Water: Structure and Dissociation
LEARNING OBJECTIVES

Describe structure of water

Justify that water is an universal solvent.

Justify that water forms the medium of cytosol, ECM and blood.

Discuss the dissociation of water and its contribution in pH.
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Discuss pH with reference to Henderson Hesselbalch Equation.
PROPERTIES OF WATER

Polar molecule
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Cohesion and adhesion
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High specific heat
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Density – greatest at 4oC
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Universal solvent of life
WATER MOLECULE
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One oxygen atom
Two hydrogen atoms
Liquid water
Tetrahedral Geometry
High
Surface Tension
Specific gravity
Boiling point
Universal Solvent
Solid water
Polarity of Water
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Two Hydrogen atoms
One Oxygen atom
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Oxygen is more electronegative, the region around oxygen has a partial
negative charge
The region near the two hydrogen atoms has a partial positive charge.
A water molecule is a polar molecule with opposite ends of the molecule with
opposite charges.
Attractions between these polar molecules.
The slightly negative regions of one molecule are attracted to the slightly
positive regions of nearby molecules, forming a hydrogen bond.
Each water molecule can form hydrogen bonds with up to four neighbors.
HYDROGEN BONDS
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Hold water molecules together
Each water molecule can form a maximum of 4 hydrogen bonds
The hydrogen bonds joining water molecules are weak, about 1/20 th as strong as
covalent bonds.
They form, break, and reform with great frequency
Extraordinary Properties that are a result of hydrogen bonds.
Cohesive behavior
Resists changes in temperature
High heat of vaporization
Expands when it freezes
Versatile solvent
UNIVERSAL SOLVENT

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Solution
o Solute
o Solvent
Aqueous solution
Hydrophilic
o Ionic compounds dissolve in water
o Polar molecules (generally) are water soluble
Hydrophobic
o Nonpolar compounds
Distribution of Water within Human Body
Most
biochemical
reactions
involve
solutes
dissolved in
water.
Most biochemical reactions involve solutes dissolved in water

There are two important quantitative proprieties of aqueous solutions.
o Concentration
o pH
DISSOCIATION OF WATER MOLECULES
The hydrogen atom leaves its
electron behind and is transferred
as a single proton - a hydrogen
ion (H+).
The water molecule that has lost
a proton is now a hydroxide ion
(OH-).
The water molecule with the
extra proton is a
hydrogen/hydronium ion
(H+/H3O+).
Water Molecule Dissociates
Water Molecule Dissociates cont….
1 mole of water= 18 G
1 Liter= 1000G = ? mole
18 Gram = 1 mole
1000 Gram = 1000/18 = 55.56 mole
Molar conc. Of H2O = 1.8 × 109 × 55.56
= Probability × [H2O]
= 1.0 × 10-7
Dissociation of Water Molecules

Water molecule dissociates into a hydrogen ion and a hydroxide ion:
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H2O <=> H+ + OH-
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This reaction is reversible.

At equilibrium the concentration of water molecules greatly exceeds that of H+ and OH-.
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In pure water only one water molecule in every 554 million is dissociated.
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At equilibrium, the concentration of H+ or OH- is 10-7M (25°C) .
Water Molecule Dissociates cont….
pH SCALE

The pH scale in any aqueous solution :
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[ H+ ] [OH-] = 10-14
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Measures the degree of acidity (0 – 14)
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Most biologic fluids are in the pH range from 6 – 8
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Each pH unit represents a tenfold difference (scale is logarithmic)
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A small change in pH actually indicates a substantial change in H+ and OHconcentrations.
ACIDS AND BASES
An acid is a substance that
increases the hydrogen ion
concentration in a solution.
Any substance that reduces
the
hydrogen
ion
concentration in a solution is a
base.
Some bases reduce H+ directly
by accepting hydrogen ions.
Strong acids and bases
completely dissociate in water.
Weak
acids
and
bases
dissociate only partially and
reversibly.
Problem
REFRENCES
Harper chapter 1
Thank you