Chapter #3 – Water And The Fitness Of The Environment – Notes
Introduction – The Molecule That Supports All Life
I. The Polarity Of Water Molecules Results In Hydrogen Bonding
1. Water is shaped something like a wide V, with two hydrogen atoms joined
to the oxygen atom by a single covalent bond.
2. Because oxygen is more electronegative than hydrogen, the electrons of the
covalent bonds spend more time closer to oxygen than to hydrogen. The
bonds are polar covalent.
3. This unequal distribution of electrons makes water a polar molecule.
4. Each water molecule can form hydrogen bonds to a maximum of four
Neighbors.
5. The extraordinary properties of water is emergent properties resulting from
hydrogen bonding. (Hydrogen bonding is a hydrogen atom is covalently
bonded to an electronegative atom & is attracted to another electronegative
atom nearby.
6. Hydrogen bonding between water molecules is the basis for water’s unusual
(extraordinary) emergent properties.
A. Four emergent Properties of Water Contribute to Earth’s Fitness for Life
1. The four emergent properties of water that contribute to Earth’s suitability as
an environment for life are as follows:
a)
b)
c)
d)
cohesive behavior
ability to moderate temperature
expansion upon freezing
versatility as a solvent
B. Cohesion
1. Cohesion is the phenomenon of water molecules staying close together as
a result of hydrogen bonding.
2. Cohesion contributes to the transport of water against gravity in plants.
3. Adhesion, the clinging of one substance to another.
4. Adhesion of water to cell walls by hydrogen bonds helps counter the downward pull of gravity.
5. Related to cohesion is surface tension, a measure of how difficult it is to
stretch or break the surface of a liquid.
C. Moderation of Temperature
1. Water stabilizes air temperature by
a) absorbing heat from air that is warmer
b) releasing the stored heat to air that is cooler.
D. Heat & Temperature
1. Kinetic energy is the energy of motion.
2. Atoms & molecules have kinetic energy because they are always moving.
3. Heat is a form of energy. Heat is a measure of the matter’s total kinetic
energy due to motion of its molecules. Heat depends in part on the matter’s
volume.
4. Temperature is a measure of heat intensity that represents the average kinetic
energy of the molecules, regardless of volume.
5. A calorie is the amount of heat it takes to raise the temperature of 1 g. of
water by 1 degrees Celsius.
6. A calorie is also the amount of heat that 1 g. of water releases when it cools
by 1 degrees Celsius.
E. Water’s High Specific Heat
1. Water has a high specific heat, which means it resists temperature changes
when it absorbs or releases heat.
2. Specific heat is defined as the amount of heat that must be absorbed or lost
for 1 g. of that substance to change its temperature by 1 degrees C.
F. Evaporative Cooling
1. Vaporization (evaporation) is the transformation from a liquid to a gas.
2. Heat of vaporization is the quantity of heat a liquid must absorb for 1 g of it
to be converted from the liquid to the gaseous state.
a) Example – As a liquid evaporates, the surface of the liquid that remains
behind cools down.
G. Insulation of Bodies of Water by Floating Ice
1. Water is one of the few substances that are less dense as a solid than as a
liquid
2. While other materials contract when they solidify, water expands.
3. The cause of this behavior is hydrogen bonding.
4. Water reaches its greatest density at 4 degrees Celsius and then begins to
expand as the molecules move faster.
H. The Solvent of Life
1. A solution is a liquid that is a completely homogenous mixture of two or
more substances.
a) Solvent – the dissolving agent of a solution.
b) Solute - the substance that is dissolved.
2. An aqueous solution is one in which water is the solvent.
a) Ionic compounds dissolve in water.
b) Polar compounds are water soluble.
c) Nonpolar compounds are NOT water soluble.
I. Hydrophilic and Hydrophobic Substances
1. Any substance that has an affinity for water is hydrophilic.
2. A colloid is a stable suspension of fine particles in a liquid. These molecules
are so large that they do not dissolve. Instead, they remain suspended in the
aqueous liquid of the cell.
3. Substances that are nonionic & nonpolar cannot form hydrogen bonds, they
actually seem to repel water; these substances are said to be hydrophobic.
An example – vegetable oil, which does not mix with water based substances.
J. Solute Concentration in Aqueous Solution
1. Most of the chemical reactions in organisms involve solutes dissolved in
water.
2. There are two important quantitative properties of aqueous solutions:
a) solute concentration
b) pH
3. Prepare 1 mol of sucrose:
a) Write the molecular or empirical formula for sucrose: C12H22O11
b) Calculate its molecular mass (the sum of the masses of all the atoms in a
molecule)
c) Molecular mass = 342 Dalton Units
d) A mol represents the exact number of objects - 6.02 x 10 twenty third
power, which is Avogadro’s number.
e) There are 6.02 x 10 twenty third power Daltons in 1 g.
f) Weigh out 342 g.
g. This is 1 mol sucrose.
4. Prepare 1 molar solution of sucrose
a)
b)
c)
d)
Weigh out 342 g of sucrose.
Gradually add water, stir until sugar is completely dissolved.
Add enough water to bring the total volume up to 1 liter (1000 ml)
This is 1 M solution of sucrose (Molarity is the number of moles of solute
per liter of solution)
III. Acidic & Basic Conditions Affect Living Organisms
1. A water molecule can transfer an H+ to another water molecule to form H3O+
and OH ion.
2. The concentrations of H ion & OH ion are equal in pure water, but adding
certain kinds of solutes, called acids & bases, disrupts this balance.
a) Concentration of each ion in pure water is 10-7 M @25 degrees C
3. Biologists use a pH scale to determine the acidity or alkalinity of a solution.
A. Acids & Bases
1. An acid is a substance that increases the hydrogen ion concentration of a
solution. An example – HCl ---- H+ plus Cl2. A base is a substance that reduces the hydrogen ion concentration of a
solution. Example – NaOH ----- Na+ plus OHB. The PH Scale
1. In any aqueous solution @ 25 degrees C, the product of the H+ and OHConcentrations is constant at 10-14.
2. A base increase the OH concentration in a solution.
3. The pH= -log(H+) Defined – pH of a solution is the negative logarithm
(base 10) of the hydrogen ion concentration.
B. Buffer
1. Buffers are substances that minimize changes in the concentration of H+ &
OH- in a solution. Buffers in a biological fluid resist changes in pH.
2. A buffer cnsists of an acid-base pair that combines reversibly with hydrogen
ions.
C. Threats to Water Quality on Earth
1. The burning of fossil fuels results in emission of oxides (leading to acid
precipitation) and increasing amounts of CO2.
2. The above react with water in the air to form strong acids, which fall to Earth
with rain or snow. Acid precipitation refers to rain, snow, or fog with a pH
lower than pH 5.2.
*Uncontaminated rain has a pH of about 5.6 (owing to the formation of
carbonic acid from carbon dioxide and water.
.