Water is the most basic and fundamental component of life on earth. Approximately ¾ of
the earth's surface is covered by water, and from 50 to 90 percent of the weight of living
organisms is in the form of water. Protoplasm, the basic material of living cells, consists of a
solution in water of fats, carbohydrates, proteins, salts, and similar chemicals. Water acts as
a solvent, transporting, combining, and chemically breaking down these substances. Blood in
animals and sap in plants consist largely of water and serve to transport food and remove waste
material. Water also plays a key role in the metabolic breakdown of such essential molecules as
proteins and carbohydrates. This process, called hydrolysis, goes on continually in living cells.
Water on earth is located primarily in the oceans (97.2%), polar caps (2.14%),
ground water(0.61%), and surface water(0.009%). The remaining 0.041% is contained in soil and
atmospheric moisture. Consider that of all the water you can see, every lake, river, stream,
swamp, creek, and puddle, only represents 0.009% of the earth's supply. Yet in 1985
approximately 79% or 275 billion gallons of the water we use for municipal and industrial
purposes came from this source, while only 21% or 75 billion gallons came from the far more
abundant ground water.
In recent years, ground water has become a central issue in protecting our water resources.
Ground water is a great source for supplying our water needs, but it is also one that is
susceptible to contamination. Unlike surface water, ground water may only move a few meters per
year. Once a ground water system is contaminated, it can take decades to recover. As human
consumption places greater demands on ground water resources, it becomes increasingly important
for us to keep these systems free from contamination.
- Composition
Water is made by combining two Hydrogen atoms with one Oxygen atom. The atoms
are held together by a covalent bond which forms between each Hydrogen
and the one Oxygen. This bond
forms due to the fact that Oxygen only has six outer electrons, but needs
eight to be stable. In addition, each Hydrogen needs two outer electrons
to be stable, but has only one. By sharing
their electrons, it is possible to satisfy both conditions, which results
in a stable water molecule. (Note: in order to make water, Hydrogen gas
and Oxygen gas must be available, then add just a
little heat. The result is a large release of energy, usually in the form
of an explosion, and stable water molecule.)
- Bi-Polarity
When the Hydrogen and Oxygen combine by the sharing of electrons, the sharing
is not equal. Consider what happens when an adult shares a candy bar with
a child. Usually the adult, being bigger, gets a bigger portion of the
candy bar. So it is with water. Oxygen, being about 32 times more massive
than Hydrogen, tends to pull the shared
electrons closer to itself, and does not share equally with the Hydrogen.
This causes a unique electrical effect. Since the Oxygen has most of the
electron, it also has more of a negative charge.
The Hydrogen, having a smaller portion of the electrons, loses its negativity
and becomes slightly positively charged. This type of bonding creates what
is known as a coordinate covalent bond, and causes the water
molecule to be polar, or charged at its ends.
- Surface Tension
In any liquid, the attractive force exerted by the molecules below the surface upon those at the surface/air interface,
resulting from the high molecular concentration of a liquid compared to the low molecular concentration of a gas (in layman's
terms, this states that the molecules on the surface of a drop are attracted towards the center of the droplet). An inward pull, or
internal pressure, is thus created which tends to restrain the liquid from flowing. The strength of the surface tension varies with
the chemical nature of the liquid. Surface tension is often measured using a capillary tube. The equation for surface tension is:
Where r = radius of tube, h = height of the liquid in tube, p = density of the liquid, and g = gravity. Note that because gravity
changes depending on location, a liquid will have a different surface tension atop Mt. Everest as compared to the same liquid's
surface tension at sea level. Also, the liquids density will change depending on its temperature. How would you expect the
surface tension to change for water at 1°C and 99°C?
