Water: The Universal Solvent

1. Water: The Universal Solvent • Polarity makes water special. • O atom is an electron “hog” in the covalent bond it forms with 2 H atoms. • Water has a positive end and a negative end. • This makes it a good solvent for ionic solids. Slightly negative O H H Slightly positive Mullis

2. Solutions • Same as a homogeneous mixture • Solvent = substance that takes in, or dissolves, another substance • Solute = substance that is taken in, or dissolved by the first substance • The 3 phases of matter can be paired together in 9 ways to make solutions. Mullis

3. Solutions and phases of matter Mullis

4. Factors which affect solution rate and solubility • Solution rate: • Temperature • Particle size • Movement • Solubility: • Temperature • Pressure Mullis

5. Solubility • Solids: Increased temp increases solubility. • Gases: Solubility in water decreases when temperature increases. • Gases: Increased pressure increases solubility. • More fizz in a soda bottle if you store with lid on (increased pressure) and in the refrigerator (decreased temperature). Mullis

6. Concentration • Relationship between solute and solvent: • Concentration = volume solute/volume solvent • Relative concentration of solutions: • Unsaturated: More solute can be dissolved at a given temp. and pressure. May be dilute or concentrated. • Saturated: Maximum solute a solvent will dissolve at a given temp. and pressure. • Supersaturated: More than the maximum solute a solvent will dissolve at a given temp. and pressure. (Usually obtained by heating a saturated solution to very high temp and cooling slowly.) Mullis

7. Concentration • Concentration is the amount of solute dissolved in an amount of solution. • Molar concentration is called Molarity, or M. • M = moles of solute volume of solution in L • If 2.0 moles NaCl is dissolved in 1 L of solution, its concentration is “two molar” and is expressed as 2.0 M NaCl. Mullis

8. Steps to make a 1 M solutionExample: 1 M NaOH solution • Calculate the mass needed. 1 mol NaOH 40.01 g NaOH = 40.01 g NaOH 1 mol NaOH • Measure this amount of NaOH into a beaker, then dissolve in some solvent to dissolve it. • Pour the solution into a 1.0 L volumetric flask. • Rinse the weighing beaker with more solvent and add to the flask. • Add solvent until the solution meets the line indicating 1.0 L. • Stopper the flask and swirl, mix thoroughly. Mullis

9. Molality • Molal concentration is called molality, or m. • m = moles of solute mass of solvent in kg • If 0.50 moles NaCl is dissolved in 1 kg of solution, its concentration is “zero point five molal” and is expressed as 0.50 m NaCl. • 1000 g = 1 kg. • For water, 1 g = 1 ml 1kg = 1000 ml = 1 L Mullis

10. Molality Example • Molality is used when studying properties of solutions related to vapor pressures and temperature changes. The expression for concentration using m is unaffected by changes in temperature. • What is the molality of a solution of 3.73g KCl in 100 g water? 3.73 g KCl 1 mol KCl = 0.05 moles KCl 74.55 g KCl m = 0.05 moles KCl = 0.5 m KCl solution 0.100 kg H2O Mullis

11. Molarity Example To produce 23.4 g of K2CrO4 from 6.0 M K2CrO4, what volume of the solution is needed? Given: 23.4 g K2CrO4 needed concentration of solution = 6 moles K2CrO4 per 1 L Want: volume of K2CrO4 in solution 23.4 g K2CrO4 1mol K2CrO4 = 0.120 mol K2CrO4 194.2g K2CrO4 0.120 mol K2CrO4 1 L = 0.020 L K2CrO4 soln 6 mol K2CrO4 Mullis

12. Molality example How much iodine (in grams) must be added to prepare a 0.480 m solution of iodine in CCl4 if 100.0 g of CCl4 is used? Given: m = 0.480 m I2 Want: mass of solute in g solvent = 100.0 g CCl4 = 0.100 kg I2 = 253.8 g/mol 0.480 mol I2 = x mol I2 1 kg CCl4 0.100 kg CCl4 (0.480 mol I2)(0.100) = x = 0.0480 mol I2 0.0480 mol I2 253.8 g I2 = 12.2 g I2 1 mol I2 Mullis

13. Suspensions and Colloids • Remember: A solution is a homogeneous mixture. • Some mixtures are not solutions—It depends on the size of the particles. • Large particles usually don’t make homogeneous mixtures. • The names of 2 types of mixtures with large particles are suspensions and colloids. Mullis

14. Suspensions • A mixture in which some particles will settle out is a suspension. • Gravity will eventually make large particles sink. • Particles are large enough to be filtered out. • Examples: Muddy water and salad dressing. • Particles are large enough to reflect light, so a beam of light will be visible in a suspension. Mullis

15. Comparing mixtures • Solution = homogeneous, small particles • Suspension = heterogeneous, large particles • Colloid = in-between suspension and solution. • Is not completely homogeneous. • Examples: fog, homogenized milk, toothpaste. Mullis

16. Colloids • How can you tell is a mixture is a colloid? • A beam of light will be visible when shone through a colloid (like a suspension). • Particles do not separate or settle out after standing. Mullis

17. Colloids: 5 types • Sol • Solid is dispersed in solid or liquid. • Examples: Clay, toothpaste, red glass • Gel • Long particles that trap liquid inside. • Examples: Jam, jelly, gelatin dessert, hair gel • Aerosol • Solid is dispersed in air. • Examples: Fog, some hair sprays • Foam • Gas is dispersed in liquid. • Examples: Marshmallows, whipped cream • Emulsion • Liquid is dispersed in liquid. • Examples: Milk, butter, some cosmetics Mullis

18. Comparing mixtures • Solution = homogeneous, small particles • Suspension = heterogeneous, large particles • Colloid = in-between suspension and solution. • Is not completely homogeneous. • Examples: fog, homogenized milk, toothpaste. Mullis

19. Solutes: Electrolytes • Electrolyte = Substance that dissolves in water to give a solution that conducts electricity • Mobile charged particles conduct electricity (ions) • Soluble ionic compounds are electrolytes (Na+ and Cl-) • Many acids are electrolytes (HCl =>H3O+ and Cl-) Mullis

20. Solute-Solvent Interactions Like dissolves like • Polar dissolves polar Water + NaCl • Nonpolar dissolves nonpolar CCl4 + motor oil • Immiscible liquids (oil and vinegar) • Do not mix • Liquid solutes and solvents that are not soluble in each other • Miscible liquids (gas and oil) • Mix • Liquids that dissolve freely in one another in any proportion Mullis

21. Henry’s LawSolubility of gas in liquid • The solubility of a gas is directly proportional to the partial pressure of that gas on the surface of the liquid. • Soda bottle: • High pressure at the surface while the bottle is closed, so lots of CO2 in the liquid • Open bottle, pressure on surface lowers to room atmosphere and CO2 leaves the liquid • High pressure = High gas concentration • Low pressure = low gas concentration Mullis