3. Chemistry
of Water
A. First Cells Evolved in Water
1. All living things are 70-90% water.
2. Because water is a polar
molecule, water molecules are hydrogen bonded to each other.
3. With hydrogen bonding, water is
liquid between 0 degrees C and 100 degrees C which is critical for life.
B. Properties of Water
1. The temperature of liquid water rises and falls more slowly than that of
most other liquids.
a. Calorie
is amount of heat energy required to raise temperature of one gram of water 1
degree C.
b. Because
water holds heat, its temperature falls more slowly than other liquids; this
protects organisms
from rapid temperature changes and helps them maintain normal temperatures.
2. Water has a high heat of
vaporization.
a. Hydrogen
bonds between water molecules require a large amount of heat to break.
b. This
property moderates earth's surface temperature; permits living systems to exist
here.
c. When
animals sweat, evaporation of the sweat takes away body heat, thus cooling the
animal.
3. Water is universal solvent,
facilitates chemical reactions both outside of and within living systems.
a. Water is
a universal solvent because it dissolves a great number of solutes.
b. Ionized
or polar molecules attracted to water are hydrophilic.
c. Nonionized and nonpolar molecules
that cannot attract water are hydrophobic.
4. Water molecules are cohesive and
adhesive.
a. Cohesion
allows water to flow freely without molecules separating, due to hydrogen
bonding.
b. Adhesion
is ability to adhere to polar surfaces; water molecules have positive, negative
poles.
c. Water
rises up tree from roots to leaves through small tubes.
1) Adhesion of water to walls of vessels prevents water column from breaking
apart.
2) Cohesion allows evaporation from leaves to pull water column from roots.
5. Water has a high surface tension
measured by how difficult it is to break the surface of a liquid.
a. As with
cohesion, hydrogen bonding causes water to have high surface tension.
b. Permits a
rock to be skipped across pond surface; supports insect walking on water
surface.
6. Unlike most substances, frozen
water is less dense than liquid water.
a. Below 4
degrees C, hydrogen boding becomes more rigid but more open, causing expansion.
b. Because
ice is less dense, it floats; therefore bodies of water freeze from the top
down.
c. If ice
was heavier than water, ice would sink and ponds would freeze solid.
C. Acids and Bases
1. Covalently bonded water molecules ionize; the atoms dissociate
into ions.
2. When water ionizes or
dissociates, it releases a small but equal number of H+ and OH- ions;
thus its pH
is neutral.
3. Water dissociates into hydrogen
and hydroxide ions: H - O - H 
H+
+ OH-.
4. Acid molecules
dissociate in water, releasing hydrogen ions (H+) ions: HCl H+
+ Cl-.
5. Bases are molecules
that take up hydrogen ions or release hyroxide ions. NaOH Na+ + OH-.
6. The pH scale
indicates acidity and basicity (alkilinity)
of a solution.
a. Measure
of free hydrogen ions as a negative logarithm of the H+ concentration (-log
[H+]).
b. PH
values range from 0; most acidic to 14; most basic.
1) One mole of water has 10 to the 7 moles/liter of hydrogen ions; therefore,
has neutral pH of 7.
2) Acid is a substance with pH less than 7; base is
a substance with pH greater than 7.
3) As logarithmic scale, each lower unit has 10 times the amount of hydrogen
ions as next higher pH unit;
as move up pH scale, each unit has 10 times the basicity
of previous unit.
7. Buffers keep pH
steady and within normal limits in living organisms.
a. Buffers
stabilize pH of a solution by taking up excess hydrogen or hydroxide ions.
b. Carbonic
acid helps keep blood pH within normal limits: H2CO3 H+
+ HCO3-.