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Amazing Water

Most of us will know quite a lot about water; for example, that water is essential to all life, which of course includes us humans; that a molecule of water is composed of 2 hydrogen atoms and 1 oxygen atom - H2O; that the salt water oceans cover much of the surface of the earth (about 70%); that solid water - ice - is common near the poles and at the top of some mountains; that water vapour, the gaseous form of water, is in the air we breathe and when it condenses it forms the clouds in the sky; that we need fresh water to drink; that water is an excellent solvent; that we use it for washing, for cooling, for dealing with our sewage, for cleaning, for producing energy, for cooking, for fighting fires, for recreation; and we use it in industry and agriculture. Indeed there is so much that could be said about water that a whole book could be written about it. But that is not the intent here, nor is it feasible.


Here are some properties and facts about water that are probably less well known but which are also very significant. In what follows water refers to the liquid phase, ice to the solid phase, and water vapour to the gaseous phase. Some terms are explained in the glossary at the end.


(1) We humans are largely made of water but the percentage depends on age. Newly born babies are 78% water dropping to 65% after a year, adults are about 60% for men and 55% for women.


(2) Water is transparent in the visible part of the electromagnetic spectrum. Thus aquatic plants, algae, and phytoplankton can live in water because sunlight can reach them. An example would be the sea-grasses which exist in St Georges Basin and Jervis Bay. In the atmosphere the water vapour allows visible light to reach Earth but longer wavelengths tend to be absorbed. Much of the energy emitted by the Earth is in the infrared but there is enough visible light to allow astronauts to view the the planet from space.


(3) The distance of planet Earth from our sun is such that water can exist in all three forms, liquid, solid, and gas. Were we closer to, or further from, the sun this condition would not hold. The mass of the earth and the gravity it generates allows it to maintain an atmosphere which includes a water vapour component. The greenhouse effect of this water vapour keeps the surface of the earth at a temperature of about 30 degrees higher than would otherwise be the case.


(4) The Celsius (or Centigrade) temperature scale is based on fresh water. The freezing point of water is defined as 0 degrees, and the boiling point as 100 degrees. Both at a pressure of one standard atmosphere. On the other hand the Farenheit scale has a zero point based on the freezing point of brine (which is water saturated with sodium chloride). On the Fahrenheit scale the freezing point of fresh water is 32 degrees while its boiling point is 212 degrees.


(5) In general water gets denser as it cools. However, the maximum density of (fresh) water occurs not at 0 degrees but at about 4 degrees Celsius. So once water attains a temperature less than 4 degrees it rises to the surface. And furthermore, once it becomes ice it expands becoming less dense than water and floats. This latter property is quite unusual although not unique. So ice forms from the top down rather than from the bottom up. The situation with salt water needs consideration. The density of salt water increases with salinity. The salinity of the oceans is about 35 g sodium chloride (mostly) per litre. At this salinity freezing occurs at about -2 degrees. But note that the ice formed is fresh because the process excludes the dissolved salts.


(6) Water has a very high specific heat (see glossary); the second highest known. It requires a lot of energy (heat) to warm it up and conversely it takes a long time to cool down. The fact that water can hold onto a lot of energy helps drive the oceanic convection currents and allows the transfer of heat from one part of the world to another - normally from the tropics towards the cooler temperate regions - for example the Gulf Stream.


(7) Considering what we know about the dangers of water and electricity it may be a surprise to learn that ultra-pure water is a very good insulator. However, since it is an excellent solvent it does not normally exist in this high purity form (in fact it requires expensive technology to make very high purity water). Water usually contains dissolved impurities from its surroundings and becomes a good conductor of electricity.


(8) The latent heat (see glossary) of freezing/melting of water is the second highest known while the latent heat of evaporation/condensation is the highest known. Evaporation, for example, facilitates the transfer of large amounts of heat from the ocean surface into the atmosphere. The latent heat of condensation is released in due course when rain-carrying clouds are formed. In the tropics with higher sea temperatures cyclones can form with sometimes drastic results. The released latent heat helps fuel the intensity of the cyclone. In the general situation over land or sea warm air will rise. If water vapour is present clouds will be formed when condensation occurs.


(9) In the introduction the chemical symbol for water was given, viz. H2O but a more detailed examination involving isotopes (see glossary) is worthwhile. Both hydrogen and oxygen have more than one stable isotope. Hydrogen has two (1H and 2H) while oxygen has three (16O, 17O, 18O). 2H is the less common isotope of hydrogen (having a neutron as well as the proton in its nucleus); it often called deuterium. 2H is 6420 times less common than 1H. 2H 2O is commonly known as heavy water and is important in some nuclear reactors. 16O is the most common isotope of oxygen; 18O is about 500 times less common than 16O, while 17O if far less common. It is 2,632 times less common than 18O and we needn't consider this isotope further.

A note on nomenclature. The subscript after the chemical symbol H indicates the number of hydrogen atoms in the water molecule. The superscript before the chemical symbol (for H and O) is the nuclear mass number (the number of protons plus the number of neutrons in the nucleus of the atom).

When water changes from one state to another e.g. liquid to gas, fractionation (see glossary) occurs. Furthermore, fractionation is a temperature dependent process. With a mass spectrometer (see glossary) scientists can measure ratios such as 2H/1H or 18O/16O in a sample. In ice cores the difference between winter and summer temperatures can be seen and the oscillations in the ratio (18O/16O) can be counted like tree rings to determine age. For very old ice where there is severe compaction scientists rely on the average value of the ratio. Such measurements in ice-cores provide information on temperature as a function of time; in other words they provide paleoclimatic (see glossary) data.

Glossary

Fractionation: In the sense used above fractionation is a process in which one isotope is treated prefentially with respect to another because of the difference in mass. For example, the lighter form of water H2O evaporates more readily than the heavier form. Photosynthesis is another process where fractionation occurs although the element carbon is involved in this case.

Isotope: Isotopes are atoms of an element which have different numbers of neutrons in the nucleus. An example would be carbon (chemical symbol C and 6 protons in the nucleus) with two stable isotopes 12C and 13C. The first 12C, has 6 neutrons, the second, 13C, has 7 neutrons. The two isotopes, therefore have different masses. A radioactive isotope 14C also exists but the above text does not involve radioactive isotopes. In similar fashion because the oxygen nucleus has 8 protons, 16O must also have 8 neutrons.

Mass Spectrometer: An instrument to separate and identify the atoms (molecules in some cases) in a sample. The classic instrument involves the movement of charged particles in a magnetic field. Particles of different mass follow different paths.

Latent heat: is an amount of heat associated with a change of state - such as from solid to liquid, or liquid to solid. If the change is from a more ordered state to a less ordered state (i.e. solid to liquid, or liquid to gas) heat must be supplied and usually comes from the surroundings. If the change is in the opposite direction heat is released. No change in temperature is involved.

Paleoclimate: The climate, or parameters such as temperature and atmospheric gases, of earlier times in the earths history. Information going back 600,000 years has been obtained from ice cores.

Specific heat: the amount of heat required to raise the temperature of a substance by 1 degree.


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