Open Writing

Unless you are living during a drought, it is very easy to take water for granted. A large fraction of our bodies are water. The food we eat contains a large fraction of the water we ingest each day. Many biologists believe that life on Earth originated in water. The atmosphere contains a large amount of water. The seas were originally pure water; however, after millions of years of rivers carrying dissolved salts to the sea, the seas are about three percent salt, by weight.

Water can be broken into its constituents, hydrogen and oxygen, by passing an electrical current through the water. A small amount of an electrolyte, such as salt, must be added to pure water before it will conduct the electricity. Water participates in a process called “osmosis.” Osmosis can be made to occur by placing a permeable membrane between two volumes of water. One volume of water contains a salt that is more concentrated than in the other volume. In this arrangement, water will flow through the pores in the membrane from the volume containing the higher concentration of water (and lower concentration of salt) to the other volume. This process is employed in the use of laxatives to alleviate constipation. By ingesting a salt, water is caused to flow from bodily fluids through the wall of the intestine, thus increasing the volume of water in the intestine. Additionally, a process called reverse osmosis is sometimes employed to purify water.

The physical properties of pure water are rather unusual compared with those for other substances. Water freezes at a temperature of 0 degrees Celsius (C). However, water has its maximum density at a temperature of about 4 degrees C. Therefore, ice will float upon the surface of a fresh water body of water. It is good for us humans that this is so. If ice were to sink to the bottom of a pond, it would accumulate there and make the pond very unhealthy for any aquatic life that happened to be present.

Then there is water at higher temperatures. Most of us know that, at sea level, water boils and becomes “steam” at a temperature of 100 C. If the water were enclosed, eventually all of the liquid would become steam (a gas). As the ambient pressure is increased, the water becomes steam at increasingly higher temperatures. If, eventually, the pressure is increased to the “critical pressure,” about 221 atmospheres (or 3250 pounds per square inch), there is no longer a distinction between liquid water and steam. The water becomes a single fluid. Steam power plants have been built and operated successfully wherein the “steam” entering the high pressure steam turbine is a fluid that is above the critical pressure. These power cycles have a very high thermal efficiency; but, are more expensive to build than are more conventional steam turbine/boiler plants.

When we think of water, we usually think of liquid water that is in some container, or existing as a body of water. There is a lot more water around us than is obvious from being in one of the above forms. The atmosphere contains water vapour that is invisible to our eyes. The amount of water vapour in a particular volume of atmosphere depends upon its temperature and the atmospheric pressure. This water vapour in the atmosphere can hold a significant amount of heat energy. This must be considered by the engineer who designs mechanical, air-conditioning systems. The water vapour in the atmosphere will come to some equilibrium with solid surfaces that contain it. Therefore, at any time solid surfaces will never be “dry.” The number of water molecules per unit area of a surface will depend upon the humidity and temperature of the atmosphere to which it is exposed. It’s a good thing too; otherwise we would constantly suffer from dry skin.

So the poet who wrote “water, water everywhere; but, not a drop to drink” had it right.

© Colin Fisher

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Colin writes for Bonzer magazine. Please visit www.bonzer.org.au