Water, that most essential of chemical substances on our planet, is a wonderful gift of nature that most living things cannot live without, especially us human beings. Water, in the form of oceans and lakes and other ‘bodies of water’ constitutes almost three-quarters of the contents of the Earth’s surface. It is because of water’s abundance on Earth that life on this planet can be sustained, though there is now a pressing need to conserve water in all its forms as a result of water shortages and water pollution caused largely by human activities.
Before we can discuss the properties of water, we need to look closely at the chemical composition of water molecules. There are slight differences in structure of the water molecules ( H2O ) in the three states of water, viz. liquid water ( the form that it exists at room temperature ), solid water or ice and gaseous water or steam/water vapour. But the basic chemical structure of the water molecule consists of two hydrogen atoms bonded to one oxygen atom, forming a molecule that is slightly polar due to the different electronegativities of oxygen and hydrogen. In every H2O molecule, there are 2 lone pairs of electrons on the oxygen atom, resulting in a slightly negatively charged oxygen atom and causing the two hydrogen atoms to be slightly positively charged.
Because of the slightly polar nature of the hydrogen and oxygen atoms on each H2O molecule, there arises the very important phenomenon known as hydrogen bonding in which hydrogen atoms from neighbouring H2O molecules would be attracted to the oxygen atom of a H2O molecule; there are also intermolecular forces such as Van der Waals that acts between H2O molecules. The consequences of hydrogen bonding in water, ice and steam are that water thus possesses unique physical properties – properties that make it the most important chemical on Earth.
UNIQUE AND IMPORTANT PROPERTIES OF WATER in the three states, liquid, solid and gas.
Because of the presence of hydrogen bonding and Van der Waals forces, water has very high melting point ( zero degree Celsius ), boiling point ( 100 degress Celsius ) and heat capacity. In non-technical language, this means water takes a lot of heat energy for it to boil ( try boiling a kettle full of water over flame using your home stove ) and it retains its heat for a long time after boiling. It also takes a long while for it to cool and it has to dissipate a lot of its energy to become ice.
A. Liquid Water: The fairly strong forces attributable to hydrogen bonding and Van der Waals’ in water causes water to have high surface tension and viscosity – meaning there is a surface force on the water that could allow a light insect ( water spiders, light drogon flies, pond skaters, etc ) to stand on water surface without ‘sinking’ or drowning in it. But the hydrogen bonds in water are constantly being broken and formed and so even though there is some form of ordered packing of water molecules, this ordered structure is not permanent but in a constant state of flux.
B. Solid water or Ice: When water freezes to become ice, the H2O molecules begin to assume a 3-dimensional, tetrahedral, lattice structure over a very open space, resulting in the fact that, compared to water at just before freezing, ice is less dense than water and therefore ice floats on water and not vice versa. Another important consequence of this fact is that when water forms ice or solidifies in a lake, it does so from the surface downwards ( because the less dense ice forms floating on still liquid water ), thus allowing aquatic living things to survive. That is why, during winter, when the top layers of lakes and rivers are frozen, fishes and other creatures still could survive in the liquid water at the lower layers of the lake.
C. Gaseous water : steam and water vapour: Again, because of hydrogen bonding, water has a very high boiling point among substances existing in liquid form at room temperature. This again is a property of water that is absolutely essential for the sustenance of life on Earth: if not of the high boiling point of water, all our lakes and oceans will become devoid of water as water would have been vaporized easily and it may not even rain because water would have remained gaseous if not for its high boiling point.
Water as a SOLVENT for Ionic compounds:
One does not have to learn a lot of chemistry to know that common salt dissolves in water. In technical jargon, salt is sodium chloride NaCl, which is an ‘ionic’ compound where the bonds between each Na atom and Cl atom are ionic, not covalent. Ionic compounds very often exist in the form of strong, compact lattice structures in which the atoms are held together by strong ionic bonds. However, the beauty of nature is that it provides us with a very versatile solvent known as water which can break up the lattice structure of ionic compounds such as sodium chloride.The slightly positive ends of the water molecule ( the hydrogen atoms ) are attracted to the negative ions ( the anions ) – the chloride ions, while the slightly negative ends of water molecules ( the oxygen atoms ) are similarly attracted to the positive ions ( cations ) – the sodium ions, and this attraction results in a bond formation, which releases energy sufficient to break up the lattice structure of sodium chloride. In the language of chemistry, we say the ions in sodium chloride have been ‘hydrated’ and the energy released is known as the enthalpy of hydration. Of course, water does not dissolve non-ionic compounds like naphthalene ( moth balls ). This important property of water ( as a solvent ) makes it possible for many nutrients ( often ionic compounds chemically speaking ) to be transported into living systems in the form of aqueous solutions, thus sustaining and nurturing life.
We count our fortunes as human beings, blessed with such miraculous gifts from nature as water. We must not therefore squander away our most essential resource on Earth; we must take immediate steps and make concerted efforts to conserve water, for the present generation as well as all generations to come.
