Solutions

1. Solutions

2. Nature of Solutions • A solution is a homogeneous mixture of two or more substances in a single physical state • The substance that is dissolved is the solute. The substance that does the dissolving is the solvent. • Substances that dissolve in other substances are said to be soluble. If they don’t dissolve they are insoluble.

3. Types of Solutions • Solid Solutions – involve at least one solid. The most common are solutions containing two or more metals, called alloys • Sterling silver, brass, bronze • Gaseous Solutions – all mixtures of gases • Air

4. Types of Solutions • Liquid Solutions – involving at least one liquid • Antifreeze, vinegar, soft drinks • Liquids that mix in any amount are miscible. Liquids that cannot mix in any proportions are immiscible. • Aqueous Solutions – solutions that have water as a solvent • Substances that dissolve in water to create electric current are called electrolytes

5. Concentration of Solutions • Concentration is the amount of solute in a given amount of solvent or solution • Molarity (M) • The molarity of a solution is equivalent to the moles of solute divided by the liters of solution

6. Concentration of Solutions • Molality (m) • The molality of a solution is equivalent to the number of moles of solute dissolved in each kilogram of solvents

7. Concentration of Solutions • Mole Fractions (X) • The mole fraction is the number of moles of one component (either solute or solvent) divided by the number of moles in solution • The sum of the mole fractions for the solute and solvent must add up to 1

8. Saturation • A solution is saturated if it contains as much solute as can possibly be dissolved under the existing conditions of temperature and pressure • A solution with less than the maximum amount of solute than can be dissolved is unsaturated. If the solution has a greater amount of solute than needed to form a saturated solution it is supersaturated.

9. How Solutions Form • Dissolving takes place at the surface of a solid • The interaction between solute and solvent molecules is called solvation • When the solvent is water it is called hydration

10. Solubility • Solubility is the amount of a solute that will dissolve in a specific solvent under given conditions

11. Factors Affecting Solubility • Nature of Solute and Solvent • “Like Dissolves Like” – polar solvents dissolve polar solutes; nonpolar solvents dissolve nonpolar solutes; ionic solutes are soluble in polar solvents • Temperature • In general, solubility increases as temperature increases

12. Factors Affecting Solubility • Pressure (for gases) • The solubility of a gas in any solvent is increased as the pressure of the gas over the solvent is increased

13. Factors Affecting Dissolving Rate • Surface Area • The greater the surface area (smaller pieces), the faster the solute will dissolve • Stirring • Stirring increases the contact between the solvent and the solute surface

14. Factors Affecting Dissolving Rate • Temperature • An increase in temperature increases the rate at which a solute dissolves

15. Colligative Properties • A property that depends on the concentration of solute particles but is independent of their nature is a colligative property • There are four: vapor point reduction, boiling point elevation, freezing point depression, and osmotic pressure

16. Colligative Properties • Vapor Pressure Reduction • When a nonvolatile solute (one that won’t vaporize or sublime under existing conditions) is added to a pure solvent, the solute molecules take up space at the surface of the liquid • This prevents some solvent molecules from leaving the liquid

17. Colligative Properties • However, there is no change in the rate at which molecules in the gas state return to the liquid • This produces a shift – because more molecules leave the gas than enter it, the gas pressure is reduced (vapor pressure reduction) • The magnitude by which the pressure is lowered is directly proportional to concentration

18. Colligative Properties • Boiling Point Elevation (ΔTb = Kbm) • Boiling point is the temperature at which the vapor pressure of a liquid is equal to the external pressure on its surface • Since the addition of a nonvolatile solute reduces the vapor pressure, a higher temperature is required to make the substance boil

19. Colligative Properties • The amount by which the boiling point is raised is the boiling point elevation • The boiling point elevation is directly proportional to molality

20. Colligative Properties • Freezing Point Depression (ΔTf = Kfm) • The freezing point is the temperature at which the vapor pressures of the solid and liquid phases are the same • If a nonvolatile solute is added, the vapor pressure of the solution is reduced in proportion to the mole fraction of the solute – therefore the freezing point is lowered • Freezing point depression is directly proportional to molality

21. Colligative Properties • Osmotic Pressure (π) • The osmotic pressure is the pressure required to prevent osmosis • When two solutions are at the same osmotic pressure, they are isotonic

22. Colligative Properties • If a solution has a lower osmotic pressure than another, it is hypotonic • Putting cells in a hypotonic solution causes them to burst • If a solution has a higher osmotic pressure than another, it is hypertonic • Putting cells in a hypertonic solution causes them to shrivel