The Anatomy of a Water Molecule

“Water, water of life…” these are the lyrics to a well-known school hymn sung by children everywhere but water really is the substance that defines life. Water is the medium in which the reactions of life take place without water there would simply be no life. But what is it about water that makes this substance so unique and so fundamental? Most of these answers lie in the unique properties of each water molecule.

Everybody knows the chemical formula of water, H20. This tells you that two atoms of hydrogen are joined to one atom of oxygen.  The water molecule is made up of two O-H bonds. The bonds between the oxygen and hydrogen in water are called covalent bonds.

As you might remember from high school science a covalent bond is the sharing of electrons between atoms in a molecule so that each atom gets itself a stable and full outer shell. So in water each of the hydrogen atom shares one electron with the oxygen atom to help both elements fill their outer electron shells. 

Covalent bonds within a molecule are very strong but in contrast the strength of attraction between individual covalent molecules within a covalent compound are weak. The forces of attraction between covalent molecules within a compound are called van der Waals.  As a consequence of these weak forces of attraction most covalent compounds have low melting points and boiling points but not water.  So why does water in comparison to other covalent compounds have such a high boiling point?

Water is a polar molecule. There is a difference in electronegativity between the negative oxygen atom and the positive hydrogen atoms. The force of attraction that exists between polar molecules containing positive hydrogen atoms and a lone pair of electrons in another molecule is called a hydrogen bond. Hydrogen bonds exist between individual water molecules and explain the high boiling point in water. 

Many of water’s unique properties that make it so fundamental for life’s reactions are due to these properties.

You may have noticed in a glass of water how it appears to have a skin. Or worse you might have mistimed a dive and ended up belly flopping in to a pool. If you have then you will have realized that water has a high surface tension. This is because of the hydrogen bonds, which pull the water molecules down together at the boundary between water and air.

Also, large amounts of energy are needed for water to change state. It is a good job too because otherwise all the water on the planet might have already evaporated. However, because there are so many hydrogen bonds in water a lot of energy is need for water to vaporize. It is exactly because so much energy is needed to evaporate water that the human body uses this mechanism for sweating and to cool down.

Water is a brilliant solvent. A solvent is a substance in which something can dissolve. Water makes such a good solvent because it is a polar molecule. The fact it is a polar molecule allows other polar molecules to dissociate into ions. Once ions dissolve in water the water molecules surround them. This is the reason why all reactions in our body’s cells take place in an aqueous solution.

Water is also unique by being denser in liquid form than it is in solid form. The reason ice floats. The fact that ice floats is again because of the unique anatomy of its molecules. When freezing takes place the two hydrogen atoms and two lone pairs of electrons in the oxygen atom results in a fixed three-dimensional tetrahedral arrangement. This means that ice has a rigid and less dense structure than liquid water. It is because ice floats and lakes freeze at the top that life can continue on below their surface.

It is because of the anatomy of the water molecule that it has so many unique properties. These properties allow water to take part in the most important chemical reactions that every human on the planet is dependent upon.