Water and the Fitness of the Environment

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Water and the Fitness of the Environment

• Chapter 3

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Water

• Life depends on water
• Human body and cells ? 70-95 water
• Abundance of water ? Earth can support life
• Water exists in three possible states ice,
  liquid, and vapor

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Properties of Water

• Polar Molecule
• Cohesion- Adhesion
• High Surface Tension
• High Specific Heat
• Low Density of ice
• Excellent solvent

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Structure of Water

• Water is a polar molecule ? uneven distribution
  of charge between the hydrogen and oxygen atoms ?
  regions of partial and charges
• Each water molecule can form H- bonds with up to
  4 neighbors.

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Structure of Water
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Cohesion - Adhesion

• Cohesion ? tendency of molecules of the same kind
  to stick together. Strongest in water.
• Adhesion ? attraction between unlike molecules

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Cohesion - Adhesion
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Surface Tension

• Surface Tension ? a measure of how difficult it
  is to stretch or break the surface of a liquid
• At the surface water molecules are H-bonded to
  one another and to the water molecules below?
  resist stretching or breaking the surface
• Water behaves as if covered by an invisible film.

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Water Moderates Temperatures on Earth

• Water stabilizes air temperatures ?absorbing heat
  from warmer air and releasing heat to cooler air.
• Water is a heat bank? can absorb or release
  relatively large amounts of heat with only a
  slight change in its own temperature

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Heat and Temperature

• Kinetic Energy ?the energy of motion
• Heat is a measure of the total quantity of
  kinetic energy due to molecular motion in a body
  of matter.
• Temperature measures the intensity of heat due to
  the average kinetic energy of molecules
• When two object of different temperature meet,
  heat passes from the warmer to the cooler until
  the two are the same temperature.

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• Temperature is measured on the Celsius scale
  (oC).
• At sea level, water freezes at O oC and boils at
  100oC.
• Human body temperature averages 37 oC.
• Heat energy calorie (cal)
• One calorie is the amount of heat energy
  necessary to raise the temperature of 1 g of
  water by 1oC.
• Biological processes? the kilocalorie (kcal)
• A kilocalorie is the amount of heat energy
  necessary to raise the temperature of 1000g of
  water by 1oC.
• The joule (J), is equivalent to 0.239 cal

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High Specific Heat

• Waters high specific heat is due to hydrogen
  bonding
• Water resists changes in temperature because it
  takes a lot of energy to speed up its molecules.

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Evaporative Cooling

• As a liquid evaporates, the surface of the liquid
  that remains behind cools - evaporative cooling.
• This occurs because the most energetic molecules
  are the most likely to evaporate, leaving the
  lower kinetic energy molecules behind.
• Evaporative cooling moderates temperature in
  lakes and ponds and prevents terrestrial
  organisms from overheating.
• Evaporation of water from the leaves of plants or
  the skin of humans removes excess heat.

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Low Density of Ice

• Water is unusual because it is less dense as a
  solid than as a liquid.
• Ice floats on the cool water below.
• This has important consequences for life.

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Low Density of Ice

• If ice sank ? all ponds, lakes, and oceans would
  freeze solid.
• The surface layer of ice insulates liquid water
  below, preventing it from freezing and allowing
  life to exist under the frozen surface.

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Water is the Solvent of Life

• A liquid that is a completely homogeneous mixture
  of two or more substances is called a solution
• The dissolving agent is the solvent and the
  substance that is dissolved is the solute.
• In an aqueous solution, water is the solvent.

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Water as an Aqueous Solution

• C6H12O6? 6(12)12(1)6(16)180g MW
• 180g 1 mole of glucose
• Add 1L H2O
• Avogadro's number 6.02x1023 molecules /mole

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Water surrounds and separates the positive and
negative ions
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The Dissociation of Water

• A hydrogen atom shared by two water molecules
  shifts from one molecule to the other.
• leaves its electron behind and is transferred as
  a single proton - a hydrogen ion (H).
• molecule that lost a proton is now a hydroxide
  ion (OH-).

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Dissociation of Water

• Water molecule dissociates into a hydrogen ion
  and a hydroxide ion
• H2O ltgt H OH- (reaction is reversible)
• At equilibrium the concentration of water
  molecules greatly exceeds that of H and OH
• In pure water only one water molecule in every
  554 million is dissociated.
• At equilibrium the concentration of H or OH- is
  10-7M (25C)
• Adding acids or bases can change this equilibrium
  and modify the the conc. of H and OH-

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Acids and Bases

• Acid ? compound that donates H ions
• Hydrochloric acid, added to water, hydrogen ions
  dissociate from chloride ions
• HCl -gt H Cl-
• Addition of an acid makes a solution more acidic

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Acids and Bases

• Base? compounds that donate OH- ions or removes
  H ions
• Some bases reduce H directly by accepting
  hydrogen ions.
• Ammonia (NH3) acts as a base when the nitrogens
  unshared electron pair attracts a hydrogen ion
  from the solution, creating an ammonium in
  (NH4).
• NH3 H ltgt NH4
• Other bases reduce H indirectly by dissociating
  to OH- that combines with H to form water.
• NaOH -gt Na OH- OH- H -gt H2O
• Solutions with more OH- than Hare basic
  solutions.

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pH Scale

• The scale that describes how acid or basic a
  solution is.
• Ranges from 0-14
• pH 2 is 10X more acidic than pH 3
• pH 7 is neutral

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pH Scale

• In any solution the product of their H and OH-
  concentrations is constant at 10-14.
• pH -logH
• H OH- 10-14
• If H 10-5 M,then OH- 10-9 M
• In a neutral solution, H 10-7 M and OH-
  10-7 M, pH 7

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pH Scale

• The pH of a neutral solution is 7.
• Acidic solutions have pH values less than7 and
  basic solutions have pH values more than 7.
• Most biological fluids have pH values in the
  range of 6 to 8.
• However, pH values in the human stomach can reach
  2.
• Each pH unit represents a tenfold difference in
  H and OH- concentrations.
• A small change in pH actually indicates a
  substantial change in H and OH- concentrations

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Buffers

• Substances that can prevent or resist sharp
  changes in pH
• Important because molecules in cells are very
  sensitive to concentrations of H or OH- ions.
  Even the slightest maybe harmful to living things

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Buffers typically consist of a weak acid and its
corresponding base

• Buffer in human blood and other biological
  solutions is carbonic acid.
• The chemical equilibrium between carbonic acid
  and bicarbonate acts at a pH regulator.
• The equilibrium shifts left or right as other
  metabolic processes add or remove H from the
  solution.

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Carbonate-bicarbonate buffer
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Acid Precipitation

• Uncontaminated rain has a slightly acidic pH of
  5.6.
• The acid is a product of the formation of
  carbonic acid from carbon dioxide and water.
• Acid precipitation occurs when rain, snow, or fog
  has a pH that is more acidic than 5.6.
• Effects of acid rain??