The wonder of water
Water, water every where; nor any drop to drink. Water is the one constant the world over. To the best of our knowledge, all forms of life – from simple bacteria to massive trees and animals – require water. It is the one compound that is essential for life.
But water is also so much more. The science of water fills volumes. Indeed, one book on water I used when a graduate student was more than 3,000 pages long. Understanding water is fascinating science.
Water is also the source of spectacular artistry. Or, maybe a better way to say it would be that in the transformation of water between its various states, nature generates some spectacular sculptures. This is readily apparent at this time of year with frost covering the landscape.
The recent blast of cold air combined with relatively high humidity generated frost on just about any surface available. Trees, bushes, buildings and cars were coated in tiny crystals formed from the direct deposition of water vapour. The view from my window lent itself to images of fairy lands as twinkling crystals of solid water floating through the air.
To understand our winter landscape, it is perhaps useful to understand water itself.
In some ways, water is a very simple molecule. It consists of two hydrogen atoms and a single oxygen atom in a V-shape with the hydrogen atoms along the arms and the oxygen at the point. It also has two lone pairs of electrons which extend from the oxygen atom forming a second V pointed in the opposite direction but twisted 90 degrees.
The resulting shape or electronic structure of water can be described as a tetrahedron with two corners occupied by hydrogen atoms and two corners occupied by electron clouds.
This “two-and-two” arrangement is important for the properties of water. This allows each water molecule to share two hydrogen atoms with its neighbours and to accept two hydrogen atoms via its lone pairs of electrons from two neighbours.
The resulting structure joining oxygens and hydrogens is called a “hydrogen bond” and consists of an O-H-O connection. It differs from the covalent bonds found within molecules where electrons are fully shared between atoms or the ionic bonds in salts which depend on the attraction of positive and negative ions.
Hydrogen bonds are weaker and more fragile than covalent and ionic bonds but are strong enough to hold water molecules in place. They are intermediates between full-fledged connections and nothing at all. They are fleeting in existence. But the consequence of this weak interaction is that water has a fairly high melting and boiling point for a molecule its size.
For us, the important consideration is that because of hydrogen bonding, water is a liquid for the most part on the surface of the Earth. If hydrogen bonds were stronger, all water would be a solid. If weaker, it would be nothing but a gas.
And the energy involved in a hydrogen bond means that water can also change to a solid by freezing or to a gas by evaporation. And it can change back – from a solid to a liquid by melting or from a gas to a liquid by condensing. All it takes is small shifts in the amount of energy available – in the temperature.
Water molecules can also do one more trick.
They can, under the right circumstances, go directly from a solid to a gas (sublimation) or a gas to a solid (deposition). It is the latter which leads to the fairy land delight of a frosty morning.
The crystals of hoar frost are generated when surfaces cool below the freezing point of water and come into contact with humid air. The cool surfaces stick the water molecules in place through hydrogen bonding. The humid air has just enough water to allow molecules to arrive one-by-one. Too much humidity and droplets are generated resulting in ice and not frost.
It is adding one molecule at a time which generates the spectacular beauty. It allows frost to grow in unpredictable directions and to take on shapes or forms that curve, branch and twist.
Frost on a window pane can generate feather or tree shapes which lend themselves to visions of surreal landscapes. On tree branches, hoar frost generates clear or translucent crystals which shimmer and shine in the sun. Flowers and plants develop beards of crystals, turning plants white.
Perhaps not surprisingly, the term “hoar” comes from Old English and means “showing signs of age.” A spectacular display of hoar frost can make trees and other obstacles look as though they have aged greatly.
There are many different types of frost or many variations on the basic theme but they all arrive because of the deposition of water vapour.
They are physical expressions of the underlying molecular geometry of water.