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  Student Reading - The Unique Properties of Water
 

(The ensuing dicussion is adapted from Campbell, N.A. 1996. Biology (4th edition). Benjamin/Cummings Publ. Co. Inc. Menlo Park, CA, USA.)

The Unique Structure of Water
Polarity 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 bonds.

Oxygen is 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.

The water 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.

An electrostatic 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.

Simple exercise to demonstrate the polar nature of water:

  1. Fill a burette with tap water attached to a ring stand over a 400 ml beaker.
  2. Rub an air filled balloon against a wool cloth.
  3. Open the valve on the burette to allow a stream of water to flow into the beaker below.
  4. Position the balloon near the stream of water.
  5. Students should share their observations.

Cohesion of Water Molecules
When water 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 of cohesion.

Cohesion 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.

Related to cohesion is surface 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 the rim.

The air-water 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.

Water's Specific Heat
Water has 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 that gain or lose energy.

The heat 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.

This resistance 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 environments.

The high 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.

Because 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.

Evaporation and Cooling
Water has a high heat of vaporization - the energy required to convert liquid water to a gas. Because of the energy needed to break the hydrogen bonds holding a water molecule to its neighbors, more energy is required to evaporate liquid water than most other substances. To evaporate each 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.

Water's 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.

Water's Liquid Temperature Range
Water remains 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.

Water as the Universal Solvent
Water is 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 . Shielded by a shell of water molecules, the ions stay in solution because they are no longer affected by attractive forces from other ions.

Frozen Lake Density
Water is 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 water molecule is bonded to the maximum of four partners .

When the 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.

Density Relationships of Water
A lake's 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 . Therefore, 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.


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date last updated: Saturday March 06 2004