(click
here to download word version of this article)
Molecules behaving
badly
Water seems to be a simple substance: two hydrogen atoms covalently
bonded to an oxygen atom, forming a boomerang-shaped molecule.
But water is no straightforward molecule and behaves badly whenever
it gets the chance.
Water exists as a solid,
a liquid and a gas at temperatures not too far off those in which
we live, while most other materials go from one extreme to the
other. Water is much more viscous than other substances because
its molecules hold on to each other relatively tightly even in
the liquid state. If you heat and squeeze it at the same time,
water can even enter a hybrid supercritical phase between gas
and liquid, in which it has properties of both but is neither
one nor the other. Unlike almost every other chemical, water expands
when it solidifies. And, to top it all, life simply would not
exist without this badly behaved molecule.
Hydrogen bonding
Much of the bad behaviour of water can be explained by the fact
that each molecule is polar. There is a residual negative charge
on the oxygen atom that makes its hydrogen neighbours slightly
positively charged. This polarisation means that water molecules
can hook on to each other, with the positive hydrogen on one molecule
linking to the oxygen of another through a connection known as
a hydrogen bond.
Water molecule
The water molecule is composed of one oxygen atom (O) joined
to two hydrogen (H) atoms with a covalent bond, sharing electrons.
The molecules join together with strong hydrogen bonds.
Your life depends on
it
While water might behave badly, it is its strange and unexpected
properties that make life on earth possible. Water has a large heat
capacity, for instance. This is the amount of heat in joules needed
to raise the temperature of a substance through one degree kelvin.
Water's specific heat capacity is 4200 joules per kilogram kelvin
(4200 J kg-1K-1). This means that every kilogram
of water needs to absorb 4200 joules of energy to raise its temperature
by one degree. The specific heat capacity of a similar-sized molecule
such as chloroform (trichloromethane), on the other hand, is only
96 J kg-1K-1.
When things heat
up
The reason water has such a large heat capacity is down to those
hydrogen bonds. If they did not form between water molecules all
the energy could be used to increase the vibration of the molecules
in the liquid, thereby raising the temperature. However, before
any heat energy can warm the water, the hydrogen bonds have to
be broken, a process that also uses energy. Once the bonds are
broken, the heat can increase the movement, or kinetic energy,
of the molecules, raising the temperature of the water.
Once it has absorbed
heat, water is equally reluctant to lose it again. As it begins
to cool, some of the energy it would otherwise release goes into
re-making those hydrogen bonds. This helps explain why the temperature
of the sea is usually little different between the height of summer
and the cold of winter. It takes a lot of energy to raise the
temperature, and a lot of energy has to be lost to lower it again.
Biologists believe the
high heat capacity of water is one of the factors that allows
living things to regulate their temperature. With each cell of
a plant or animal composed mainly of water, when the outside temperature
goes up, a lot of energy has to be absorbed by the water in the
cells to break the hydrogen bonds before the water's temperature
rises. So, relatively small changes in the temperature outside
don't have a big impact on the inside of the cells.
Watery solution
Water's polarity also endows it with another unique property -
its ability to dissolve a wide variety of other chemicals well.
The hydrogen bonds between water molecules in the liquid can easily
be displaced when the charged particles making up sodium chloride,
for instance, (sodium and chloride ions) are present. The positively
charged sodium ions can quickly latch on to the negative oxygens,
and the negative chlorides seek out the positive hydrogens.
The picture is complicated
by the fact that the water molecules can actually split in the
presence of such ions releasing hydroxide (OH+) and
hydrogen (H+) ions, which form H3O+
oxonium ions. These surround the dissolving sodium and chloride
ions, forming a solution. Water's polarity means it can dissolve
almost any ionic solid and many polar materials, such as ethanol.
Indeed, water and ethanol are said to be miscible because they
dissolve in one another. Adding ethanol, or other compounds such
as ethylene glycol, actually lower the freezing point of the water,
so they are used in antifreeze solutions.
How water dissolves salt i) Sodium (Na+) and chloride (Cl-)
ions in solid sodium chloride (NaCl, or common salt) are in
a cubic crystalline formation.
ii) When immersed in water, Na+ ions are
attracted by a slight negative charge on the water's oxygen
ions, while Cl- ions are attracted by a slight
positive charge on the hydrogen ions.
iii) Once all the Na+ and Cl-
ions are attached to water molecules, they are hydrated and
the solid NaCl is dissolved
Hydration
Water can also hydrate non-ionic and non-polar molecules, such
as the huge natural polymers found in living cells - proteins
and nucleic acids (DNA and RNA). Water molecules attach themselves
to the long chains of amino acids that make up proteins, through
the attraction of residual charges on certain amino acids and
the positiveness of water's hydrogens and its negative oxygen.
The hydration of proteins helps them fold up into their active
shape in the body.
The ability of water
molecules to dissolve almost everything from the simplest ionic
compound to hydrating the very stuff of life, DNA and proteins,
have led to it being described as a universal solvent. Many processes
in living cells rely on the universal solvent's ability to dissolve
ionic chemicals and polar molecules and carry them across cell
membranes, as well as allowing them to interact with other compounds
such as enzymes and receptors, the biological sensors.
The changing phases
of water
Water exists as a solid, liquid and gas all within a hundred-degree
range under everyday conditions. But its melting and boiling points
are much higher than scientists would expect for a small, supposedly
simple molecule. For the sake of comparison, chloroform melts
at -63° Celsius. The picture is similar with other small molecules.
Frozen water, however, melts at 0° Celsius and above.
The structure of water
Water molecules are attracted to one another because of their
polarity. In liquid water, molecules are joined together in
small groups.
The structure of ice
Ice has an open lattice structure, with the molecules further
apart than in liquid water. This makes ice greater in volume
than water but less dense, so it can float.
The structure of steam
Water vapour, or steam, has a random structure, since the
molecules are in a state of high energy and move too fast
to form permanent bonds.
Again, hydrogen bonds
are behind water's behaviour. In the solid state - ice - each
water molecule is connected to four neighbours by hydrogen bonds,
which lock them in place. When ice is heated these hydrogen bonds
have to be unlocked, or broken, to allow enough movement for the
liquid to flow. The energy to break them apart comes from the
heat. But because it is being used to break bonds rather than
increase the kinetic energy of the water molecules, more energy
is needed to melt it than for a similar solid that has no hydrogen
bonds. Therefore, the melting point of ice is higher than expected.
Although the bonds are
broken in melting water, the liquid still has an ever-changing
network of hydrogen bonds between molecules. This means that more
heat energy is needed to release molecules from the surface of
the liquid when it boils, which explains why water's boiling point
at sea level is high at 100° Celsius, whereas chloroform boils
at a balmy 60° Celsius. Living things would freeze to death or
boil dry if water did not behave so badly.
The catalogue of water's
bad behaviour seems almost endless. It expands when it freezes,
its melting point falls when the pressure increases, the heat
capacity of the liquid is twice that of ice or steam, and there
exist a wide range of different crystalline and amorphous forms
of the solid, ice. Most other materials fit a simple description.
Water is indeed very different and its exceptional behaviour makes
it the liquid of life.
About the author…
David Bradley is a freelance science writer specializing in chemistry.
He can be reached through his Elemental Discoveries website: http://www.sciencebase.com
Water molecule 
