Structure and Properties of Water

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Structure and Properties of Water

• Molecular Structure
• Physical Characteristics
• Implications for Life on Earth
• www.sbu.ac.uk/water/anmlies.html

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Why do we care?

• Molecular structure and physical characteristics
  determine important properties
• Physical characteristics serve as a foundation
  for measurement systems
• Physical characteristics support life on the
  planet

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Bipolar molecule

• H2O in purest form
• Two hydrogen atoms and one oxygen atom
• Bipolar structure due to orientation of hydrogen
  and oxygen atoms held in covalent bond

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Significance of bipolar structure

• Covalent bonds result when electrons are shared
  by atoms
• Oxygen is larger than hydrogen so sharing is
  unequal
• The result is an asymmetric distribution of
  charge, leading to a positive and negative end of
  the molecule

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Solubility of Chemical Compounds

• Efficient solvent
• Two mechanisms ionic and hydrogen bonding
• Ionic bonding charged ions of a compound are
  isolated by oppositely charged poles of water
  molecules and separated (the compound
  dissociates)
• Hydrogen bonding compounds are partially
  dissociated and held in solution

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Water as solvent

• Solvent a substance that dissolves other
  substances, thus forming a solution. Substances
  may mix thoroughly so that solute cannot be seen,
  or the solution is visibly changed
• Almost all chemical substances have been found in
  solution in water
• Transfers gases to earth surface
• Dissolved nutrient transfers sustain life

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Attraction between ions and partial charges of
poles
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Hydrogen bonding

• Charge-based bonds
• Weaker than covalent bonds and temporary
• Strong enough to develop forces that determine
  many physical properties

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Hydrogen bonding

• Results in surface tension
• Determines density and molecular structure of
  water and ice
• Determines wetting ability and solubility of
  surfaces and compounds

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Surface tension

• The cohesive force of water or the work that
  needs to be done to pull it apart
• Affects
• Groundwater flow
• Infiltration into soils
• Ability of water surface to support an object
• Examples
• water rise in a capillary tube
• needle floating on water
• the Jesus lizard

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Density and molecular structure of water

• Density Mass per unit volume
• Typically used density of water 1 g/cm3
• Actual range of densities of liquid 1 - .987
  g/cm3 (at 4º and 100º C)

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Density and Molecular Structure

• As many liquids cool, they decrease in volume
  (alcohol)
• Hydrogen bonds are temporary, and the overall
  number of bonds formed is dependent upon
  molecular motion
• Molecular motion decreases as temperature
  decreases

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Density and Molecular Structure

• Increased rigidity and alignment lead to slight
  expansion below 4ºC
• Implications
• Ice is less dense than liquid water and floats
• Water bodies freeze from the surface down, rather
  than the bottom up

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Why is this important?

• Very few life forms can withstand freezing and
  thawing cycles
• Many can withstand cold conditions
• Ice remains on the surface of water bodies
• Life on the bottom is preserved
• Lakes turn over
• Allows for aquatic life in the higher latitude
  regions

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Trends in boiling points in similar compounds
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Wetting Hydrophobic and hydrophilic surfaces

• Hydrogen bonding can occur between molecules of
  chemically diverse substances
• Tendency for water to coat or bead on a surface
  is related to hydrogen bonding that takes place
  on the surface
• Hydrophilic surfaces allow coating because they
  have oxygen with unpaired electrons
• Surface tension forces, caused by hydrogen
  bonding, cause beading on hydrophobic surfaces

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Water as a reference substance

• Bounds for liquid state of water define
  temperature measurement scales
• Example Fahrenheit and Celsius scales
• Energy specific heat and specific heat capacity

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Fahrenheit Scale

• Developed by Gabriel Fahrenheit a thermometer
  maker
• Scale replaced a previous scale based on body
  temperature
• Divided temperature difference between water in
  solid state and in gas state into 180º, with 32º
  as the reference for ice
• Depended upon changes in density of fluids
  (mercury and alcohol)

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Celsius (Centigrade) Scale

• Introduced in 1742 by Andreus Celsius
• A decimal system, with 100º between solid and gas
  phases
• Reference temperature is 0º at freezing
• Question at what temperatures are Celsius and
  Fahrenheit readings the same?

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Modulus of Expansion

• Liquid
• Density varies with temperature (mass/volume)
• Liquid to solid volume expands 9 upon
  freezing
• If temperature changed from 4ºC ? 4ºC, and we had
  a completely full container, how would the volume
  change?

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Modulus of Compression

• Relatively incompressible
• A 1 psi decreases volume by about 3.410-4
• ? a pressure of 100 psi would decrease volume by
  about 3.4 10-2
• Water is efficient in transferring kinetic energy
  when enclosed in a rigid structure

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Water and Energy Transfer

• In liquid phase, water absorbs 1 calorie per
  temperature rise of 1º C/cm3
• An additional 539 calories are needed to go from
  liquid to gas phase
• Going from liquid to solid phase releases 80
  calories per cm3
• Change in state releases or absorbs significant
  amounts of energy
• Liquid water can absorb and store large amounts
  of energy

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Applications

• Sweating vaporization for cooling
• Liquid water on a hot free surface requires 539
  calories per cm3 to become vapor
• The result is substantial loss of thermal energy
  (cooling)
• Heat transfer car cooling systems
• Flowing water efficiently transfers heat away
  from hot spots created by friction

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Global energy transfer

• Equatorial zone receives much more solar energy
  than poles
• Energy transfer from equator is accomplished by
  evaporation, condensation and ocean currents
• Oceans have tremendous capacity to absorb and
  transfer solar energy

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Summary

• Chemical and physical properties of water are
  important and different than many others
• Hydrogen bonding leads to internal cohesion and
  helps in dissolution of some chemicals
• Stability in some properties makes it an ideal
  reference substance
• Also a very good energy transfer medium,
  especially between liquid and vapor states

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Next time

• Watershed Delineation
• Readings p. 175-180
• Homework due Friday
• NOAA visit next Wednesday 9/14