1. CHAPTER 15: SOLUTIONS

2. What are solutions? Recall that solutions are homogeneous mixtures containing two or more substances called the solute and solvent –Solute: the substance that dissolves –Solvent: the dissolving medium IT IS NOT EASY TO DISTINGUISH SOLUTE FROM SOLVENT!!!!

3. May exist as a solid, liquid or gas –Ex. Air is a solution with nitrogen as its solvent Most solutions are liquids Water is most common solvent (universal solvent) Solutes may be solids, liquids, or gases as well

4. Types and Examples of Solutions Type of soln.ExampleSolventSolute Gas in gasAirNitrogenOxygen Gas in liquidCarbonated water WaterCarbon dioxide Liquid in liquidAnti-freezeWaterEthylene glycol Solid in liquidOcean waterWaterSalt Liquid in solidDental amalgamSilverMercury Solid in solidSteelIroncarbon

5. Solubility A substance that dissolves in a solvent is said to be soluble. –Ex. Sugar is soluble in water (it will dissolve) A substance that does not dissolve in a solvent is said to be insoluble. –Ex. Sand is insoluble in water. (it won’t dissolve)

6. Miscibility If two liquid substances are soluble in each other, then they are miscible. –No layers form –Ex. Anti-freeze and water If not, then they are said to be immiscible. –Ex. Oil and vinegar –Will come together through agitation but will soon separate out.

7. Solvation Why are some substances soluble and others are not? –In forming a solution, solute particles must separate from one another and mix with the solvent particles. –Mixing is based on the attractive forces between solute particles –Weak attractive forces between solute particles  allow solvent particles to move in and mix therefore, creating a solution.

8. The process of surrounding solute particles with solvent particles to form a solution is called solvation. Solvation in water is called hydration. “Like dissolves Like” is the rule to determine whether solvation will occur. To determine “Likeness” must know about bonding and polarity.

9. Aqueous Solutions of ionic compounds Water –Dipoles (positive and negative ends) –In constant motion (kinetic molecular theory) –When a crystal is placed in water, water molecules collide with the surface of the crystal –Solvation occurs and continues until crystal is entirely dissolved

10. Example of a Solution not forming Gypsum (calcium and sulfate) is mixed with water to form a plaster mixture. Gypsum is insoluble in water due to the attractive forces being too strong to be overcome by the solvent. Solvation will not occur

11. Aqueous Solutions of Molecular Compounds Water is a good solvent for molecular compounds, too (such as sugar) Sucrose molecules are polar and have many –OH bonds Hydrogen bonding will occur between water and –OH bonds in sucrose Easily solvated

12. Factors that Affect Rate of Solvation Solvation occurs between solute and solvent that are in contact with each other Three common ways to increase collisions: –1. agitation (stirring) –2. increase surface area (granulated sugar versus a sugar cube) –3. increase temperature of solvent (hot tea vs. iced tea)

13. Heat of Solution Energy is required for a solute to separate into particles (likewise for solvent) Attraction of particles is exothermic (heat of solution)

14. Factors that Affect Solubility By definition, solubility refers to the maximum amount of solute that will dissolve in a given amount of solvent at a specified temperature and pressure. Rates of solvation and crystallization may eventually equalize leading to types of solutions Saturated  contains maximum amount of solute/solvent Unsaturated  contains less dissolved solute (more can be dissolved)

15. Temperature and Solubility More soluble at higher temperatures Supersaturated solutions contain more dissolved solute than a saturated solution at same temperature. –To make, a saturated solution is formed at high temperatures and then cooled slowly. –Very unstable –A seed crystal (one tiny particle) can cause all of the excess solute to precipitate out quickly.

16. Pressure and Solubility The solubility of a gas increases as its external pressure increases –Carbonated beverages depend on this The dissolved gas is CO 2 and it is dissolved at a high pressure When drink is opened, the pressure above the liquid decreases Bubbles form, rise to top, and escape If not recapped, then goes flat (all CO 2 escapes)

17. KI NaNO 3 KNO 3 Na 3 PO 4 NaCl Solubility (g solute / 100 g H 2 O) 200 180 160 140 120 100 80 60 40 20 0 406080100 The general rule of thumb is that solubility of solids increases with increases in temperature. Maximum amount of a solute that can dissolve in a solvent at a specified temperature and pressure is its solubility. We can use these graphs to predict how well a material will dissolve and can identify a solution as saturated, unsaturated, or supersaturated.

18. 10 20 304050 6070 80 90 100 0 0 10 20 30 40 50 60 70 80 90 100 160 150 140 130 120 110 190 180 170 NaNO 3 LiCl Li 2 SO 4 NH 4 Cl NaCl CsCl RbCl SO 2 KCl NaC 2 H 3 O 2 Solubility in grams per 100 g of water Temperature ( o C) Questions: If I have dissolved 130 g of RbCl in water at 85 o C, is it a saturated, unsaturated or supersaturated solution? If I dissolve 60g of LiCl at 20 o C? If I dissolve 70g of KCl at 60 o C?

19. Gas Solubility CH 4 O2O2 CO He Temperature ( o C) Solubility (mM) 2.0 1.0 01020304050 Higher Temperature …Gas is LESS Soluble

20. Solution Concentration Concentration is a measure of how much solute is dissolved in a specific amount of solvent or solution –Concentrated- large amount of solute per solution –Dilute – small amount of solute per solution Various ways of quantitatively expressing concentration: Percent by mass, Percent by Volume, Molarity, Molality, and mole fractions.

21. Using Percents to Describe Concentration Ratio of measured amount of solute to a measured amount of solution. Percent by mass generally describes solids dissolving in liquids Percent by mass = mass of solute x 100 mass of solution

22. Example Percent by Mass In order to maintain a sodium chloride concentration similar to ocean water, an aquarium must contain 3.6 g NaCl per 100.0 g of water. What is the percent by mass of NaCl in the solution? Mass of solution = 3.6 g + 100.0 g = 103.6g % mass = 3.6 g x 100 = 3.5 % 103.6 g

23. Percent by Volume Percent by volume = volume of solute x 100 volume of solution Example: What is the percent by volume of ethanol in a solution that contains 35 mL of ethanol dissolved in 115 mL of water? % volume = 35 mL x 100 = 23% 150 mL

24. 15.3: Colligative Properties Physical Properties of solutions that are affected by the number of particles but not by the identity of dissolved solute particles

25. Vapor Pressure Lowering Pressure of the gas on the surface of the liquid is called the vapor pressure Less solvent at the surface, so there is less pressure A solution has a lower vapor pressure than a pure solvent (less solvent particles so less vapor is produced and the vapor pressure is lower) The greater the solute particles, the lower the vapor pressure

26. Boiling Point elevation When the vapor pressure equals the atmospheric pressure, water boils More heat is needed to boil the substance with a solute in it The boiling point is elevated higher by the more solute particles in the solute –The temperature of the water is hotter and food cooks faster (i.e. salt in water for noodles) The greater the number of solute particles, the greater the boiling point elevation