Water
Importance of Water
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More than 70% of our total body weight is water
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Necessary for photosynthesis
H becomes incorporated into organic compounds
 Oxygen released for us to breathe
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Solvent for most biochemical reactions
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Important reactant/product
Water is a polar molecule
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Due to differences in electronegativities, water is
slightly charged at its poles
Oxygen takes on a slight – charge
 Hydrogens take on a slight + charge
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http://programs.northlandcollege.edu/biology/
Biology1111/animations/hydrogenbonds.html
Hydrogen Bonding
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Water can form H-bonds with up to 4
neighboring water molecules
It is water’s polarity that gives it
many of its unique properties
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Less dense as a solid than a liquid
Universal solvent
Adhesion and cohesion
Capillary action
 Surface tension
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High heat of vaporization
High specific heat
Water is the universal solvent

Water can dissolve many hydrophilic substances
Ionic compounds
 Other polar compounds
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Form “spheres of hydration”
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http://www.mhhe.com/physsci/chemistry/esse
ntialchemistry/flash/molvie1.swf
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Some substances do not dissolve readily in water
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Hydrophobic – “water-fearing”
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Non-polar substances like lipids
Cohesion

Water molecules have a strong tendency to stick
to one another
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Cohesion allows water to have a high degree of
surface tension

Any force is transmitted to the column of water as a
whole
Adhesion

Ability of water to stick to other substances, esp.
charged atoms or molecules
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Together, cohesion and adhesion allow for
capillary action
How Unique is Water??
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Water is one of only 3 naturally occurring
inorganic liquids (mercury and ammonia)
Only chemical compound that exists in all 3
states—solid, liquid, and gas
Extremely large liquid range (0oC - 100oC)
Expands, becomes less dense as a solid
Water’s 3 states differ in
the degree of H-bonding
http://mutuslab.cs.uwindsor.ca/schur
ko/animations/waterphases/status_wa
ter.htm
Liquid water has
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H-bonds that form and break constantly
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http://www.stolaf.edu/people/giannini/flashanimat/w
ater/water.swf
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Allows water to have a high specific heat

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Amount of energy required to raise temp of 1 g 1o C
1 cal/1 g
Water Vapor

As water moves from liquid to gaseous state, Hbonds are broken, allowing water molecules to
escape
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Water has a high heat of vaporization:
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It takes 540 cal for 1 g of water to move from liquid
to gaseous state
Allows for evaporative cooling
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As fast-moving liquid water molecules escape as
vapor, they take their heat energy with them
Ice
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Solid water is less dense than liquid water,
allowing it to float
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H-bonds lock into lattice structure
Acids and Bases
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Water molecules have slight tendency to
ionize:
H2O < -- > H+ + OH-
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The H+ then joins another water molecule
resulting in H3O+ (hydronium)
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The pH scale is a measure of hydronium
concentration expressed in moles/liter
pH scale
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http://www.johnkyrk.com/H2O.html
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pH = -log10[H+]
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Acids are proton donors
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Increase the # of H+ ions
Bases are proton acceptors
Acid + Base  Salt
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Acid = H+ + anion
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Base = OH- + cation
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H+ joins with OH- to get H2O
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Anion can combine with cation to make a salt
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Ex. HCl + NaOH  H2O + NaCl
Buffers
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Substance(s) that resist pH changes when an
acid or base is added
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Usu. weak acid or base, do not completely ionize
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Example: Blood in Vertebrates
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CO2 + H20 < -- > H2CO3 < -- > H+ + HCO3-
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Will stay at dynamic equilibrium unless stressed
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If add excess H+ system shifts left and forms carbonic acid
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If add OH- they combine with H+ forming water, system shifts
right
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http://www.tvdsb.on.ca/westmin/science/sbioac/biochem/buf
fer.htm
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http://www.mhhe.com/physsci/chemistry/essentialch
emistry/flash/buffer12.swf