Properties of Water

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Properties of Water
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Water is unique among all liquids in terms of

• Density
• Melting Boiling Points
• Viscosity
• Specific Heat
• Surface Tension
• Solvent Properties
• Absorption of Radiation

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Structure of Water

• H2O is a polar molecule
• Oxygen end is relatively negative
• Hydrogen end is relatively positive
• H atoms form hydrogen bonds with the O atoms of
  adjacent water molecules
• In liquid water, H-bonds break and form
  constantly, lasting only 10-12 seconds
• In liquid water, on average 3 of four possible
  H-bonds are formed
• Polarity and H bonds result in most of the other
  unique properties of water

U. Texas
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Ice lattice

• In ice, 4 of 4 possible H-bonds are formed and
  molecules are locked in a hexagonal crystalline
  lattice
• Polarity and H bonds result in most of the other
  unique properties of water

Madsci network
www.makingthemodernworld.org.uk/
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Density

• For most substances, density increases as the
  liquid phase cools to become a solid
• In water, density decreases as liquid water
  freezes to become a solid

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Density (contd)

• With decreasing temperature , the density of
  water increases to a maximum (1 gm/cm3) at about
  4 oC.
• Below 4 oC, density decreases slowly, then
  rapidly, as ice forms.
• Significance?
• Lakes freeze from the top down instead of from
  the bottom up. This preserves life.
• Note that above 4 oC, the Temp vs. density curve
  is not linear. This has significance for
  tropical lakes.

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Factors that affect density

• Temperature
• dissolved salts ??
• particulates ??
• dissolved gases - increase (CO2) or decrease
  (CH4) depending on partial molar volume
• pressure - water not very compressible, but ?P
  ?T?max so that _at_ 100 atm P (1000 m) the
  temperature of the density maximum is reduced to
  T?max 2.91?C.

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A note on Units

• In the metric system, water is used to bring
  together units of weight, length, volume, and
  energy.
• This is extremely convenient for aquatic
  ecologists!

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Units

• 1 gram is defined as the mass of 1 cubic
  centimeter of pure water (at 4oC)

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This makes for easy conversions

• For example
• 1 liter of water weighs 1 kilogram
• 1 cubic meter of water contains 1000 liters and
  weighs 1 metric tonne
• 1 liter of water weighs 1 million milligrams,
  therefore the units mg/L and parts per
  million (ppm) are equivalent.
• Also, a calorie is the energy needed to raise the
  temperature of 1 ml of water by 1 oC (at 15 oC)

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• Thus, a Snickers bar (273 kcal) contains enough
  energy to heat 273 L of water from 15 oC to 16 oC

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Properties of water (contd)

• High boiling point means that water can exist as
  liquid over a wide range of temperatures
• High Specific Heat means that water bodies tend
  to maintain a more constant temperature than
  their surroundings useful in agriculture.

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Properties of water (contd)

• High Surface Tension allows a community of
  organisms to live on (and just under) the water
  surface.

R. Suter
Water strider
Mosquito larvae
Chemistryland.com
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• High absorption of infrared radiation causes
  surface of water to heat faster than deeper
  layers. Causes thermal layering.

www.lsbu.ac.uk/water/vibrat.html
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Solvent Properties. Many inorganic molecules
(salts) dissolve in water as well as polar
organic compounds (amino acids, sugars, alcohols)
Wiley.com
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Global Fresh Water Resources
Kalff, Table 4-1
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Water Residence Time

• Days

Years
Decades
Millennia
Atmosphere, Rivers
Wetlands, Soil Moisture
Lakes
Groundwater Glaciers Icecaps

• What are the implications for
• Pollution?
• Aquatic organisms?

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Global Water Availability

• Availability of surface water is NOT growing
• Water usage increases 4-8 per year

United Nations, 2003
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Distribution of Surface water
Number of Lakes
Surface area of Lake (km2)

• There are many more small lakes than large lakes
• But, surface area of 19 great lakes is about 30
  of the total surface area of all lakes combined
• And about 95 of total surface-water volume is
  contained in only 145 lakes

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Great Lakes of the World

• Caspian Sea is the worlds largest saline lake
  (78K km3)
• Lake Baikal is the largest freshwater lake (23K
  km3),with volume equal to the combined
  Laurentian Great Lakes, although its surface area
  is little more than Lake Erie
• Lake Baikal and the LGLs contain about 40 of
  worlds surface fresh water
• Lake Superior is the largest FW lake by surface
  area (82K km2)

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Biases in Lake Studies

• Historical preference for research on small,
  relatively deep, transparent lakes which can be
  studied by an individual researcher. This leads
  to an emphasis on the pelagic zone rather than
  the littoral zone.
• Bias for summer research
• Great Lakes study require more effort (ships,
  expensive equipment, teams of researchers)
• Shallow, muddy lakes were of less interest (less
  vertical structure, less attractive)
• In stream studies, also bias for convenient
  stream size, high clarity, and summer research.