Figure 3-01
Water is Central to Life
• Water has several properties that life is
dependent upon including:
• Cohesion
• High heat of vaporization
• High specific heat
• Less dense as a solid than a liquid
• A “universal solvent”
• All of these properties are due to the polar
nature of water molecules and hydrogen
bonding
LE 3-3
Water-conducting cells
100 µm
Figure 3-04
LE 3-6
–
Na+
+
+
–
–
+
–
–
Na+
–
+
+
Cl–
Cl–
+
–
+
+
–
–
–
–
Dissociation
• Many molecules will dissociate when the
are placed in water
• Ions are separated and “released” into the
water environment
• Table salts dissolves in water (dissociates
into sodium and chlorine ions)
Acids
• When hydrogen chloride is placed in water
it dissociates into Hydrogen ions (H+) and
chlorine ions (Cl-)
• HCl is an acid
• Acids are substance that release H+ ions
(protons) when they dissociate in water
• The more acidic a solution, the higher
concentration of H+ ions (protons) it has.
Bases
• When sodium hydroxide is placed in
water it dissociates into sodium ions (Na+)
and hydroxide ions (OH-)
• NaOH is a base
• Bases are substance that release OHions when they dissociate in water
• The more acidic a solution, the higher
concentration of OH- ions it has.
Water Dissociates
• Water molecules are not completely stable
• They spontaneously dissociate (at a very
slow rate) into H+ and OH- ions.
• At any given moment only 1/554,000,000
water molecules in pure water dissociates
LE 3-UN53
Hydronium
ion (H3O+)
Hydroxide
ion (OH–)
Spontaneous Dissociation of Water
• Is water and acid? Is it a base?
• Water is amphoteric
• At equilibrium, the [H+] concentration is
1X10-7 M.
• At equilibrium, the [OH-] concentration is
1X10-7 M.
pH Scale
• pH is the –log [H+]
• The pH scale measure the acidity (or
basicity) of an aqueous solution
• The pH scale range from 0-14
• The lower the pH, the higher the [H+]
• The higher the pH the lower the [H+]
• A pH of 7 is considered to be neutral
Ph Scale
LE 3-8
pH Scale
0
Increasingly Acidic
[H+] > [OH–]
1
Neutral
[H+] = [OH–]
Battery acid
2 Digestive (stomach)
juice, lemon juice
3 Vinegar, beer, wine,
cola
4 Tomato juice
5 Black coffee
Rainwater
6 Urine
7 Pure water
Human blood
8
Increasingly Basic
[H+] < [OH–]
Seawater
9
10
Milk of magnesia
11
Household ammonia
12
Household bleach
13
Oven cleaner
14
The importance of pH to life
• Living things typically exist is an aqueous
environment
• The pH of the environment influences the
charge of molecules present in that
environment
• The charge of molecules influences their
shape (structure)
• The shape of molecules determines their
function
Case study-pH and Blood
• The normal pH of blood is about 7.4
• What are the consequences of a change
in that pH?
• Effect on hemoglobin?
• Why is hemoglobin affected by a change
in pH?
• What homeostatic mechanisms does the
blood have to keep its pH constant?
Buffering Systems