Lecture 8

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Chapter 4

Water Chemistry

 

After learning the rules of chemistry, the student learns to his or her dismay that water often seems to disobey these rules. In fact, the most abundant substance on the earth's surface has more anomalous properties than any other common substance. Strange as it is, materials and organisms on our planet behave the way they do because water behaves the way it does.

  1. Water is essential, scarce and plentiful. All earthly organisms need some water. Plants need water, usually in surprisingly large quantities. One single corn plant may use 1 liter of water per day; a large tree might use 200 or 300 liters per day. Water is known as the universal solvent, and is the solvent in the soil solution. To a great extent, soil water controls soil aeration and temperature. Water affects landscape erosion. Water on our planet is unevenly distributed. The wettest place on earth is a location on the big island of Hawaii, where it rains 460" per year. The wettest place in the 48 states is the Olympic peninsula of Washington, where it rains 140" per year. In general, the eastern states receive between 30 and 80" per year. The plains receive between 15 and 30" per year. Rainfall varies greatly among locations in the west. The state of Nevada averages less than 7" per year. About 84% of cloud water comes from oceans, but only 75% falls on oceans. Therefore, we have a hydrologic cycle, with surface water flowing from the continents to the oceans.

  2. Water is a unique substance. The water molecule would appear to be small enough to be a gas at room temperature, and yet it is a liquid. Indeed, most gases are considerably larger than water. Ice floats. This is a strange phenomenon among chemical substances. Usually the solid phase would be more dense than the liquid. If winter ice were to sink to the bottom of bodies of water, life on earth would be radically different. Water has a very high surface tension, and therefore tends to bead up. It also has a very high specific heat. Water has the highest heat of vaporization of all known substances.

  3. Hydrogen bonding accounts for water's behavior. Because both of the hydrogens in a water molecule bond to the same side of the oxygen, water is polar HOH with a + end on the hydrogen side and a - end on the oxygen side. Salts readily dissolve in this medium. Hydrogen bonding is a force between H and either N, O, or F. These three elements are the most electronegative. Because water is only H's and O's, the stage is set for the ultimate example of hydrogen bonding. In a pool of water, each H is covalently bonded to an oxygen, but retains a strong attraction (Hydrogen bond) for the nearest adjacent oxygen. In other words, each water is attracted to other waters. This phenomenon is called cohesion (the attraction of water to water). Soil minerals are also a source of oxygens to which water's hydrogens are attracted. Water, therefore, is strongly attracted to O-rich solids. This attraction of water to other materials is called adhesion. Adhesion and cohesion are best observed in the phenomenon of capillarity. You have probably observed water or an aqueous solution rising against gravity in a capillary tube, perhaps you have experienced this at the doctor's office as he or she pricks your finger, and then collects a sample of blood in a small glass tube. In capillarity, water rises until weight of column equals the attractive force between the water and the glass. For pure water the height of rise is approximately the following function of tube radius:

    h .15/r (in cm) or h 15/r (in mm)

    Soil and water attract for two reasons. First the soil is porous, and the pores behave much like capillaries. This is actually a minor consideration because natural drainage of water through a soil is strong enough to drain pores that are larger than 0.009 mm in diameter. The more important attraction is between water and solid surfaces. Surface films of water are always present in soil. The difference in water content between any two soils hinges on the question, how thick is the film?

  4. The tendency of water to move or react or do work is determined by potential. The laws of thermodynamics tell us that spontaneous changes result in reduced potential energy states. Any given parcel of water has a particular potential energy. Potential is the work water can do relative to water at zero state. The zero state is pure water that is unattached to any surface and exists at the reference elevation in a gravitational field. Negative potential means work must be done to bring water up to the zero state. Usually, soil water has a negative potential. Suction or tension are terms used to refer to negative pressure. These terms are used to avoid negative numbers. A positive tension, means a negative pressure. Many other units are used to describe water potential. Hydraulic head is the unit used by engineers. Head units are in length, as the height of a water column. A pump, for example, might deliver 90 feet of hydraulic head. In science, as opposed to engineering, water potential is the preferred term. Water potential is expressed in energy units; but the question is, energy per what? If we express potential as energy per mass, typical units might be Joule/kg. Because it is more convenient, we often use energy per volume. This is convenient because energy per volume equals pressure, something we are familiar with. Typical pressure units in use are pascals, kilopascals, megapascals, and bars.

  5. Water potential is the sum of four components. The first component is gravitational. This one is easy to visualize because we have lots of experience with gravity. The symbol used to depict gravitational potential is yg, and the value can be + or — .

    Pressure potential is also easy to visualize. Water will move from a high pressure environment to a low pressure environment. The symbol used to depict pressure potential is yp. Ponded or flooded sites are pressurized for example. The pressure component in soil is either positive or zero.

    Matric potential is the most important component in soil, but is more difficult to visualize. Water will not freely leave soil unless soil is very wet. This is because of adhesion and cohesion. Imagine placing a clump of dry soil on a table, then dropping a drop of water onto the clump of dry soil. If you elevate the soil off of the table, will the water leap out of it, dropping to the table? Of course not. It will stay in the soil, held my adhesion, or what we often call matric forces. The symbol for matric potential is ym. These values are negative or zero, but never positive, because this water is not free to move to the zero state.

    Often these three components are sufficient. For instance, to predict hydraulic flow, such as liquid flow through soil pores, one need consider no other component. However, in certain circumstances, another component is important. This other component is called solute potential. The tendency for water to undergo phase changes or to pass through membranes is controlled by the presence of solutes in the water. This is important in soil for two reasons: (1) evaporation of soil water is an important phase change, (2) water flow from soil into cells of organisms, including plant roots, requires transport through membranes. The symbol for solute potential is ys. As with matric potential, solute potential is never positive; it can be negative for impure water, or approximately zero for very pure water. Solute potential is also called osmotic potential, because the process of passing selectively through a semi-permeable membrane is called osmosis.

  6. Matric potential is the component of greatest concern. In wetlands, pressure and gravity are most important. But usually,

    total potential (yT) matric potential (ym)

    A normal soil may have ym = -5 bars. It actually ranges from nearly 0 to about -20 bars. For prospective, 5 bars of pressure is about equal to the pressure of 50 meters of water. In other words, a force equal to the weight of 50 meters of water would be required to remove water from a soil in which it was held with a matric potential of -5 bars. This magnitude of matric potential usually eclipses the small effect of the other components. The matric potential of a soil refers to the potential of the most easily removed molecule (see Figure 4-2 in the textbook).

 

Students are encouraged to look up the following vocabulary words in the textbook glossary or elsewhere and to browse the following web sites.

 

Vocabulary

matric potential
solute potential
pressure potential
gravitational potential
solvent
osmosis
phase change
semi-permeable membrane

 

Web sites

Theory and measurement of water potential; a website from a company selling instruments that measure water potential.
URL: www.focuscorp.co.kr/decagon/W-potenT.htm

A website with good diagrams illustrating principles of osmosis and water potential.
URL: www.biology.demon.co.uk/Biology/mod1/osmosis/osmosis.htm

 

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