures

  2. Key Definitions • Solute • Dissolved particles in a solution • The lesser component • Solvent • The dissolving medium in a solution • The greater component • Example: Saline Solution for contact lenses • Solute = sodium chloride • Solvent = water

  3. More definitions • Miscible • Liquids that will dissolve in each other • Ex. Oil and gasoline for 2 stroke engines • Immiscible • 2 liquids that are insoluble in each other • Ex. Oil and water

  4. How much solute is dissolved? • Saturated solution • Contains the maximum amount of dissolved solute • If you add more solute, it will fall to the bottom • Unsaturated solution • Contains less than the maximum amount of dissolved solute • If you add more solute, it will dissolve

  5. Relative Humidity • A measure of how much water vapor is in the air, compared to maximum amount of water the air can hold at that temperature • 100% humidity: rain is possible! • People feel most comfortable indoors when the relative humidity is between 40-60%.

  6. Supersaturated Solutions • Sometimes solutions can be prepared that have MORE dissolved solute than a saturated solution • Unstable! • Often used in candy making processes • Video clip:

  7. Solubility Diagrams • Solubility information is often presented graphically

  8. Interpreting Solubility Diagrams • Most substance become more soluble as temperature increases. • Gases typically become LESS soluble as temperature increases. • Note the units on the y axis: often given as grams of solute/100 g solvent.

  9. To speed up the solution process: • Increase the surface area of the solute • Which would dissolve faster: a sugar cube or powdered sugar? • Stir the solution • Increase the temperature of the solution

  10. Rule of thumb for solubility • “Like dissolves like” • Solutes tend to dissolve in solvents with similar properties. • Nonpolar substances typically dissolve in nonpolar solvents. • Ionic solids tend to dissolve in polar solvents.

  11. Solution Concentrations • Ratio of solute to solvent • Concentrated: high ratio of solute to solvent • Dilute: low solute/solvent ratio • Several different ways to calculate • Most common: • Molarity symbol: M • Percent solutions: m/m, m/v, v/v • Parts per million • Molality

  12. Molarity • Definition: • Since mol = g/gfm,

  13. Percent Solutions • Easy and reliable to calculate • Mass/mass, mass/volume, volume/volume • Frequently used in biology & medicine

  14. Molality • Used to calculate changes in physical properties of solutions • Note subtle differences from molarity formula! • “molal” solutions

  15. Vapor Pressure • Particles at the surface of a liquid may evaporate or vaporize, if they have enough kinetic energy. • If this happens in a sealed container, the particles of escaped liquid (now in the gas phase) will collide with the container walls and exert a “vapor pressure” above the liquid

  16. Vapor Pressure Diagrams • Vapor pressure data is often presented graphically

  17. Boiling Point • Boiling • when bubbles of gas form anywhere in the liquid and then rise to the surface • Boiling point • the temperature at which the vapor pressure of the liquid is just equal to the external pressure • Normal boiling point • the boiling point of a liquid at a pressure of 101.3 kPa (1 atm)

  18. Colligative Properties • Addition of a solute changes the properties of the system: • The solution will have different properties than the pure solvent • Adding solute changes the freezing point, boiling point, vapor pressure • These changes depend only on the number of particles added (NOT the identity of the particles)

  19. Phase Diagrams

  20. Phase Diagrams • Closed system • Triple Point • Where solid, liquid, and gas can coexist • Critical Point • Above this point, gas and liquid are indistinguishable • “supercritical fluids”

  21. Colligative Properties • Freezing Point Depression • The solution will freeze at a lower temperature than the pure solvent • Why we put salt on sidewalks during winter • Antifreeze in cars • Boiling Point Elevation • The solution will boil at a higher temperature than the pure solvent • Cool simulation

  22. van’t Hoff Factor (i) • Some substances dissociate into ions, resulting in more solute particles dissolving • van’t Hoff Factor = the theoretical (maximum) number of particles formed when a substance dissociates • The real degree of ionization is typically slightly less than predicted by the van’t Hoff factor

  23. Predicting van’t Hoff factors • For all covalently bonded substances, • i = 1 • For ionic substances • I = number of ions present in formula • Ex. • NaCl i = 2 • Al(NO3)3 i = 4 • MgCl2 i = 3

  24. Calculating Colligative Properties • Tbp = kbpim where kbp is a constant for each solvent • Calculate the new boiling point by adding Tbp to the boiling point of the pure solvent • Tfp = kfpim where kfp is a constant for each solvent • Calculate the new boiling point by subtracting Tbp from the freezing point of the pure solvent

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