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Structure and Properties of Water. Molecular Structure Physical Characteristics Implications for Life on Earth Why do we care?. Molecular structure and physical characteristics determine important properties

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structure and properties of water
Structure and Properties of Water
  • Molecular Structure
  • Physical Characteristics
  • Implications for Life on Earth
why do we care
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
bipolar molecule




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
significance of bipolar structure
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
solubility of chemical compounds
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
water as solvent
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
hydrogen bonding
Hydrogen bonding
  • Charge-based bonds
  • Weaker than covalent bonds and temporary
  • Strong enough to develop forces that determine many physical properties
hydrogen bonding9
Hydrogen bonding
  • Results in surface tension
  • Determines density and molecular structure of water and ice
  • Determines wetting ability and solubility of surfaces and compounds
surface tension
Surface tension

The cohesive force of water or the work that needs to be done to pull it apart


  • Groundwater flow
  • Infiltration into soils
  • Ability of water surface to support an object


  • water rise in a capillary tube
  • needle floating on water
  • the Jesus lizard
density and molecular structure of water
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)
density and molecular structure
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
density and molecular structure14
Density and Molecular Structure
  • Increased rigidity and alignment lead to slight expansion below 4ºC


  • Ice is less dense than liquid water and floats
  • Water bodies freeze from the surface down, rather than the bottom up
why is this important
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
wetting hydrophobic and hydrophilic surfaces
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
water as a reference substance
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
fahrenheit scale
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)
celsius centigrade scale
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?

modulus of expansion
Modulus of Expansion


  • 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?

modulus of compression
Modulus of Compression
  • Relatively incompressible
  • A 1 psi decreases volume by about 3.4•10-4%
  •  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
water and energy transfer
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

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
global energy transfer
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
  • 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
next time
Next time
  • Watershed Delineation
  • Readings: p. 175-180
  • Homework due Friday
  • NOAA visit next Wednesday 9/14