Water (H2O)

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Water (H2O)

• Why water exists and why is it important
• Waters unusual properties
• Hydrogen bonding and polarity
• Liquid state
• Cohesion
• Solvent
• Water and solutions
• Hierarchy theory and the emergent properties of
  water

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Take Home Message

• Water is unique among all liquids and the
  uniqueness plays a major role in life on Earth
  and elsewhere in the solar system
• Waters unusual and unique properties give it a
  place all its own among compounds and molecules
• Water solutions and pH are critical concepts in
  chemistry and biology
• Emergent properties of H2O are a special case
  study of the significance of hierarchy theory

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Elements of the Solar System Role in Living
Systems

• 25 of 92 natural elements are known to be
  essential to life on Earth
• Four of the above make up 96
• Carbon (C), Oxygen (O), Hydrogen (H) and Nitrogen
  (N)
• Compare to solar abundance and discuss relative
  to origin/evolution of life

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Elements of the Periodic Table
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Water and Its Properties Composition

• Elemental composition of H2O
• Hydrogen is the most abundant element in universe
• Oxygen is the 3rd most abundant element
• Hypothesis H2O is common throughout the universe

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Liquid Solution for Living Systems

• Introduction
• Life on Earth in water 4 billion years ago
• First 3 billions years of life in water alone
• All life tied to watery medium (plants, animals
  and microbes)
• Simplicity and complexity of the nature of the
  water molecule
• Deceptively simple in structure
• Exquisite in function

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Water and Its Properties Polarity

• Composition and structure a polar molecule
• Features
• Attraction is electrical
• Hydrogen bonding among two or more molecules of
  H2O
• Exquisite properties of H2O arise from chemical
  attractions among molecules because H2O is a
  polar molecule

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Water and Its Properties Polarity
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Water and Its Properties Phase Diagram
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Water and Its Properties Liquid State

• Water is liquid over broad range of temperatures
• 0oC to
  100oC
• Comparison with other compounds
• Compound Chemical Formula Freeze Vapor Range
  (oC) (oC) (oC)
• Water H2O
  0 100 100
• Ammonia NH3 -78
  -33 45
• Methane CH4 -182
  -164 18

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Water and Its Properties Liquid State

• Significance of broad range of temperatures for
  water to exist as a liquid
• Water in a liquid state is not too cold to slow
  reactions and not too hot to break bonds
• Could life evolve in another phase (i.e., gas,
  solid)?

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Water and Its Properties Cohesion of Molecules

• Cohesion H2O molecules are sticky
• Each hydrogen bond last 1 x 10-12 seconds
• Constant formation and breakage of bonds any
  moment, a substantial number of H2O molecules are
  bonded to its neighbors
• Consequence more structure than other liquids

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Water and Its Properties Freezing

• Water is unusual in that H2O is less dense as a
  solid than a liquid (it floats)
• Mechanism
• H2O expands when it solidifies
• Due to hydrogen bonding
• Consequence
• Ponds, lakes and ocean freeze from the top down
• Consequence for living system?

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Water and Its Properties Freezing
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Water and Its Properties Solvent of Life

• Water is versatile solvent, due to its polarity
  and dissociation
• H2O H
  OH-
• H2O H H3O
  (hydronium ion)
• In pure water, 1 molecule in every 554 million is
  dissociated (10-7 molar concentration)pH scale
• Statistically very rare BUT exceedingly important
  in chemistry of life (e.g., proteins,
  macromolecules)
• H and OH- are very reactive and affect much of
  the chemistry of living systems (e.g., membranes,
  metabolism)

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Water and pH

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Water and Solutions

• Solutions mixture of ions or molecules of 2 or
  more substances
• Air as a mixture (O2 in N2)
• Tea as a mixture (aqueous if water))
• Coffee as a mixture (aqueous)
• Solvent (majority) and solute (minority)

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Water and Solutions Concentrations

• Concentrations of solutes and solvents
• Parts per million (ppm) or billion (ppb)
• Atmosphere and carbon dioxide (370 ppm)
• Percentage by mass
• Salinity of ocean water - mass of salts in 1,000
  grams of water (35 g in 1000 g water)
• Chemistry molarity of solution
• moles of solute dissolved in 1 liter of solvent
  (H2O)

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Water and Solutions Concentrations

• Moles
• Little furry critters with no eyes that live in
  tunnels in the ground 3 moles per acre is a 1
  molar solution
• Moles to a chemist are not furry animals
• 1 mole is 1 gram molecular weight of a substance
  (atom, molecule), and it always contains the
  exact same number of atoms or molecules
• 6.02 x 1023 atoms Avogadros Number (not
  avocado)
• 1 mole of C is 12 grams and it contains 6.02 x
  1023 atoms
• 1 mole of Na is 23 grams and it contains 6.02 x
  1023 atoms
• 1 mole of H2O is 18 grams and it contains 6.02 x
  1023 molecules of H20
• Very precise way of measuring concentration

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Hierarchy Theory and Emergent Properties of H2O

• Principle of hierarchy theory
• Principle of emergent properties
• Example of H2O in Periodic Table
• a priori combine one atom of O with two atoms of
  H
• Emergent properties
• Liquid
• Hydrogen bonding and polarity
• H and OH- in solution
• Solvent
• Range of temperature at which liquid
• Three phases (gas, liquid and solid)

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Question

• Substance A has a pH of 2 and Substance B has a
  pH of 3. This means that ____.
• A. Substance A is more basic than Substance B
• B. Substance B is more acidic than Substance A
• C. Substance A is 10 times more acidic than
  substance B
• D. Substance B is 10 times more acidic than
  substance A

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Questions

• A deep lake in Minnesota is covered with ice.
  What is the water temperature at the bottom of
  the lake?
• In the summer in Virginia, pond water
  temperature is very warm (30 0C). In the fall,
  the drop in night time air temperature results in
  surface water temperature falling (e.g., 20 0C).
  What would you expect to happen in small ponds as
  the surface water temperature is cooler than the
  water temperature at the bottom of the pond (30
  0C)?

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Question

• How does ocean water in proximity to continents
  affect the climate of the adjoining land mass?
  Explain your analysis based on the physical and
  chemical properties of water.

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Question

• In the search for life on other planets and in
  other solar systems, why is the presence and/or
  absence of water regarded as one of the cardinal
  indicators of the possibility of life?

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Take Home Message

• Water is unique among all liquids and the
  uniqueness plays a major role in life on Earth
  and elsewhere in the solar system
• Waters unusual and unique properties gives it a
  place all its own among compounds and molecules
• Water solutions and pH are critical concepts in
  chemistry and biology
• Emergent properties of H2O are a special case
  study of the significance of hierarchy theory in
  the natural sciences