Water & Life
CAMPBELL AND REECE
CHAPTER 3
Hydrogen Bonding
 2 characteristics contribute to H-
bonds
1. polar covalent bond between H & O
2. molecule is bent (V-shaped)
Water in Liquid State
 H-bonds ~ 1/20th as strong as
covalent bond
 bonds constantly being
formed/broken (each lasts a few
trillionth of a sec.)
 @ any given moment a substantial %
of all water molecules are part of a Hbond
H-bonds in Water
4 Emergent Properties of Water
 all 4 contribute to suitability for life
on Earth
1. Cohesion
2. Moderation of Temperature by
Water
3. Ice Floats
4. Water is Universal Solvent
Cohesion of Water Molecules
 H-bonds hold water molecules
together = cohesion (like molecules
sticking together)
 contributes (with adhesion) to the
transport of water & nutrients against
gravity in plants
Water & Nutrients Move up the Xylem
Surface Tension
 a measure of how difficult it is to
stretch or break the surface of a liquid
 Water has greater surface tension
than most other liquids due to Hbonding
Moderation of Temperature by Water
 Water moderates temperature by
absorbing heat from air that is
warmer and releasing stored heat
energy to air that is cooler.
 Water can absorb or release a
relatively large amount of heat with
only a slight change in its own
temperature.
Citrus Trees Sprayed with Water to
Prevent Frost Damage
Heat & Temperature
 heat: a measure of matter’s total KE
due to motion of its molecules (so heat
depends on volume of matter)
 temperature: a measure of heat
intensity; represents the average KE
of its molecules regardless of volume
Calorie
 the amt of heat it takes to raise the
temperature of 1 g water 1 ºC

or
 the amt of heat that 1 g water
releases when it cools by 1ºC
Kilocalories
 a kilocalorie (kcal): quantity of heat
required to raise temperature of
1,000 g of water by 1ºC
 Food packaging calls a kcal “calorie”
 1 joule (J) = 0.239 cal
 1 cal = 4.184 J
Specific Heat
 is the amt of heat that must be
absorbed or lost for 1 g of the
substance to change its temperature
by 1ºC
 specific heat of water is high which
explains its ability to stabilize
temperature
Specific Heat of Water
 1 cal/g x ºC
 specific heat of water high compared
to other materials, water changes its
temperature less when it absorbs or
loses a given amt of heat
Specific Heat of Water
 due to H-bonding
 absorbed
heat must 1st break all the Hbonds b/4 it increases the average KE of
its molecules
 when temperature drops slightly, many
additional H-bonds form  releases
additional energy in form of heat
Water’s High Specific Heat
 allows water to absorb and store a
large amt of heat from Sun & warm up
only a few degrees
 @ night & during winter the gradually
cooling water warms the air
Water’s High Specific Heat
 the water that covers most of Earth
keeps temperature fluctuations on
land & water w/in limits to permit life
on Earth
Moderating Effects of Large Bodies of
Water on Temperatures
Evaporative Cooling
 temperature is average KE of
molecules
 some evaporization of water occurs @
any temp because some individual
molecules of water my gain enough
KE to  gas phase
Heat of Vaporization
 = quantity of heat a liquid must
absorb for 1 g liquid phase  gas
phase
 water has high heat of vaporization
relative to most other liquids because
of H-bonding: absorbed energy must
1st break H-bonds, then increase KE of
molecules
High Heat of Vaporization of Water
 Global scale: helps moderate Earth’s
climate
Evaporative Cooling
 = as liquid evaporates the surface of
the liquid that remains cools down
 contributes to stability of temperature
in lakes & ponds
 provides mechanism to keep
terrestrial organisms from
overheating
Evaporative Cooling
Ice Floats
 water one of few substances that is
less dense in its solid phase than in its
liquid phase
 water expands when goes from liquid
 solid
 > 4ºC water acts like other liquids:
expands as it warms up, contracting
as it cools
Ice Floats
 important factor in suitability of
Earth’s environment to life
 if ice sank:
lakes & ponds (even oceans) could freeze
solid (from bottom to top)
 with summer temps only top upper meter
or so would melt

Ice Floats
 so ice insulates water below it,
preventing it from freezing & allowing
life to exist under frozen surface
Global Warming
 average air temperature in Arctic has
increased by 1.4ºC in the 50 years
from 1961 -2011
 ice forms later in fall and melts earlier
to cover smaller area
Water: Solvent of Life
 aqueous solution: one in which water
is the solvent
 water is best solvent on Earth
because …
water
is
polar
Water as Solvent
 salt in water dissociates and each
cation and anion is surrounded by
water molecules called a hydration
shell
Water as Solvent
 sea water & cytosol in cells contain
variety of ions all in hydration shells
 polar molecules dissolve in water
 sugars
 proteins
surface
with ionic or polar regions on
Hydrophilic Substances
 any substance that has an affinity for
water
 substances too large to dissolve but
still hydrophilic  remain
suspended in aqueous liquid (a
colloid)
Hydrophobic Substances
 nonionic, nonpolar substances
 repel water
Molecular Mass/Moles
 sum of masses of all the atoms in a
molecule
 not practical to use #s of molecules so
use moles (mol)
1 mole = 6.02 x 10²³
Molar Mass
 molecular mass of glucose is 144
daltons
 6.02 x 10²³ daltons in 1 g
 so molecular mass of glucose can be
written as 144g in 1 mole of glucose
 measuring in moles allows you to
combine substances in fixed ratios of
molecules
Molarity
 1 mole of glucose in 1 liter water =
1-molar or 1M solution of glucose
 Molarity: the # of moles of solute per
liter of water (for aqueous solutions)
Water on Mars
 Astrobiologists have looked for life
elsewhere based on: Is water present?
 Mars has polar ice caps, ice under
surface and enough water vapor in
atmosphere to form frost

still looking for life….
Dissociation of Water
Dissociation of Water Molecules
 only 1/554 million molecules of water
are dissociated (in pure water)
 @ 25ºC the concentration of H+ or
OHis 10-¹ M
 concentration of H+ or OH- is very
low but they are each very reactive
pH Scale
 Acid a substance that increases the
[H+]
 overall
has more H+ than OH-
 Base a substance that reduces the
[H+]
 overall
has more OH- than H+
pH Scale
 bases:
1. reduce [H+] by accepting H+
or
2. reduce [H+] indirectly by
contributing OH-
Weak Acids Form Reversible Reactions
pH Scale
 In any aqueous solution @ 25ºC, the
product of [H+] x [OH-] = 10ˉ¹
 the pH scale compresses the range of
H+ and OHˉ concentrations by using
logarithms
 pH = the negative logarithm of [H+]
or pH = -log [H+]
pH Scale
 pH < 7.0 = acid
 pH > 7.0 = base
 most biological solutions are pH 6-8
 exceptions:
 Each
stomach acid is pH
pH unit = 10x differences in [H+] or
[OHˉ] (pH of 4 has 100x more H+ than
pH of 6)
Buffers
 molecules that are able to resist
changes in pH
 consists of an acid-base pair that
combines reversibly with H+ when H+
in excess & donate H+ when they have
been depleted
Buffers
Buffers
Acidification
 burning fossil fuels
 increases CO2 in atmosphere
 CO2 dissolves in water
 fresh water & sea water pH decreases
Acidification of Oceans
Acid Precipitation
 rain, snow, sleet, fog with a pH < 5.2
 normal rain has pH 5.2
 adversely affects:
1. life in ponds & streams
2. land plants
3. soil chemistry
Acid Precipitation