Water and the Fitness of the Environment

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Water and the Fitness of the Environment
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Polarity of Water

• 2 hydrogen atoms joined to oxygen by covalent
  bonds
• Oxygen is more electronegative than hydrogen
• Electrons spend more time closer to the oxygen
  atom
• Polar covalent bonds
• Oxygen region partially negative
• Hydrogen region partially positive
• Opposite ends of molecule have opposite charges-
  polar molecule

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Water Molecule
Neighboring molecules held by a hydrogen bonds
Oxygen is slightly negative (more electronegative)
Hydrogen is slightly positive (less
electronegative)
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Water Properties

• Cohesion
• Water molecules stick to each other due to
  hydrogen bonding
• Hydrogen bonding is weak (in liquid water)-
  continually breaking and reforming
• Cohesion allows transport of water against
  gravity in plants
• Adhesion
• Clinging of one substance to another
• Water adheres to the walls of vessels
• Surface tension
• Measure of how difficult to stretch or break the
  surface of liquids
• Water has a greater surface tension
• Ex some insects can walk on water

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Water Properties
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Water Properties

• Water is a heat sink
• Can absorb or release a great amount of heat,
  with very little change in own temperature
• Due to H bonding when heat is added, a lot of
  energy goes into breaking H bonds and not
  directly to making molecules move faster
• (result water can absorb large amounts of heat
  while its temp. changes very little)
• Allows body to maintain constant temp.
• Temps in aquatic environments change little
• Oceans absorb and redistribute heat (moderate
  climate on Earth)

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Heat is absorbed in the bonds
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High Specific Heat

• High Specific Heat
• Specific heat is the amount of heat that must be
  absorbed or lost for 1g of substance to change
  its T by 1 degree C
• Specific heat of water is 1 calorie p/g/C
• Ex ethyl alcohol has a specific heat of .6
  cal/g/C
• Due to hydrogen bonding (absorbed heat is used to
  disrupt H bonds before water molecules begin
  moving faster)

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Water absorbs a lot of heat when it evaporates

• Heat of vaporization is the quantity of heat a
  liquid must absorb to be converted from liquid to
  a gaseous state
• Water has a high heat of vaporization
• Breaking H bonds to cause evaporation takes a
  large amount of energy
• Ex water heated on a stove (molecules have more
  kinetic energy and hydrogen bonds break more
  readily)
• This property helps moderate the climate (lg.
  amt. of solar heat absorbed by oceans is consumed
  during evaporation of surface water)

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Evaporative cooling

• Evaporative cooling
• As liquid evaporates, the surface of the liquid
  that remains behind cools down
• hottest molecules those w/ greatest kinetic
  energy, are more likely to leave as gas
• Cooling of organisms (plants, animals, lakes.)

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Water expands when it freezes

• Ice floats
• In liquid water, H bonds are continually breaking
  and reforming when water freezes, hydrogen
  bonds become frozen in place in a crystalline
  structure
• This causes the molecules to be further apart
  than in the liquid state
• As a solid, it is less dense
• Allows life to exist in bodies of water during
  cool seasons (insulates bodies of water beneath
  frozen surface)

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Water is a solvent

• A liquid that is a completely homogenous mixture
  of 2 or more substances is a solution
• Dissolving agent - solvent
• Substance being dissolved - solute
• Aqueous solution is when water is the solvent

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Bottom Line.

• Water molecules are polar
• Water expands when it freezes
• Water absorbs a great deal of heat when it
  evaporates
• Water absorbs a large amount of heat when it is
  heated
• Water is cohesive and has high surface tension
• Water is an excellent solvent for a large variety
  of molecules
• Water dissociates to form protons and hydroxyls
  in solution

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Hydro- philic and phobic

• Hydrophilic- water loving
• Any substance that has an affinity for water
• Hydrophobic- water fearing
• Substances that repel water (non-ionic or non
  polar)

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Biological Chemistry

• Most chemical reactions in biology occur in
  aqueous solutions - involve solutes dissolved in
  water
• Substances are measured in units called moles
  (mol)
• A mole is equal in number to the molecular weight
  of the substance, but up scaled from daltons to
  grams
• Molarity is the number of moles of solute per
  liter of solution

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Example

• To measure 1 mol of table sugar
• Formula C12H22O11
• Determine molecular weight of entire molecule
• Carbon 12 daltons (12 x 12 144)
• Hydrogen 1 daltons (1 x 22 22)
• Oxygen 16 daltons (16 x 11 176)
• Total 342 daltons
• To obtain 1 mol of sucrose, we weigh 342 g
• Molecular weight expressed in grams
• To obtain 1 M solution of sucrose in water
• Dissolve 342 g. until sugar dissolves
• Bring total volume to 1 L
• This would give us 1 M solution of sucrose

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Dissociation of Water Molecules

• Hydrogen atoms shared by 2 water molecules may
  shift from one molecule to another
• Causes the H atom to leave its electron behind
• Hydrogen ion is transferred (single proton )
• Water molecule that lost a proton is now a
  hydroxide ion (OH-)
• Proton binds to another water molecule, which
  then becomes a hydronium ion (H3O)
• Simplified it is referred to as
• H2O H OH-
• Water dissociates into hydrogen ion and
  hydroxide ion

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pH Changes

• Hydrogen and hydroxide ions are very reactive
• Changes in concentration greatly affect a cells
  proteins and other molecules
• Concentrations are equal in pure water, adding
  solutes (acidic or basic) can alter balance
• pH scale is used to determine concentration of
  these ions in a solution

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Acids and Bases

• When acids dissolve in water they donate H to
  the solution
• An acid is a substance that increases the H
  concentration
• A substance that reduces the H concentration is
  a base
• By accepting H
• Or by dissociating to form hydroxide ions that
  combine with H to form water

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PH Scale
Higher concentration of Hydrogen ions H
Higher concentration of Hydroxide ions OH-
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pH scale

• In any solution, the product of H and OH-
  concentrations is constant at 10 -14
• HOH- 10 -14
• Neutral solution (7) H 10-7 OH-
  10-7
• If H increased to 10-5 M, then OH- would
  decline to 10-9 M
• 10-9 x 10-5 10-14
• Using logarithms, pH scale is simplified
• Negative logarithm (base 10) of the hydrogen ion
  concentration pH -logH
• H is 10-7, this gives us -log 10 -7 - (-7)
  7

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10
x
10
100
Each step is ten fold Ex How many more H at
pH of 1 than pH of 3?
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Buffers

• Internal pH of living cells is close to 7
• Slight changes can be harmful
• Buffers are substances that minimize changes in
  the concentrations of H and OH- in a solution
• Buffers work by accepting H when in excess or
  donating H if depleted
• Buffer found in human blood (carbonic acid
  H2CO3)
• Dissociates into HCO3- and H (bicarbonate ion
  and hydrogen ions)

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