The Structure and Properties of Water

Essential for life and covering approximately seventy per cent of the Earth’s surface, water really is a remarkable substance. Existing in a multitude of forms, whether as snow, ice, cloud or vast deep oceans, water has a number of unique physical properties. These physical properties of the macroscopic world are dictated by the structure and composition of the water molecule and the way each molecule interacts with its neighbors.

A water molecule consists of two atoms of hydrogen bonded to an atom of oxygen, thus giving us the familiar formula H2O. The two hydrogen atoms are bonded to either side of the central oxygen atom, but the molecule is not linear. The three atoms form an angle of approximately 107° . It is the angularity of each individual molecule which leads to the amazing hexagonal shapes of snowflakes and crystals when water is in its frozen, solid state. When arranged in a frozen lattice, water molecules align themselves such that there is more space between the molecules than in the less ordered, liquid state. On a macroscopic level, this means that ice is less dense than water and will float, an unusual property; most substances are denser in their solid form.

Water has a maximum density at 4° C which means that even at the bottom of the deepest darkest oceans the water temperature is always a couple of degrees above freezing. One of the reasons life on Earth has evolved and prospered is the fact that oceans, lakes, rivers etc. if deep enough, do not freeze solid. On the Celsius scale pure water freezes at 0° C and boils at 100° C, these being defined at sea-level and normal atmospheric pressure. Reducing the outside pressure, by going up a mountain for example, will cause water to boil at a lower temperature. Impurities, such as dissolved salt, will cause water to boil at a higher temperature, a property utilized in cooking. Impurities will also cause water to freeze at a lower temperature; the salt and grit put on winter roads attest to this.

Water has the ability to act as a solvent for a wide variety of substances, and is often known as the ‘universal solvent’ by chemists. Water molecules exhibit a property known as hydrogen bonding, which is an electrostatic attraction between molecules. This comes about because of the difference in size and electrical properties of the relatively small hydrogen atoms of a molecule and the much larger oxygen atom. Without going into too much detail the hydrogen atoms each obtain a slight positive charge while the oxygen atom has a balancing negative charge. Thus there are slight attractive forces between the water molecules in liquid form, but also water molecules will be attracted to and surround any dissolved substance. Pure water is neither acidic or alkaline, it has a pH of 7. It is dissolved substances which cause a change in the pH of water.

The strong surface tension of water, which allows insects such as pond skaters to ‘walk’ on water is another macroscopic physical property, resulting from interactions between individual water molecules. Strong surface tension plays a vital role in enabling plants to transport water, and therefore dissolved nutrients, around their systems. Water forms droplets and can cling to windows or other surfaces because of surface tension.

Water requires a lot of heat energy to raise its temperature compared to other substances. This means that the oceans are slow to warm but also slow to cool. For a liquid, water also conducts heat rather well, and this results in a fairly even distribution of temperature in the oceans. Thus the oceans play an important role in regulating the Earth’s temperature and climate. Because water remains a liquid over such a large temperature range (0-100° C), liquid water can be found in most places on Earth. Water is unique in being the only substance that exists naturally on Earth in the three physical states of solid, liquid and gas. The cycles that govern the changes between these states, the evaporation of seawater to form clouds for instance, involve the exchange of enormous amounts of heat so again water is intrinsic to temperature and climate.

This article has tried to convey the vital role that water plays in our very existence, from our obvious requirement for water on a biological level, to its importance in regulating the global climate. Water is made from simple molecules, each with a simple structure, but they come together to form wonderfully complex physical systems with unique properties.