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CHAPTER 3

• Water Is The Biological Medium Without It,
  Life As We Know It, Cannot Exist.

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All 3 Phases Exist Simultaneously
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Life on Earth Began in Water

• 70-95 of a cell is water
• Cells spent their first 3 billion years in water
• Fossil cyanobacteria on land 1.2 bya
• Land plants 500 mya
• Exists in the environment in all 3 phases at the
  same time

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WATERS PROPERTIES ARE DUE TO ITS POLARITY
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WATERS POLARITY RESULTS IN HYDROGEN BONDING
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Hydrogen Bonds

• 1/20th the strength of a covalent bond
• Dipole-dipole interaction
• Results in Emergent properties

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EMERGENT PROPERTIES OF WATER

• Cohesion
• Adhesion
• Temperature Moderation
• Prevents Freezing of Oceans and Lakes
• Medium For Life

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Heat And Temperature

• Heat Total kinetic energy due to molecular
  motion
• Temperature Average kinetic energy
• Heat is measured in calories or joules 1 calorie
  4.18 Joules
• Temperature is measured in degrees

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Properties of Water

• High Specific Heat 1 cal/g/ºC resists
  temperature change
• High boiling point
• High Heat of Vaporization 540 cal/g
• High Heat of Fusion 80 cal/g
• Allows evaporative cooling
• Most dense at 4 ºC
• Ice floats on liquid water
• Excellent solvent for salts and polar substances

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Living Things And pH

• Organisms are extremely sensitive to changes in
  pH (H concentration)
• Slight changes is pH can kill
• Acid rain threatens the environment

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Water Molecules Dissociate
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Kw Pure H2O at 25ºC
Kw H OH-
Kw (10-7)(10-7) Kw 10-14
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What Are Acids And Bases?
Acids Are Proton Donors Bases Are Proton Acceptors
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Whats pH ?
pH -logH For a Neutral Solution, H 10-7
M -log 10-7 -(-7) 7 pH 7 H OH- pH
pOH 14
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Indicators Change Color
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What Are Buffers

• Substances that resist pH changes
• Keep living things within their tolerable pH
  range
• Conjugate acid/base pairs
• Essential to life

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Dissociation Constant
Proton
Salt Form or Conjugate Base
Weak Acid
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Bicarbonate Equilibrium
Ka of H2CO3 4.3 x 10-7 The equilibrium shifts
right or left in response to H
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Extra For Experts Others, Hide Your Eyes!
Henderson-Hasselbach Equation
A- Neutralizes Acid While HA Neutralizes
Base Maximum Buffering Occurs When A- HA or
When the pH equals the pKa Buffering works within
1 pH unit of pKa
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Acid Precipitation

• Rain, Snow or Fog More Acidic Than pH 5.6
• Caused By Pollution of Air With Nonmetal Oxides
  Sulfur Oxides, Nitrogen Oxides
• Source Burning Fossil Fuels
• Increases Mineral Solubility Ca, Mg, Al
• Especially Affects Reproduction

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