Topic 8 Properties of Water

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Topic 8Properties of Water

• GEOL 2503
• Introduction to Oceanography

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H20

• Englishman, Sir Humphrey Davey discovered the
  formula for water H2O
• 3 Atoms two hydrogen one oxygen
• H H O2-

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Polar Molecule

• Positive end
• Hydrogens on tips of V
• Negative end
• Oxygen on other end
• Molecules attract one another

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Properties of Water

• Thermal (Heat Capacity)
• Cohesion
• Surface Tension
• Viscosity
• Transmission of light, sound

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Thermal Properties of Water

• Phase changes
• Heat capacity

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Changes of State
Solid - ice
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
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January 8, 2010 8 AM
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January 8, 2010 2 PM
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To change a state of matter

• Need time to add or remove heat
• Need time for the hydrogen bonds to break

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Pure Water

• 100 waternothing else
• No suspended particles
• No dissolved substances including gases

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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

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For example

• Take solid water (ice)
• Add heat
• Temperature rises above freezing point
• Ice melts
• Forms liquid water

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Another example

• Start with liquid water
• Add heat
• Temperature rises
• Water evaporates to form gaseous water (water
  vapor)

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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

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Heat is not the same as temperature
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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

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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

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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

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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

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Why?

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

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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
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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

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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

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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 ?
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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

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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)

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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)

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Evaporation is a cooling process

• Youve probably heard that phrase
• To evaporate water takes 540 calories of heat per
  gram
• Essentially absorbing heat
• Examplecooling our bodies (removing heat energy)
  by evaporation of perspiration

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Heat Capacity Summary

• Day/night change of water temperature is very
  small
• Helps redistribute heat around Earth
• Helps stabilize Earths surface temperatures

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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

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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

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Viscosity

• Resistance to flow
• High viscosity means high resistance to flow
• Affected by temperatureslow as molasses in
  January

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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

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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 astronautsnormal weight on Earth,
  weightless in space, 1/6th of Earth weight on
  moon, but their mass is constant

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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)

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Coke Can Density Experiment

• Can of regular Coke sinks in water
• Same size can of Diet Coke floats
• WHY?

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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

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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

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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

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Note how water molecules are farther apart than
in liquid water
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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

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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

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Water Universal Solvent

• Essentially everything dissolves in water

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Light

• Visible light is narrow band of electromagnetic
  spectrum
• Visible light is broken into rainbow ROY G BIV
• Red absorbed within 33 ft. in ocean
• Blue-green light spectrum goes deeper
• Objects seen in natural color at the surface
• Objects in deeper water appear dark because
• illuminated by blue light

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The Electromagnetic Spectrum

Violet Blue Green Yellow Orange Red
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Visible Light Seawater

• 60 is absorbed in 1st meter
• 80 is absorbed by 10 m (33 ft)
• 99 is absorbed by 150 m (500 ft)
• 100 is absorbed by 1000 m (3300 ft)

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Why is the Sea Blue?

• Color reflected light
• Red end of spectrum absorbed
• Blue-green end reflected back
• We see the reflected blue end of the spectrum
• ROY G BIV
• Absorbed Reflected back

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Transmission of light in water is measured using
a Secchi disk
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The Secchi Disk measures water clarity. Simply
lower the disk and record water depth at which
you can no longer see it. Very low-tech, but
effective measure.
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Sound

• Travels faster in water than air
• Velocity 1500 m/s (5000 ft/s)
• In dry air 334 m/s (1100 ft/s)
• Speed increases if
• Temperature increases
• Pressure increases
• Salt increases

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PDR and DSL

• PDR Precision depth recorder
• Uses narrow sound beam traces bottom while ship
  is in motion
• Echo Finders fish finders
• DSL Deep Scattering Layer
• Organisms/fish found by depth recorders (creates
  false bottom on depth recorder)

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SONAR(Sound Navigation Ranging)

• Underwater location system
• Locates objects and finds targets
• Echo reflected from target
• Sound shadows formed
• Sound changes speed as passes through different
  densities of water

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SOFAR Sound Channel

• Sound is transmitted more effectively and rapidly
  in water than it is in air
• Sound velocity in the ocean is about 1500 m/s, or
  about 4-5 times that in air
• Sound is focused into the SOFAR channel because
  of its low velocity region
• Sound is transmitted best through this channel --
  important for submarines and cetaceans

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SOFAR Channel Experiments