Polar liquids have high surface tension (water = 73 dynes/cm @ 20°C); non-polar liquids have much lower values (ethyl alcohol
= 22.3 dynes/cm), thus they flow more readily than water. Mercury, a metal that is a liquid at room temperature, has the highest
surface tension of any liquid (480 dynes/cm). Mercury has such a high surface tension that it actually does not flow. Rather than
flowing, it disintegrates into small droplets that roll, similar to the way a water balloon rolls across a floor. Be advised that
Mercury is a neurotoxin that is highly lethal to humans through skin contact and inhalation of vapors. The toxic level dose of
Mercury is only 0.05 mg/m3 of air.
DO NOT TOUCH OR PLAY WITH MERCURY.
Once commonly used in thermometers and dental fillings, Mercury is now tightly regulated and seldom
used in consumer products. The term "mad hatter" stems from the fact that hat makers who used mercury
to cure beaver pelts often went insane. Their insanity was a direct result of their contact with
mercury. The following diagram illustrates the beading affect caused by surface tension for equal sized
drops of Mercury, water, and alcohol.
- Effects of Turbulence on Surface Tension
Surface tension can be altered. A substance that breaks-up surface tension is referred to as surface acting agents, or
surfactants. Soaps and detergents are common surfactants that you will learn more about later. Another way to reduce surface
tension is through physical means. If you have ever done a belly flop off the diving board you should be quite familiar with how
water's surface tension effected you. Platform divers feel the same sting even when they land properly. To help reduce surface
tension, a hose with a nozzle is positioned to spray water into the pool at the spot where the diver enters the water. The
turbulence of the sprayed water on the surface of the pool helps to reduce the sting felt when breaking the water's surface
tension upon entry.
- Forming Solutions
A solution is formed when a solvent dissolves a solute in a process called
solvation. Several types of solutions can be formed. The most common solution
is the aqueous solution,
which is formed when a substance (solute) is dissolved (known as solvation process) in water
(solvent). Salt water, Kool-Aid, and hot chocolate are all examples of aqueous
solutions. Another type of solution you may be familiar with is a tincture. In a tincture, the solvent is
alcohol. A once common example of a tincture is methiolate (the red stuff your mom put on your
cuts and scrapes to kill germs), which is iodine dissolved in alcohol.
The solvation process is the method by which solvents dissolve solutes. Water (solvent) dissolves substances by using its polarity
to attach to other substances (solutes). If you place common table salt (NaCl) in water the positive end of the water molecule
will attract to the negative Chlorine atom in the salt's crystalline lattice structure and pluck it out of the lattice. This leaves a
positively charged Sodium atom exposed to the water. The negatively charged end of a water molecule will attach to this
Sodium atom and pluck it from the lattice. The process continues and dissolves away the salt crystals.
Solvents can only hold so much solute. When a solvent has dissolved all the
solute it can it is said to be saturated. At this point it cannot hold any
more solute. You may have witnessed this condition by noticing the sugary
ooze that is left in the bottom of a
bowl when you put too much sugar on the cereal. Actually, some solvents can
dissolve more solute than they are theoretically supposed to. In this case
the solution is said to be supersaturated. If you would like to know more
about supersaturation go to
Eric's Treasure
Trove of Chemistry.
- Dissociation of Water
Water will naturally "break up" into H+ and OH- in a process known as dissociation. When water dissociates the lone
hydrogen atom breaks its bond with oxygen and leaves behind its electron. The hydrogen atom is now positively charged and
properly called a hydrogen ion. The remaining hydrogen is still connected to the oxygen, which now has an extra electron,
giving this pair a negative charge. The OH- molecule is properly called a hydroxide ion. The amount of dissociated water
molecules in relation to all the water molecules is very small, and since the overall amounts of H+ and OH- are equal, they
cancel each other out.
If, for some reason, the H+ and OH- are not balanced an acid or base is formed. The acidity or alkalinity (baseness) of the
solution is rated on the pH scale. The range of this scale is from 0 to 14 with 7 being the neutral point. Numbers below seven
are acidic, while numbers above seven are basic. It is important to note that this scale is logarithmic. Thus, a pH of 2 is not
twice as acidic as a 4, but rather 100 times as acidic. That same pH of 2 is not three times as acidic as a pH of 6, but rather
10,000 times as acidic.
