Ch3, U402

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Water and the Fitness of the Environment

• Ch3, U402

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• Three-quarters of the Earths surface is
  submerged in water
• The abundance of water is the main reason the
  Earth is habitable

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Human density results

• Why is the density of a human very close to 1.0?
• Why are males generally slight more dense than
  females? (physically ?).
• Do note the higher the fat content, the more a
  human will float. Muscle is more dense than fat.
  Championship swimmers often sink because of
  their higher density (more muscle)

For most organisms, over 2/3 of mass is water
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Why water?

• NO living thing can live without it
• In the evolution of life, ubiquitous water
  provided the medium in which molecules could
  interact
• Contrast the amount of life in tropical rain
  forests (water plentiful) to deserts (water poor)

So what makes water SO special?
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• Concept 3.1 The polarity of water molecules
  results in hydrogen bonding
• The water molecule is a polar molecule

• The polarity of water molecules
• Allows them to form hydrogen bonds with each
  other
• Contributes to the various properties water
  exhibits
• Underlies its chemistry and directly and
  indirectly accounts for waters special properties

Water itself is electrically neutral, yet the
molecule has charged regions
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• Concept 3.2 Four emergent properties of water
  contribute to Earths fitness for life

Cohesion Moderation of Temperature Insulation of
Bodies of Water Water as a solvent
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Cohesion Moderation of Temperature Insulation of
Bodies of Water Water as a solvent

• Water molecules exhibit cohesion
• Cohesion
• Is the bonding of a high percentage of the
  molecules to neighboring molecules
• Is due to hydrogen bonding
• Makes water molecularly sticky

Wait! Arent hydrogen bonds weak? Arent they
only 5-10 as strong as covalent bonds? Arent
they very transient?
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• Cohesion
• Helps pull water up through the microscopic
  vessels of plants

Incidentally, water also ADHERES to the inside of
the xylem as well
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• Surface tension
• Is a measure of how hard it is to break the
  surface of a liquid
• Is related to cohesion

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Cohesion Moderation of Temperature Insulation of
Bodies of Water Water as a solvent

• Water moderates temperature changes
• By absorbing heat from air that is warmer and
  releasing the stored heat to air that is cooler
• Water stores heat it heats up slowly

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Heat and Temperature

• Kinetic energy
• Is the energy of motion

• Heat
• Is a measure of the total amount of kinetic
  energy due to molecular motion
• Temperature
• Measures the intensity of heat

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Waters High Specific Heat

• The specific heat of a substance
• Is the amount of heat that must be absorbed or
  lost for 1 gram of that substance to change its
  temperature by 1ºC
• Water has a specific heat of 1.0, ethanol 0.6,
  iron 0.1
• It would take 1 cal to heat 1 g of water 1ºC
• It would take 0.6 cal to heat ethanol the same
  amount
• Touch iron pan on stove iron hot, water not

• Water has a high specific heat, which allows it
  to minimize temperature fluctuations to within
  limits that permit life
• Heat must be absorbed to break hydrogen bonds, so
  that water molecules can begin to move more
• Heat is released when hydrogen bonds form (fast
  moving water molecules slow down enough to form
  hydrogen bonds

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True or False

• The more polar a substance, the higher its
  specific heat.

TRUE
FALSE
NOPE!
YEP!
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Evaporative Cooling (also relates to waters
moderation of temperature)

• Evaporation
• Is the transformation of a substance from a
  liquid to a gas

• Heat of vaporization
• Is the quantity of heat a liquid must absorb for
  1 gram of it to be converted from a liquid to a
  gas

• Evaporative cooling
• Is due to waters high heat of vaporization
• Allows water to cool a surface (carries away
  heat remember, heat caused water molecules to
  break hydrogen bond

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Other hot topics

• Since organisms have a high water content, they
  dont experience rapid changes in temperature
  (including, and especially, ectotherms)
• Water molecules in solution do not have the same
  exact kinetic energy
• Ever watched water boil?
• The average kinetic energy is referred to as

temperature
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Cohesion Moderation of Temperature Insulation of
Bodies of Water Water as a solvent

• Solid water, or ice
• Is less dense than liquid water
• Floats in liquid water

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• The hydrogen bonds in ice
• Are more ordered than in liquid water, making
  ice less dense
• 10 fewer water molecules by volume

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• Since ice floats in water
• Life can exist under the frozen surfaces of lakes
  and polar seas
• Water below ice stays liquid ice insulates water
  below

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Cohesion Moderation of Temperature Insulation of
Bodies of Water Water as a solvent

• Water is a versatile solvent due to its polarity
• It gathers around molecules or compounds that
  have an electrical charge
• It can form aqueous solutions

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• The different regions of the polar water molecule
  can interact with ionic compounds called solutes
  and dissolve them

Water molecules surround something forms a
hydration shell which prevents it from
reconnecting
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• Water can also interact with polar molecules such
  as proteins

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Hydrophilic and Hydrophobic Substances

• A hydrophilic substance
• Has an affinity for water

• A hydrophobic substance
• Does not have an affinity for water (tends to be
  high in C-H bonds, which are non-polar)

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• Concept 3.3 Dissociation of water molecules
  leads to acidic and basic conditions that affect
  living organisms

• Water can dissociate
• Into hydronium ions and hydroxide ions (one H
  leaves, the electron stays behind
• Changes in the concentration of these ions
• Can have a great affect on living organisms

This tends to lose its H
At 25 C, 1 molecule of water about 550 million
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• A liter of water contains 1/10,000,000 moles of
  H
• This can be expressed as 10-7 moles
• So with a concentration of 10-7 moles/liter, we
  can report the hydrogen ion concentration as pH
• pH - log H
• 1/10,000,000 7
• 1/1,000,000 6
• 1/100,000 5

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The pH Scale

• The pH of a solution
• Is determined by the relative concentration of
  hydrogen ions
• Is low in an acid
• Is high in a base

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• The pH scale and pH values of various aqueous
  solutions

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What would cause a solution to have an imbalance
in its H (or OH-) concentration?

• An acid
• Is any substance that increases the hydrogen ion
  concentration of a solution
• HCl ? H Cl-
• A base
• Is any substance that reduces the hydrogen ion
  concentration of a solution
• NH3 H ? NH4
• NaOH ? Na OH-

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Buffers

• The internal pH of most living cells
• Must remain close to pH 7

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• Buffers
• Are substances that minimize changes in the
  concentrations of hydrogen and hydroxide ions in
  a solution
• Act as a reservoir for hydrogen ions, donating
  them when hydrogen ion concentration falls and
  taking them up when it increases
• Consist of an acid-base pair that reversibly
  combines with hydrogen ions
• EXAMPLE
• H2O CO2 ? H2CO3 (hydrogen ion donor) ? HCO3-
  (takes up hydrogen) H
• A salt is an ionic compound formed when an acid
  reacts with a base.
• An example would be the reaction of hydrochloric
  acid (HCl) with sodium hydroxide (NaOH) to
  produce sodium chloride (NaCl) or table salt plus
  water.
• Body fluids contain ions which help to maintain
  homeostasis.
• In the absence of regulation, the pH can change
  too much and cause metabolic failure and death

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The Threat of Acid Precipitation

• Acid precipitation
• Refers to rain, snow, or fog with a pH lower than
  pH 5.6
• Is caused primarily by the mixing of different
  pollutants with water in the air

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• Acid precipitation
• Can damage life in Earths ecosystems

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Effects of acid precipitation