ensuing dicussion is adapted from Campbell, N.A. 1996. Biology (4th edition).
Benjamin/Cummings Publ. Co. Inc. Menlo Park, CA, USA.)
of water molecules results in hydrogen
bonding. The water molecule is relatively simple in structure. Two
hydrogen atoms are joined to a single oxygen
atom by single covalent
more electronegative than the hydrogen atoms which allows the electrons
of the polar bonds to spend more time closer to the oxygen side of the
molecule. The oxygen side becomes more negative in charge, and the hydrogen
atoms have a slight positive charge. This forms the polar
molecule is shaped like an isosceles triangle, with a slight bond angle
of 104.5 degrees at the oxygen nucleus. The weak Coulombic characteristics
of the bonding of hydrogen atoms to the weakly electronegative oxygen
atom result in both ionized and covalent states that simultaneously
maintain the integrity of water. Water is one of the only compounds
that possess these characteristics.
attraction occurs between the polar water molecules. The slight positive
charged hydrogen atom is attracted to the slight negative charged oxygen
atom of another water molecule. This weak attraction is called a hydrogen
bond. Every water molecule is hydrogen bonded to its
four nearest neighbors.
to demonstrate the polar nature of water:
- Fill a burette with tap water attached to a ring stand over a 400
- Rub an air filled balloon against a wool cloth.
- Open the valve on the burette to allow a stream of water to flow
into the beaker below.
- Position the balloon near the stream of water.
- Students should share their observations.
is in liquid form, its weak hydrogen bonds are about one-twentieth as
strong as a covalent bond. Hydrogen bonds constantly form and break.
Each hydrogen bond lasts for a fraction of a second, but the molecules
continuously form new bonds with other water molecules around them.
At any time a large percentage of water molecules are bonded to neighboring
water molecules which gives water more structure than most other liquids.
Collectively, the hydrogen bonds hold water together by the property
due to hydrogen bonding contributes to the formation of waves and other
water movements that occur in lakes. Water movements are integral components
of the lake system and play an important role in the distribution of
temperature, dissolved gases, and nutrients. These movements also determine
the distribution of microorganisms and plankton.
tension, a measure of how difficult it is to stretch or break the
surface of a liquid. Water has a greater surface tension than all other
liquids except mercury. At the interface between water and air is an
ordered arrangement of water molecules which are hydrogen bonded to
one another and the water below. The result is an interface surface
or film under tension. Students can observe the surface tension of water
by overfilling a glass of water to the point where water stands above
interface forms a special habitat for organisms adapted to living in
this surface film. This community is called the neuston.
Water's high surface tension serves as a supporting surface for many
organisms. Many aquatic organisms have evolved adaptations that allow
them to spread their body weight over a large surface area to prevent
breaking water's surface tension.
a high heat capacity. Specific
heat a measure of heat capacity, is the heat required to raise
the temperature of 1 gram of water 1°C. Water, with its high heat
capacity, therefore, changes temperature more slowly than other compounds
gain or lose energy.
capacity of water stems directly from its hydrogen bonded structure.
Although hydrogen bonds are weak, their combined effect is enormous.
As heat is added to ice or liquid water, the energy first breaks hydrogen
bonds, which allows the molecules to move freely. Since temperature
is a measure of the average kinetic energy of molecules (the rate at
which they move), the temperature of water rises slowly with the addition
of heat. When the temperature of water drops slightly, many additional
hydrogen bonds form and release a considerable amount of energy in the
form of heat.
to sudden changes in temperature makes water an excellent habitat because
organisms adapted to narrow temperature ranges may die if the temperature
fluctuates widely. The heat requiring and heat retaining properties
of water provide a much more stable environment than is found in terrestrial
situations. Fluctuations in water temperature occur very gradually,
and seasonal and diurnal extremes are small in comparison to terrestrial
specific heat can have profound effects on climatic conditions of adjacent
air masses. When it warms only a few degrees, a large lake can absorb
and store a huge amount of heat from the sun in the daytime and summer.
At night and during winter, the gradually cooling water can warm the
air. This is the reason Michigan and areas east of the Great Lakes have
more moderate climates than the Great Lakes region. Mild winters with
higher precipitation rates and moist, cool summers are common in Michigan
and areas east of the Great Lakes.
of water's high specific heat, the water that covers most of the earth's
surface keeps temperature fluctuations within limits that allow living
organisms to survive. Also, because organisms consist mostly of water,
they are more able to resist changes in their own temperatures.
a high heat of vaporization - the energy required to convert liquid
water to a gas. Because of the energy needed to break the hydrogen
holding a water molecule to its neighbors, more energy is required
to evaporate liquid water than most other substances. To evaporate
gram of water at room temperature, about 580 calories
of heat are needed, which is nearly double the amount needed to vaporize
a gram of alcohol or ammonia.
high heat of vaporization helps moderate the earth's climate. A considerable
amount of energy from the sun is absorbed by lakes during the evaporation
of its surface waters. As water evaporates, the remaining surface water
cools. This evaporative cooling occurs because the warmest molecules
are those with the greatest kinetic energy and are most likely to leave
in the gaseous state. Evaporative cooling of water contributes to the
stabilization of temperature in lakes.
liquid over a wide temperature range, from 0 100°C. Most
other substances remain liquid over a narrower range. Since the chemical
reactions of metabolism
depend on interactions between molecules moving about in liquid water,
the limits of life are set by water's freezing and boiling points. This
property of water makes possible a wide variety of aquatic habitats.
Some fish species survive in temperatures at or near freezing while
some bacteria and algae
survive in hot springs where the water temperature is near boiling.
a substance that can almost dissolve anything. Salts such as sodium
chloride (NaCl), dissolve in water by dissociating as each ion
becomes surrounded by the polar water molecules
by a shell of water molecules, the ions stay in solution
because they are no longer affected by attractive forces from other
one of the few substances that are less dense as a solid than as a liquid.
While most substances contract when they solidify, water expands. This
property is due to the hydrogen bonding. When water is above 4 °C it
behaves like other liquids; it expands as it warms and contracts when
it cools. Water starts to freeze when the temperature
approaches 0°C and the molecules no longer move vigorously enough
to break their hydrogen bonds. As the temperature reaches 0°C
the water molecules become locked into a crystalline lattice, and each
molecule is bonded to the maximum of four partners
surface temperature in a lake reaches 0°C, ice forms and floats
on top of the lake. The ice becomes an insulating layer on the surface
of the lake; it reduces heat loss from the water below and enables life
to continue in the lake. When ice absorbs enough heat for its temperature
to increase above 0°C, the hydrogen bonds can be broken and allow
the water molecules to slip closer together. If ice sank, lakes would
be packed from the bottom with ice, and many of them would not be able
to thaw out, since the energy from the air and the sunlight does not
penetrate very far.
physical, chemical, and metabolism dynamics are governed to a very great
extent by differences in density.
The density of ice is almost ten times lighter than liquid water. Water's
density increases to a maximum at 3.98°C
warmer waters are always found on top of cooler water in lakes and produce
layers of water called strata. This is typical of a lake that is stratified
during the summer. In winter the density differences in water cause
a reverse stratification
where ice floats on top of warmer waters.