Chapter 3: WATER AND THE FITNESS OF THE ENVIRONMENT
Water contributes to the fitness of the environment to support life. Life on earth probably evolved in
water. Living cells are 70% - 95% water. Water covers about three-quarters of the earth. In nature,
water exists in all three physical states of matter.
I. WATER MOLECULES AND HYDROGEN BONDING
A. Water has polar bonds between the two hydrogens and the oxygen. These two bonds are
arranged asymmetrically. Water is a polar molecule.
B. Water molecules are held together by hydrogen bonds. Each water molecule can hydrogen bond
to four other water molecules. These hydrogen bonds are transient. At 37ºC, about 15% of water
molecules are hydrogen bonded at any instant.
II. SOME EXTRAORDINARY PROPERTIES OF WATER
A. Liquid water is cohesive.
1. Cohesion is the phenomenon of a substance being held together by hydrogen bonds. Water
molecules stick to each other and liquid water has more structure than other liquids. Cohesion
contributes to the upward transport of water in plants.
2. Surface tension is a measure of how difficult it is to stretch or break the surface of a liquid.
Water has a greater surface tension than most liquids; this causes water to bead and form spheres.
3. Adhesion is the clinging of water to another substance, e.g. the walls of plant vessels and
B. Water has a high specific heat.
1. Heat and temperature
a. Kinetic energy is the energy of motion.
b. Heat is the total kinetic energy due to molecular motion in a body. It is the flow of energy
due to a difference in temperature from high to low until the two objects become the same temperature.
c. Calorie is the amount of heat it takes to raise the temperature of one gram of water by one
degree Celsius (kilocalorie-one kilogram of water).
d. Temperature is a measure of the average kinetic energy of molecules.
e. Specific heat is the amount of energy that must be absorbed or lost for one gram of a
substance to change its temperature by one degree Celsius (see calorie). The specific heat of water is
one calorie per gram per degree Celsius (1 cal / g / ºC).
2. How water stabilizes temperature
a. Water has a high specific heat value which means it resists temperature changes when it
absorbs or releases heat.
b. A large body of water can act as a heat sink. This keeps temperature fluctuations within a
suitable rang for life, coastal areas have milder climates than inland, and the marine environment has a
relatively stable temperature.
C. Water has a high heat of vaporization
1. Vaporization is the transformation from the liquid state to the gas state. If this happens at the
surface of a liquid it is called evaporation. This occurs when molecules have enough kinetic energy to
overcome the attraction forces in the liquid.
2. Heat of vaporization is the amount of energy needed to vaporize one gram of a substance at its
boiling point. Hydrogen bonds must be broken for water to vaporize. This requires approximately 540
3. Evaporative cooling is the cooling of a surface when a liquid evaporates.
a. Moderates the earth’s climate.
b. Stabilizes temperature in aquatic ecosystems.
c. helps organisms from overheating.
D. Water expands when it freezes
1. Water is densest at 4ºC. As water cools to 4ºC, it contracts. Between 4ºC and 0ºC, water
expands and becomes less dense than liquid water (ice floats).
2. Expansion of water contributes to the fitness of the environment for life because it prevents
deep bodies of water from freezing solid. The ice at the surface serves to insulate the layers below. It
also makes the transitions between the seasons less abrupt.
E. Water is a versatile solvent.
1. A solution is a homogeneous mixture of two or more substances. The solvent does the
dissolving and the solute is the substance being dissolved. When the solvent is water, the solution is
called an aqueous solution.
2. Water is a versatile solvent because of it’s polarity and ability to form hydrogen bonds.
a. A hydrophilic substance has an affinity for water. Ionic compounds and polar molecules generally
dissolve in water.
b. A hydrophobic substance does not have an affinity for water. Nonpolar substance do not
dissolve in water.
III. AQUEOUS SOLUTIONS: most biochemical reactions involve solutes dissolved in water.
A. Solute concentration is a quantitative measure of how much solute is in a given amount of
solution or solvent. One way to express this is in terms of molarity. Molarity is the number of moles of
solute divided by the volume of the solution. Square brackets, [ ], refer to the molarity of a substance in
1. Mole is a measure of the amount of a substance. It is numerically equivalent to the molecular
mass in grams.
