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. Why do we care?. Molecular structure and physical characteristics determine important properties

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Structure and properties of water l.jpg
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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104.5°

+

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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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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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 3.4•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


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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


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