Biology 107
Water
August 29, 2005
Water
Student Objectives: As a result of this lecture and the assigned
reading, you should understand the following:
1. For all living organisms, life depends upon water, and living things
are made up mostly of water molecules (H2O)
2. The dipoles produced by the polar covalent chemical bonds
between the hydrogens and oxygen of the water molecule allow for
hydrogen bonding between water molecules, and the properties of
water are related to these intermolecular bonds.
Water molecules "stick" to each other, cohesion, and to
charged substances, adhesion. The cohesive and adhesive
properties of water allow it to rise against the force of gravity
through capillarity.
Water
Related to cohesion is the property of surface tension - a
measure of how difficult it is to stretch or break the surface of a
liquid.
Water’s hydrogen bonds moderate temperature and decrease
water evaporation and vaporization.
Ice is less dense than liquid water.
Water's versatility as a solvent results from the polarity of its
molecules; substances that dissolve in water are called
hydrophilic (i.e., water-liking), while those that do not dissolve
in water are hydrophobic (i.e., water-hating).
Water
3. A certain proportion of the water molecules dissociate into ions
because of their polarity. The two ions formed in the chemical
reaction that dissociates water are the hydrogen cation (H+), which
is simply a proton, and the hydroxide anion (OH-)
4. Some substances when dissolved in water dissociate and release
H+ ions. By definition, these substances that release H+ ions when
dissolved in water are called acids. Acids increase the H+
concentration of the solution. That is, the more acidic a solution, the
higher its concentration of H+ ions, and the lower its pH.
5. A base is any substance that accepts H+ ions when dissolved in
water.
Inventory of Water at the Earth's
Surface.
Reservoir
Volume (cubic km x 1,000,000)
Percent of Total
1370
97.25
Ice Caps and Glaciers
29
2.05
Groundwater
9.5
0.68
Lakes
0.125
0.01
Soil Moisture
0.065
0.005
Atmosphere
0.013
0.001
Streams and Rivers
0.0017
0.0001
Biosphere (Animals & Plants)
0.0006
0.00004
Oceans
Water Molecule Dipoles
Hydrogen Bonding of Water Molecules
Water Adhesion and Cohesion
Cohesion and Adhesion
Cohesion and adhesion of
water produces: 1) surface
tension and 2) capillary action.
Mechanism by which plants
draw water upward from roots
to leaves through small
vessels. Mechanism by which
water moves through
groundwater reservoirs
between the particles of rock.
Water Movement in Plants
Cucumber stem in cross
section showing vascular
bundles that convey water
by capillary action.
Hydrogen Bonding Helps Water Move
Through Plants
Some trees can move water
from their roots to their leaves
at a rate of 25 mm (1 inch)
per second.
Hydrogen Bonding Holds Waters Molecules
Together and Produces Surface Tension
High tea in space
High Heat Holding Capacity
1. Water has a high boiling point, so water is a liquid over a wide
temperature range.
2. Water has the ability to absorb much heat, so large bodies of
water can minimize temperature changes.
3. Water produces evaporative cooling. The heat loss
associated with evaporation makes evaporation an effective
way for organisms to shed excess heat.
High Specific Heat of Water
Evaporative Cooling
Hydrogen Bonding of Water Molecules to Other
Molecules
Water as a Polar Solvent
Water as a Polar Solvent
Water as a Polar Solvent
Water as a Polar Solvent
Why Ice Floats
Ice Floats
Frozen water and frozen benzene
Ionization of Water
In pure water only 10-7 moles/liter of water are in dissociated, ionized form
pH Scale
pH measures the hydrogen ion
concentration of a solution
The higher the H+ concentration the
lower the pH number
Acids donate H+ , while bases accept
H+