2. Molecular mass is the sum of the masses of the elements in a compound.
3. A mole of a substance has the same number of particles (molecules) in it as a mole of any
other substance. This number is referred to as Avogadro’s Number and is equal to 6.02 x 1023.
4. Using moles allows scientists to connect the atomic scale (daltons) with the macroscopic scale
(grams), and to combine substance in fixed ratios of molecules.
B. Acids, Bases and pH
1. Dissociation of water molecules. Water ionizes only slightly according to the following
equation: H2O(l) H+1 + OH–1 . This produces equal numbers of H+1 and OH–1 . The H+1
that is produced is very reactive and attaches itself to another water molecule to form the hydronium ion,
H3O+1 . This leaves the hydroxide ion behind, OH–1. The ionization of water can now be written as:
H2O(l) + H2O(l) H3O+1 + OH–1.
2. Acids and bases: The ion product constant of water (or the ionization constant of water) is
given the symbol Kw and numerically is 1.00 x 10–14 , and is [H3O+1] . [OH–1]. When there are
unreacted H3O+1 in a solution it is considered acidic and when there are unreacted OH–1 in a solution it
is considered basic. So if the [H3O+1] = [OH–1] the solution must be neutral and numerically [H3O+1]
= [OH–1] = 1.00 x 10–7 because [H3O+1].[OH–1] = 1.00 x 10–14 . So if you know either the [H3O+1]
1 00 10 14 1 00 1014
or the [OH–1] you can determine the other: H 3 O1 and OH
OH 1 H 3O
If you add an acid to a neutral solution, the [H3O+1] increases and the [OH–1] decreases so [H3O+1] >
[OH–1] and it is now considered to be an acidic solution. If you add a base to a neutral solution, the
[OH–1] increases and the [H3O+1] decreases so [H3O+1] < [OH–1] and it is now considered to be a
3. The pH scale is a simple way to state what the [H3O+1] is in a solution. It identifies the relative
acidity or basicity of the solution. pH is expressed as a power of ten, usually from 100 to 10-14 . If
[H3O+1] = [OH–1] = 1.00 x 10–7 is considered neutral than the pH = 7. Definition: pH = –log10
[H3O+1]. The pH scale is usually expressed from 0 to 14. A neutral solution has a pH = 7. An acidic
solution has a pH< 7 and a basic solution has a pH >7.
4. A buffer is a substance that can absorb moderate amounts of acid or base without significantly
altering the pH. A buffer is a solution of a weak acid and it's conjugate base or a weak base and it's
a. Buffers are made by:
1). adding the salt of a weak acid to a solution of the weak acid
2). adding the salt of a weak base to a solution of the weak base
3). titrating half of a weak acid solution with a strong base and adding it to the untitrated half
of the weak acid
4). titrating half of a weak base solution with a strong acid and adding it to the untitrated half
of the weak base
b. how buffers work
1). adding a acid to a buffered solution
a). weak acid buffer: H+1 + A-1 HA
b). weak base buffer: H+1 + B BH+1
2). adding a base to a buffered solution
a). weak acid buffer: OH-1 + HA A-1 + H2O
b). weak base buffer: OH-1 + BH+1 B + H2O
c. Buffering systems are found in many living organisms where they are responsible for
homeostatic control. An example in humans is the blood buffering system using the HCO3-1.
IV. ACID RAIN: UPSETTING THE FITNESS OF THE ENVIRONMENT
A. Acid rain is rain that is more strongly acidic than normal rain. Normal rain has a pH of 5.6 due
to water combining with the carbon dioxide in the air and forming carbonic acid.
B. Acid rain occurs when sulfur oxides and nitrogen oxides in the atmosphere react with water
vapor in the air. When the water vapor condenses it rains acid rain. The major source of these
pollutants is the burning of fossil fuels by industry and cars.
C. Acid rain lowers the fitness of the environment to support life.
1. Acid rain lowers the soil pH which affects the solubility of minerals. It can leach out the
minerals or increase their concentrations to toxic levels.
2. Acid rain lowers the pH of lakes and ponds and runoff carries leached out soil minerals into
aquatic ecosystems. This adversely affects aquatic life.
D. What can be done to reduce acid rain?