Understanding Water: Properties, Behaviors, and Solutions

1. Chapter 17 Water

2. Section 17.1 • Water is a very polar molecule. • The oxygen is electronegative. • The hydrogens are electropositive. • Water molecules are attracted to each other by Hydrogen bonds. The results of these bonds are that water has a: • High surface tension • High specific heat capacity • High heat of vaporization • High boiling point.

3. Why does water form a ball on a sheet of paper? • Hydrogen bonds! • Water has a high surface tension: • The inward force or pull that tends to minimize the surface area of a liquid. • You can decrease the surface tension by adding a surfactant: • A wetting agent such as soap or detergent. • These detergents interfere with the hydrogen bonding.

4. Why does water have a low vapor pressure? • Hydrogen Bonds! • The water molecules hold onto each other so that water doesn’t escape into the gaseous phase.

5. Why does water have a high specific heat capacity? • Hydrogen Bonds!! • It takes 4.18 J (1 cal) to raise 1 g of water 1˚C. • Do metals have a higher or lower heat capacity? • Lower • Why does this factor help to moderate? • Our temperatures – summer – relatively mild and winter relatively mild.

6. Section 17.2 • Heat of Vaporization: • Amount of energy needed to convert 1 g of a substance from a liquid to a gas at the boiling point. • Condensation: • Going from a gas to a liquid – the amount of energy removed in order to convert 1 g of a substance from a gas to a liquid at the boiling point. • Molecular compounds that have low molecular mass are usually gas or liquids at room temperature and have low boiling points. (NH3 boils at -33˚C) • Water, a molecular compound with low molar mass, is high (boils at 100˚C). Why?

7. What happens when a liquid cools? • The molecules move closer and closer, the density increases, and eventually the liquid turns to a solid and sinks. • Water behaves like this for only so long. At 4˚C, water is the densest it will be. Below this temperature water is actually decreasing in density. • When water reaches 0˚C, water turns to a solid and floats. • Why? • When ice forms, the hydrogen bonds make the molecules form a honeycomb type of structure. This makes it less dense. • What would happen if ice were to become more dense than liquid water in real life?

8. Section 17.3 • Aqueous solutions: • Water samples containing diessolved substances. • Solvent: • Dissolving medium • Solute: • Dissolved particles • Substances that dissolve readily in water: • Ionic compounds • Polar covalent molecules • Nonpolar covalent molecules don’t (oil, grease) • Solvation: • The process that occurs when a solute dissolves.

9. “Like Dissolves Like” + = → - = H2O = This occurs until all of the solid chunk have been “carried off”, dissolved, by the dissolving medium.

10. Electrolytes: • Compounds that conduct an electric current in aqueous solutions or the molten state. • Ex: ionic compounds • Strong Electrolytes: • Almost all of the solute exists as separate ions and conducts a strong current. • Weak Electrolytes: • Only a fraction of the solute exists as ions and conducts a weak current. • Nonelectrolytes: • Compounds that do not conduct an electric current in either the aqueous or molten states. • Ex: molecular compounds

11. Electrolyte vs. Nonelectrolyte Electrolytes conduct electricity Non-electrolytes do not

12. Water of hydration: • The water in a crystal or water of crystallization. • A compound containing water of hydration is a hydrate. • Ex: copper (II) sulfate pentahydrate • CuSO4ּ5H2O • Effloresce: • If a hydrate has a vapor pressure higher than that of the water vapor in the air, the hydrate will effloresce by loosing the water of hydration. • For simplicity sakes: if its humid, water stays with the compound and if it’s dry, the water evaporates. • Example: CuSO4ּ5H2O has a vapor pressure of 1.0 kPa. The water vapor at room temperature is ~1.3 kPa. Will the compound effloresce? • No, the vapor pressure is too high.

13. Drying Agents • Some compounds have a low vapor pressure and remove water from the air. • Hygroscopic: • Salts and other compounds that remove moisture from the air. • CaCl2 • Hygroscopic substances are used as drying agents or desiccants. • Ex.: silica gel • Deliquescent compounds: • Remove sufficient water from the air to dissolve completely and form solutions. • Ex: NaOH and Damp Rid

14. Section 17.4 • Suspensions: • Mixtures from which particles settle out upon standing. • Ex.: sandy water • The particles are bigger than in solutions (100 x bigger) • Exhibits Tyndall Effect: • Scattering of light in all directions • Heterogeneous mixtures – 2 substances clearly identified.

15. Colloids • Colloids: • Heterogeneous mixtures containing particles that are intermediate in size. They are between those of suspensions and true solutions. (between 1nm and 100nm) • Particles are the dispersed phase. • Spread throughout the dispersion medium. (they can be solids, liquids, or gases) • Ex.: glue, jello, paint, aerosol sprays, smoke, fog

16. Colloid cont. • Cloudy or milky appearance when concentrated, but when dilute appear clear. • Exhibit the Tyndall Effect • Particles exhibit Brownian Motion: • Chaotic movement of colloidal particles • These movements are the result of bumping into water molecules and keeps them from settling out. • Ex: bumper cars

17. Tyndall Effect

18. Comparison Table for solutions, colloids and suspensions.

19. Emulsions • Emulsions: • Colloidal dispersions of a liquid in a liquid. • i.e. particles group together because of the same charge. • Ex.: oil in water • The soap and detergents are emulsifying agents (they cause an emulsion to form) O Polar end Attracted to the water Non polar end Attracted to grease OH Palmitic acid – used in making soaps