Water may be one of the most familiar substances on the planet, but it certainly isn't ordinary. In fact, water's unique chemical properties make it so complicated that after decades of research, scientists still have much to learn about this remarkable and versatile substance.
The
Three States of Water
Water
can exist in three different forms, known as States. These states
refer to the form that water takes depending on what is happening
to the water molecules. Water molecules change from one state to another
when heat energy is added or lost. The three states of water are Liquid
(flowing water), Gas (steam), and Solid (ice).
-
Gas State
The bond between the hydrogen and oxygen that form the water molecules is a weak attraction. When water is heated to boiling, and steam begins to form, the molecules separate and are no longer attached. This causes a gas to form. In this gaseous state, the water molecules have been heated by energy and are moving very fast. Water in the form of gas also forms when it Evaporates from a liquid . This occurs in the Water Cycle as oceans, seas, lakes, and smaller bodies of liquid water evaporate in the heat of the sun.. -
Liquid State
Water in the liquid state flows. In a liquid state the molecules are moving more slowly than they move in the gaseous state because they have lost some of the heat energy. -
Solid State
Water that is frozen is in the solid state. When water is cooled down below its freezing point it becomes a transparent crystalline solid. As the water cools, the molecules in the water are slowed by the loss of heat energy and do not move very fast. Water in the solid state has an interesting property, as it freezes it expands, and as it returns to the liquid state it contracts.
Water
Is A Chemical
Indeed! Water is one of our most plentiful chemicals. Its chemical
formula, H20, is probably the most well known of all chemical formulas.
Water plays an important role as a chemical substance. Its many important functions include being a good solvent for dissolving many solids, serving as an excellent coolant both mechanically and biologically, and acting as a reactant in many chemical reactions. Blood, sweat and tears... all solutions of water.
Chemists consider water from many perspectives. It is their role to use physical and mathematical laws in application for useful purposes, including diverse perspectives such as living systems, materials and energy. The world of the chemist is a small world - atomic, molecular - which plays a large part in making our lives healthy, comfortable, and hopeful. Because of the diversity of the chemical world, it would be difficult to touch upon all of the applications of water. And for the same reason, it would be impossible to discuss the chemical aspects of water without touching upon the physical, mathematical, and biological aspects of the subject.
Let's start the discussion of water as a chemical with a look at its structure. From a molecular perspective, structure is one of the important features of a substance. Just as you might say that the shape of a key determines its function - which doors it can and cannot open - the structure of a molecule and its composition absolutely determines its functions and properties.
Chemists have a vested interest not only in understanding how a substance may be used and broken down, but also in knowing how that substance is created. From this perspective let's look at the chemistry which creates water from its elements, hydrogen and oxygen, and the chemistry of water's breakdown, also known as Electrolysis.
Water may
be a substance so common that we scarcely make note of it - We waste
it, pollute it, let it run down the drain, flush it away... Certainly
we take it for granted! However, chemically speaking, water is really
not common at all. When compared to other compounds of similar size,
composition, and structure - it is absolutely unique! In fact its properties
are so unusual that it would be irreplaceable. Let's take a chemical
look at these unusual properties, how they arise and what their implications
are.
What
does the chemical formula tell us?
The
formula H20
tells
us that one molecule of water is comprised of 2 atoms of hydrogen
and one atom of oxygen bonded together. The bonds which hold the hydrogen
and oxygen together are called covalent bonds - they are very strong.
Let's look at a picture of a molecule of water: In this picture the two hydrogens are represented by white spheres and the oxygen by a red sphere.
Water
is a "polar" molecule, meaning that there is an uneven distribution
of electron density. Water has a partial negative charge (
)
near the oxygen atom due the unshared pairs of electrons, and partial
positive charges (
)
near the hydrogen atoms.
Why
does the water molecule look bent?
The water molecule maintains a bent shape (bent at 107.5 degrees actually)
because of two considerations. First the tetrahedral arrangment around
the oxygen and second the presence of lone pair electrons on the oxygen.
| What
are Lone Pair Electrons? These are the electrons that are not involved in the covalent bonds. The pairs of electrons are left alone. In our picture they are represented by the double dots. These lone pairs are very negative - containing two negative electrons each - and want to stay away from each other as much as possible. These repulsive forces act to push the hydrogens closer together. |
 |
| Did you say "Tetrahedral" - What does that mean? | |
 |
Tetrahedral means "four-sided". In chemistry we interpret this in our imaginations. Draw the central atom in an imaginary space. Next put the atoms attached to the central atom around it such that the distance between them is maximized. The arrangement you'll adopt will be the form of a regular tetrahedron. This molecular shape is shown here. It has regular bond angles of 109.5 |
If we do a similar arrangement of water, putting oxygen in the center, and using the two hydrogens and two lone pairs at the corners, we also come up with a tetrahedral arrangement. However, there is one important difference - the bond angles for water are not 109.5. Because of the presence of the very negative lone pair electrons, the two hydrogens are squeezed together as the two lone pairs try to get away from each other as far as possible. The resulting angle gives water a 104.5 bond angle. Because we don't "see" the electrons, the resulting tetrahedron "looks" BENT!
What's
your point?
