Water

1. Introduction: the presence of water assures the presence of life

• 95% water

• 15% water

2. Good old H2O: Molecular structure of water

• sp3 hybridization

• electronegativity again

• Key concept: THE HYDROGEN BOND

• hydrogen bonds have 10% covalent character

3. Water Structure

• water behavior movie

4. Properties of water explained by hydrogen bonding

Table 1. Physical properties of water compared with other molecules of similar size

• surface tension (Water Striders; Capillary action: How water gets to the top of a tall tree)

• specific heat or heat capacity: amount of heat that must be absorbed or lost for 1g of that substance to change its temperature by 1 degree C

 (Consequences: Why the lake is warmer in the Autumn; How high specific heat helps mammals to maintain body temperature; Protecting enzymes)

• heat of vaporization: quantity of heat a liquid must absorb for 1 g of it to be converted from the liquid to the gaseous state 

(Consequences: How do plants and animals keep cool?)

• Temperature and density of water; The structure and density of ice  (Ice is less dense and occupies more volume than liquid water) 

Consequences:  Ice floats; Expanding water ruptures cell membranes; cells dehydrate

• How can ice crystal formation be prevented?

•  Fertility Clinics

•   Cryonics and Ted Williams)

• Fish Antifreeze proteins (trout without antifreeze proteins at 0 C)

5. Water as a solvent

• water is called the "universal solvent" (wrong!)

• Hydrophilic and Hydrophobic

• the shape of enzymes in water

6. pH and Buffers

• How ionization of water occurs

• What is a mole?

• The pH scale  

• Buffers:

a solution that resists changes in pH

consists of weak acid, plus conjugate base

       (acid - H⁺ donor; base - H⁺ acceptor)

Example:

bicarbonate buffer in blood

• a weak acid (carbonic acid H₂CO₃) is formed when CO₂ dissolves in water (blood serum):

       CO₂ + H₂O ---> H₂CO₃ 

       This acid is 'weak' because it slowly dissociates into:

        H₂CO₃ ---> H⁺ (protons) + HCO₃^- (bicarbonate ion)

        (bicarbonate ion is the conjugate base)

        These reactions are in equilibrium with the direction determined

        by the concentrations of the reactants

           If acid (H+) is added to the buffer, this will 'push the

        reaction in the direction of carbon dioxide and water:

        H⁺  + HCO₃^- ↔ H₂CO₃ ↔ CO₂ + H₂O  

        If base (OH-) is added, then bicarbonate ion and water is

        formed: 

        OH^- + H₂CO₃ ↔ HCO₃^- + H₂O

• Why are buffers needed in biological fluids?

           enzyme activity and pH

           what happens to enzymes as the pH changes?