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2. Properties of Water Polar molecule Forms hydrogen bonds High Cohesion and surface tension Density – greatest at 4 o C Universal solvent Forms hydrates 2

3. Polarity of Water 2 H 2 + O 2  2H 2 O – O is more electronegative – The two H atoms have a partial positive charge. A water molecule is a polar molecule – negative at the oxygen end and positive at the hydrogen ends 3

4. Water has unusual properties because of its polarity which allows it to form a hydrogen bond. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 3.1 Polarity of Water Hydrogen bonding is the attraction of a slightly positive hydrogen in one water molecule to a slightly negative oxygen in another water molecule 4

5. HYDROGEN BONDS Hold water molecules together Each water molecule can form a maximum of 4 hydrogen bonds The hydrogen bonds joining water molecules are weak, about 1/20 th as strong as covalent bonds. They form, break, and reform with great frequency Extraordinary Properties that are a result of hydrogen bonds. – Cohesive behavior – Resists changes in temperature – High heat of vaporization – Expands when it freezes – Versatile solvent 5

6. Surface tension, a measure of the force necessary to stretch or break the surface of a liquid, is related to cohesion. – Water - greater surface tension than most other liquids because of hydrogen bonds – Water behaves as if covered by an invisible film. – Some animals can stand, walk, or run on water without breaking the surface. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 3.3 6

7. Surface Tension Surfactant – a substance that interferes with the hydrogen bonding between water molecules. “It makes water wetter.” ex. soaps and detergents 7

8. Solid water... Ice, Ice, Baby! Due to hydrogen bonding, when water solidifies it expands into a hexagonal lattice work and becomes less dense. It forms a “honeycomb”. This is why snowflakes have six sides. 8

9. Density of Water Ice (solid) has a lower density than water (liquid). That’s why ice cubes float. The density of water: 1.Prevents water from freezing from the bottom up. 2.Ice forms on the surface first—the freezing of the water releases heat to the water below creating insulation. 3.Makes transition between season less abrupt. 9

10. Aqueous solution - water that contains dissolves substances Solvent – in a solution, the dissolving medium Solute – in a solution, the dissolved particles 10

11. “LIKE DISSOLVES LIKE” Substances that dissolve easily into water are ionic compounds and polar covalent molecules. – think salt water Nonpolar compounds (oil, grease and gasoline) do not dissolve in water. 11

12. HYDRATES A compound that contains water of hydration (crystalline form). Copper (II) sulfate pentahydrate CuSO 4 · 5 H 2 O Cobalt (II) chloride hexahydrate CoCl 2 · 6 H 2 O 12

13. HYDRATES When heated, they lose their water of hydration and become anhydrous. CuSO 4 · 5 H 2 O → CuSO 4 (s) + 5 H 2 O (g) CoCl 2 · 6 H 2 O → CoCl 2 (s) + 6 H 2 O (g) 13

14. % Water in a Hydrate Sodium carbonate decahydrate Na 2 CO 3 · 10 H 2 O 14

15. Try these: CuSO 4 · 5 H 2 O CoCl 2 · 6 H 2 O 36.1% 45.4% 15