Chemistry, Life and Water II
Think/Pair/Share
What properties of water enabled
evolution of life on earth?
Life
Importance of water
What properties of water enabled
evolution of life on earth?
Water dissolves most things
– Because water is polar, it is an excellent
solvent
Water keeps its cool
– Water’s heat capacity is among the highest of
all materials – resists temperature change
– Oceans warm and cool much more slowly
than the atmosphere
What properties of water enabled
evolution of life on earth?
As water approaches freezing, water gets
less dense
– Freezes from the top down (floats when
frozen)
Cohesiveness
Hydrogen bonds between water molecules
Ice, water, and steam
Hydrogen-Bond Lattice of Water
Water lattice
– Neighboring water molecules form hydrogen
bonds temporarily
– Difficult for nonpolar substances to penetrate
the lattice
– Polar or charged substances penetrate easily
Ice Lattice
Ice lattice
– A rigid, crystalline structure
– Water molecules in the ice lattice are spread
farther apart than in liquid water
– Ice is less dense than water and floats
Hydrogen-Bond Lattice of Water
Fig. 2-12, p. 33
Animation: Structure of water
What properties of water enabled
evolution of life on earth?
Water dissolves most things
– Because water is polar, it is an excellent
solvent
Water is the solvent of life
Water is the solvent of life
Hydrophilic – water loving
Hydrophobic – water repelling
– Non-ionic, non-polar
Polar
(hydrophilic)
regions at the
surface of the
micelle
Nonpolar
(hydrophobic)
ends are
oriented toward
the interior of
the micelle
15
Hydration Layer
Water forms hydration layer over
surfaces of polar and charged biological
molecules, particularly proteins
Separates ions and molecules from each
other so they can enter a solution
– Water = solvent
– Dissolved substance = solute
Hydration Layer
Hydration
layers around
Na+ and Cl–
ions keep salt
in solution
Fig. 2-14, p. 34
Animation: Spheres of hydration
What properties of water enabled
evolution of life on earth?
Water keeps its cool
– Water’s heat capacity is among the highest of
all materials – resists temperature change
– Oceans warm and cool much more slowly
than the atmosphere
High Specific Heat
Water temperature increases slowly as
heat is added
– Must break hydrogen bonds to allow water
molecules to move faster
– Helps moderate and stabilize temperature of
organisms and environment
– c (calorie) = heat to raise 1g of water 1°C
– C (Calorie) = 1,000 calories = 1 kilocalorie
(kcal)
Water moderates temperatures on Earth
Water can absorb or release a large
amount of heat with only a slight change in
its own temperature
Water’s high specific heat
– Water, 1 cal/g/C
– Iron, 0.1 cal/g/ C
High Heat of Vaporization
Water absorbs a large amount of heat to
break loose from liquid water and form a
gas
– Some animals sweat (water evaporation cools
skin and underlying blood vessels)
– Plants evaporate water from leaves (cools
heat absorbed from sunlight)
What properties of water enabled
evolution of life on earth?
As water approaches freezing, water gets
less dense
– Freezes from the top down (floats when
frozen)
Ocean and lakes do not freeze solid
What properties of water enabled
evolution of life on earth?
Cohesiveness
– Hydrogen bonds hold the water molecules
together
– Organisms depend on the cohesion of water
molecules
– Ex. Water transport in plants
Water Molecules Resist
Separation
Cohesion
– Attraction between water molecules
Adhesion
– Attraction of water molecules to surfaces with
charged or polar molecules
Surface Tension
Forms at
surface of
water in
contact with air
Hydrogen
bonds resist
stretching,
giving surface
strength
Fig. 2-15, p. 36
Water has an unusually high specific heat. This is
directly related to which one of the following?
1. At its boiling point,
water changes from
liquid to vapor.
2. More heat is required
to raise the
temperature of water.
3. Ice floats in liquid
water.
33%
1
33%
2
33%
3
Which of the following explains what is happening
when sodium chloride dissolves in water?
1. More hydrogen
bonds are forming
between water
molecules.
2. Sodium and chloride
atoms are separating
from one another.
3. Hydration shells are
forming around the
sodium and chloride
ions.
33%
1
33%
2
33%
3
Predict how life on earth would be different if water were
less polar. Which one of the following would result if
organisms lived in and consisted of a less polar medium
than water?
Heavier insects than
water striders would be
able to walk on the
surface of a pond.
2. Sweating would be a
less effective means of
keeping cool.
3. The water temperature
of ponds and pools
would increase more
slowly when in sunlight.
1.
33%
1
33%
2
33%
3
Water, Lipids, and Membranes
Membrane molecules have one polar end
and one nonpolar (lipid) end
In water, lipid molecules are forced into a
double layer (bilayer) that forms the
membrane
– Hydrophilic ends face water
– Hydrophobic ends associate inside
Membranes: The Lipid Bilayer
Fig. 2-13, p. 34
Water Ionization and
Acids, Bases, and Buffers
Substances act as acids or bases by
altering the concentrations of H+ and OH−
ions in water
Buffers help keep pH under control
Water Ionization
Water dissociates to form ions:
H2O ↔ H+ + OH–
H+ (protons) = hydrogen ions
OH– = hydroxide ions
In pure water, concentration of H+ = OH–
Acids and Bases
Acids release H+ as they dissolve in water
– Solution becomes acidic
HCl ↔ H+ + Cl–
Acids and Bases
Bases gather H+ or release OH− in
solution
– Solution becomes basic
NaOH ↔ Na+ + OH–
pH Scale
Measures relative concentrations of H+
and OH− (acidity) in a water solution on a
scale of 0 to 14
pH = –log10[H+]
Pure water: [H+] = [OH−] = 1 X 10-7 M
– pH 7 (neutral) = –log10[1 X 10-7]
– pH < 7 is acidic, pH > 7 is basic
pH Scale
Fig. 2-16, p. 37
pH animation
pH sliding scale
The hydrogen ion concentration of a solution
whose pH is 9.0 is greater than that of blood
whose pH is 7.3.?
1. True
50%
50%
2. False
1
2
In humans, blood pH is around 7.4, and a
decrease in blood pH to 6.4 would be fatal. A drop
by 1 pH unit represents which of these?
1. twice as many H+
ions in the solution
2. 1/10 as many H+ ions
in the solution
3. ten times as many H+
ions in the solution
33%
1
33%
2
33%
3
The hydrogen ion concentration
of a solution whose pH is 9.0 is
greater than that of blood whose
pH is 7.3. Is this statement true
or false? Explain.
Buffers
Regulate pH of living cells
Absorb or release H+ to compensate for
changes in H+ concentration
Biocarbonate ion (HCO3–), important
base
H2CO3 ↔ HCO3– + H+
Buffers
If cell is too acidic
– Push reaction to the left
– Remove some H+ ions
If cell is too basic
– Push reaction to the right
– Add more H+ ions
Help keep cells close to neutral pH
Blood pH critical
Blood pH 7.35 – 7.45
Breathing too fast removes buffering
capacity from the blood
Carbonic acid-bicarbonate system
Fig. 2-18, p. 39