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Topic 8 Properties of Water

Topic 8 Properties of Water. GEOL 2503 Introduction to Oceanography. H 2 0. Englishman, Sir Humphrey Davey discovered the formula for water: H 2 O 3 Atoms: two hydrogen & one oxygen H+ H+ O 2-. Polar Molecule. Positive end Hydrogens on tips of “V” Negative end

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Topic 8 Properties of Water

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  1. Topic 8Properties of Water GEOL 2503 Introduction to Oceanography

  2. H20 • Englishman, Sir Humphrey Davey discovered the formula for water: H2O • 3 Atoms: two hydrogen & one oxygen • H+ H+ O2-

  3. Polar Molecule • Positive end • Hydrogens on tips of “V” • Negative end • Oxygen on other end • Molecules attract one another

  4. Properties of Water • Thermal (Heat Capacity) • Cohesion • Surface Tension • Viscosity • Transmission of light, sound

  5. Thermal Properties of Water • Phase changes • Heat capacity

  6. Solid - ice Changes of State Liquid – liquid water Gas -water vapor Water is the only common substance to exist in all three states of matter at normal Earth surface temperatures

  7. January 8, 2010; 8 AM

  8. January 8, 2010; 2 PM

  9. To change a state of matter • Need time to add or remove heat • Need time for the hydrogen bonds to break

  10. Pure Water • 100 % water—nothing else • No suspended particles • No dissolved substances including gases

  11. State Change Temperatures • Pure water melts/freezes at 0° C (32 ° F) • Pure water boils/condenses at 100 ° C (212 ° F) • [at standard air pressure]

  12. For example: • Take solid water (ice) • Add heat • Temperature rises above freezing point • Ice melts • Forms liquid water

  13. Another example: • Start with liquid water • Add heat • Temperature rises • Water evaporates to form gaseous water (water vapor)

  14. Also works in reverse: • Start with water vapor (gas) • Remove heat (cool) • Temperature falls • Water vapor condenses to form liquid water • Remove more heat • Freezing • Solid water (ice) forms

  15. Heat is not the same as temperature

  16. What is Heat? • Heat is energy • Measured in calories • A calorie is the amount of heat needed to raise the temperature of 1 gram of water by 1° C • [Kcal (1000 calories) = 1 food calorie]

  17. Remember: • It takes 1 calorie of energy (heat) to raise the temperature of 1 gram of liquid water by 1° C • Also, if you remove 1 calorie of energy the temperature of 1 gram of liquid water will then drop 1° C

  18. But look at state (phases) changes: • To change 1 gram of ice at 0° C to liquid water at 0° C • Requires 80 calories per gram not 1 calorie • Called the latent heat of fusion

  19. And: • To change 1 gram of liquid water at 100° C to gas (water vapor) at 100° C • Requires 540 calories per gram not 1 calorie • Called the latent heat of vaporization

  20. Why? • Not changing temperature • Breaking chemical bonds • That takes energy (heat) • Works in reverse by taking heat away (chemical bonds are formed)

  21. Solid Gas Liquid

  22. A--hydrogen bond B--oxygen atom C--hydrogen atom D--positively charged side of the water molecule E--negatively charged side of the water molecule

  23. State Change Terminology • Solid to liquid: melting • Liquid to gas: evaporation • Gas to liquid: condensation • Liquid to solid: freezing • Solid to gas: sublimation • Gas to solid: deposition

  24. Heat Capacity • Is defined as the quantity of heat required to produce a unit change of temperature in a unit mass of that material • Heat capacity of water is 1 calorie per gram per degree Celsius • Heat capacity of water is higher than most other liquids due to hydrogen bonding

  25. Heat capacities: Liquid water = 1 cal/g/ºC Ice = 0.5 cal/g/ºC Water vapor = 0.5 cal/g/ºC How many calories does it take to raise the temperature of 1 gram of water from -100 ºC to +150 ºC ?

  26. High heat capacity helps move heat around the Earth, moderating climate • Water evaporates from oceans, absorbing heat from oceans • Winds move water vapor • Water returns to liquid form by precipitation, adding heat to the atmosphere

  27. Range of normal Earth Temperaturescontrolled by latent heat • Deserts +50° C (122° F) • Antarctica -50° (-58° F) • Land range 100° C (180° F) • Tropical oceans +28° C (82° F) • Polar oceans -2° C (28° F) • Ocean range 30° C (54° F)

  28. Land versus Ocean Temperatures • Land: • +50° C (122° F) to -50° (-58° F) • Land range 100° C (180° F) • Water: • +28° C (82° F) to -2° C (28° F) • Ocean range 30° C (54° F)

  29. “Evaporation is a cooling process” • You’ve probably heard that phrase • To evaporate water takes 540 calories of heat per gram • Essentially absorbing heat • Example—cooling our bodies (removing heat energy) by evaporation of perspiration

  30. Heat Capacity Summary: • Day/night change of water temperature is very small • Helps redistribute heat around Earth • Helps stabilize Earth’s surface temperatures

  31. Cohesion • Water has more structure than other liquids due to the hydrogen bonds - hold water together • Water molecules stay close together due to polarity • Makes water relatively sticky

  32. Surface Tension • Water molecules form a film because of sticky nature of water • Paper clip floats, water striders • Overfill glass of water • High surface tension is important for waves • Decrease temperature increases surface tension and vice versa

  33. Viscosity • Resistance to flow • High viscosity means high resistance to flow • Affected by temperature—“slow as molasses in January”

  34. Density • Less dense floats on denser liquids • Example: Ocean water is denser than fresh water so fresh floats on salt water • Defined: mass per unit volume of a substance • Measured in grams per unit centimeter cubed

  35. What is mass? • Mass is the amount of matter present • It is not the same as weight • Weight changes depending on gravity • Mass remains constant • Think of astronauts—normal weight on Earth, weightless in space, 1/6th of Earth weight on moon, but their mass is constant

  36. Density • Pure water: D = 1.0000 g/cm3 • At 4° C (39.2° F) • Sea water has salt dissolved in it, so it is denser than pure water • Sea water: D = 1.0278 g/cm3 • At 4° C (39.2° F)

  37. Coke Can Density Experiment • Can of regular Coke sinks in water • Same size can of Diet Coke floats • WHY?

  38. Effect of Temperature on Density • As heat is added, molecules move faster and move farther apart • End up with less mass per unit volume • Therefore, as temperature increases, density decreases

  39. Less dense substances float on denser substances • Dry pine wood floats on water • Oil floats on water • Fresh water floats on salt water • Warm water floats on cold water

  40. Ice and Density • As pure water cools, density increases • Maximum D at 4° C ( D = 1 g/cm3) • Below 4° C solid crystal ice structure starts to form and molecules are pushed apart • Same mass but occupying larger volume so lower density • Ice is 10 % less dense than water at 4° C • Ice floats

  41. Note how water molecules are farther apart than in liquid water

  42. Effect of Salt on Density • Dissolved materials increase density of water (more mass per unit volume) • Pure water: D = 1.000 g/cm3 • Fresh water: D ≈ 1.000 g/cm3 • Sea water: D = 1.0278 g/cm • [at 4° C]

  43. Pressure • Increases with depth • For every 10 m (33 ft.) in depth the pressure increases by 1 atmosphere • 1 atmosphere = 14.7 pounds per square inch

  44. Water = Universal Solvent • Essentially everything dissolves in water

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