Like many things in the chemical world, the shape and structure of
a molecule is an important determinant of its function. The importance
of the bent structure of water is that it provides water with two
distinct "sides": One side of the water molecule has two negative
lone pairs, while the other side presents the two hydrogens.
Does
this make water unusual?
YES! But it's not just that the molecule is bent that makes it special.
Water is also highly polar - the two sides of water have very different
charge.
The
lone pairs are negative - Are the Hydrogens positive?
The
hydrogens are slightly positive. They get this way because
of the "electronegativity" of oxygen. Electronegativity is a measure
of how much one atom wants to have electrons, and oxygen wants to
have electrons more than hydrogen does. Oxygen has a higher electronegativity.
Because of this difference in electronegativity, the electrons in
the covalent bonds between oxygen and hydrogen get pulled slightly
toward the oxygen. This leaves the hydrogens a little bit electron-deficient
and thus slightly positive. We can draw this polarization like this:
What does the polarization have to do with the properties of water?
Everything! Because water has a slightly negative end and a slightly
positive end, it can interact with itself and form a highly organized
'inter-molecular' network. The positive hydrogen end of one molecule
can interact favorably with the negative lone pair of another water
molecule. This interaction is call "Hydrogen Bonding". It is a type
of weak electrostatic attraction (positive to negative). Because each
and every one of the water molecules can form four Hydrogen Bonds, an
elaborate network of molecules is formed.
The Hydrogen and Oxygen of Water
Hydrogen
+ Oxygen = Water
The simple statement that water is made from hydrogen and oxygen doesn't give us a very clear picture of what really goes into the creation of a molecule of water. A quick look at the chemical equation for the formation of water tells us more.
2H2 + O2 = 2H2O
It takes two molecules of the diatomic hydrogen gas, combined with one molecule of the diatomic oxygen gas to produce two molecules of water. In other words the ratio of hydrogen to oxygen is 2:1, the ratio of hydrogen to water is 1:1, and the ratio of oxygen to water is 1:2.
There's something more though that doesn't show up in the equation. Energy. The formation of water from it's elements produces, in addition to water, a tremendous amount of energy, 572 kJ to be exact.
This is an example of an exothermic reaction, a reaction that produces energy. It is also an example of what is called a combustion reaction, where a substance (in this case hydrogen gas) is combined with oxygen. You are probably familiar with this reaction through two tragic examples of the unleashed energy of the combustion reaction of hydrogen, the Hindenburg, and the spaceshuttle Challenger.
Ions
At The Edge
In 2004, scientists tackled the question of where ions—charged
particles such as chloride from the salt sodium chloride (Cl - of
NaCl), for example—go in a body of water. Conventional wisdom
says the surface layers of water repel ions, which are abundant in
salty seawater. Consequently, scientists thought such molecules might
get buried, going deep into the interior of solutions. But new experimental
and computer-generated models from several different research teams
indicate the current thinking is wrong. Although they disagree on
some of the details, everyone involved concludes that at least some
ions are present in the surface layers of water particles. And where
there are accumulated ions, chemistry can occur.
In fact, exposed ions on the ocean surface and in aerosols could potentially bind and react with all sorts of chemicals from the atmosphere. Consequently, fog and ocean spray droplets may be more chemically reactive than previously thought. Indeed, recent atmospheric research indicates that is the case. For example, reports suggest that two ions found in seawater—bromide and chloride—trigger chemical reactions that destroy ozone in the Arctic atmosphere. These destructive but natural events occur after wind and waves deposit the chemicals on polar ice and expose them to sunlight. If the 2004 results hold up, atmospheric chemists who have long ignored the contributions of surface ions when modeling conditions such as air quality will have to rethink their calculations.
And...
so much more
There is so much more that can be spoken about in
regards to water - as there are entire books dedicated to unraveling
the mysteries of water. Hopefully this page has given you just
a sampling of that mystery.
In
Conclusion
Water is one of the most plentiful and essential of compounds, which
is vital to life, participating in virtually every process that occurs
in plants and animals. In fact, life is believed to have originated
in the worlds complex brews, the oceans. It is no wonder, that due
to its prominence, water has long played an important religious and
philosophical role in human history. The Chinese alchemist Tsou Yen
(3rd century BC) placed water among the five divine elements (together
with fire, earth, wood and metal) that make up the universe. And the
Greek Thales of Miletus (6th century BC) regarded water as the sole
fundamental building block of matter:
"It is water that, in taking different forms, constitutes the earth, atmosphere, sky, mountains, gods and men, beast and birds, grass and trees, and animals down to worms, flies and ants. All these are different forms of water. Meditate on water!"
The great
Aristotle included water among the four elements alongside earth, air
and fire, which belief persisted for more than 2000 years until experiments
in the second half of the 18th century showed that water is a compound
made up of the elements hydrogen and oxygen. Unfortunately, nowadays
practically none of us sees anymore the magic in such things as our
planet being covered with oceans of liquid water, in solid rocks of
ice floating on water, in sugar cubes disappearing in water whereas
milk blends with water (not to mention the behavior of tea leaves in
water), in clouds of water flying in the air or even in its beautiful
appearance in a rainbow and in the water crystals in snowflakes. Yet,
especially in the eyes of physicists and chemists, water is an extraordinary
substance with very unique and mysterious properties.
