Water - The Essential Substance

        Water is a simple chemical compound, each molecule composed of one oxygen and two ____________ atoms. Why, then, is it so important?         Most fundamentally, water is essential for life. Every living organism on earth must have water in some form in order to survive. Every living cell is composed primarily of water. When space probes are dispatched to other planets in search of other life forms, they look for the presence to water to determine if life, as we know it, is possible there.

What properties or characteristics of water make it so essential? Special Properties         Water is a special chemical substance. In chemical terms, the water molecule is strongly dipolar. This property makes water an effective ____________, particulary for crystalline salts. The small size of hydrogen atoms makes it possible for molecules of water to effectively bond together or chemically associate, particularly at lower temperatures. However, water also partially dissociates into very minute concentrations of acid [H3O⁺] and base [OH^-] ions, a characteristic which leads to the use of the pH scale to measure relative acidity or alkalinity.

 The chemistry of water leads to some very interesting, and important, physical properties. At the temperatures and pressures found on the surface of this planet, water can be found in all three physical states: solid, liquid and ____________. While pure water freezes at 0 degrees Celsius, because of molecular association its maximum density is reached at 4 degrees C. This permits solid water (ice) to float on liquid water. This molecular association also accounts for the strong surface tension exhibited by liquid water, which permits certain animals to move about on its surface and aids water to penetrate tiny spaces in the soil and elsewhere. In its liquid form, water has relatively high viscosity, thereby slowing its rate of runoff from the land and keeping it more available for terrestrial life.

Water has a much higher boiling point than its nearest chemical relatives, making it relatively efficient in the high temperature conversion of heat into work. Greater heat is required to vaporize water than that needed to vaporize an equal weight of any other liquid. This makes water vapour a good carrier of energy and is responsible for its dominant effects in modifying the ____________ of the earth. Indeed, more heat is required to raise the temperature of water than is required to raise the temperature of an equivalent weight of any other liquid. This means that water, even in its liquid state, can transport great quantities of heat and have major effects on climate.

 Pure water is transparent to the visible wavelengths of light, with maximum transmission of light in the blue portion of the visible spectrum. It also transmits reasonably well many of the wavelengths used by plants for ____________, thereby supporting plant growth in oceans and other water bodies. Water vapour in the atmosphere, unlike liquid water, absorbs the sun's ultraviolet radiation which otherwise would be extremely harmful to life on earth. Both liquid water and water vapour are nontransparent to the infrared radiation which is given off by the earth, thereby preventing much of this heat from being lost to space and keeping earth's temperatures at habitable levels.

 While oxygen atoms comprise almost 80% of water by weight, this oxygen is chemically bound and not available to plants and animals. However, some molecules of oxygen do dissolve in water, particularly at lower temperatures. This dissolved oxygen is available for ____________ and permits animals to inhabit lakes, streams and oceans.

These special properties of water would be interesting, but not nearly as important, were not water so abundant here on earth. With an estimated 1.4 billion cubic kilometres of water on this planet, its special properties become extremely important. Not all water, however, is readily available. The oceans, which cover well over two-thirds of the earth's surface, also contain about 97% of all its water. However, there are also substantial quantities of fresh water in various forms and locations. Almost 90% of this fresh water is locked up in polar ice-caps and glaciers. Only about 0.3% of all water on earth is currently available fresh water, and almost all of this is located in natural underground reservoirs or ____________. Less than 1 of every 5,000 litres of water on earth can be found as either surface or atmospheric fresh water. However, this minute fraction still represents an estimated total volume of more than 200,000 cubic kilometres, enough to cover all of Canada to a depth of 17 metres.         How, then, is all this water so essential? Why, with so much water around, do we have so many problems finding it and using it?

As we noted earlier, water readily dissolves most salts. However, water is a poor solvent for proteins and other large organic molecules in living cells. This means that liquid water can transport nutrient elements to and within living cells without dissolving and destroying the organic molecules of which cells are made. Water also transports waste elements away from cells. Thus, all living cells contain water, with the proportion ranging from 3-5% in resting seeds to perhaps 95% in very young plant tissues and jellyfish. This water penetrates the cell protoplasm, but does not dissolve it. Increased water content leads to increased chemical or ____________ activity within the cell. Without water, there would be no life.         This means that the distribution of life on earth is highly dependent on the distribution of water. Our needs, both metabolically and otherwise, are often affected by the distribution of water and by competition for available water.

Of course, water is not evenly distributed, and this has a major impact on us, both as living organisms and as a complex social species. Most water is concentrated in the oceans. Even on land, the distribution of water varies greatly because of weather patterns, evaporation rates, and other factors.         We humans often choose to live in areas with dry, sunny climates. At the same time, we require large quantities of water for domestic, recreational, agricultural, and industrial uses. This inevitably leads to water ____________ problems and water redistribution schemes are undertaken. In other situations, human populations settle on fertile coastal plains or in river valleys. These areas are susceptible to flooding during storms and spring run-off. Again, issues of water management become important.         In either case, engineering solutions are usually available to alter water distribution, although often at high cost. We must attempt to weigh fairly the costs and benefits, both short- and long-term, before undertaking such engineering projects.

Direct support of life, while most important, is only one of a multitude of roles played by water on our planet. As noted earlier, water is extremely important as a modifier of climate. It stores heat in great quantities, transports it around the globe, both in ocean currents and in atmospheric vapour, and helps to retain stored ____________ at the earth's surface by reducing its radiative loss to space.

 As an effective solvent, and as a physical mass releasing energy as it moves downhill, water is a major modifier of the earth's surface. It erodes and breaks down rock, carries materials downstream, and deposits these eroded mineral particles in depressions where they can combine with organic materials to form ____________. It can also erode these soils, leaving a greatly altered landscape. The very nature of our planet's surface is a product of water and its distribution.         Of course, the very ability of water to run downhill and shape the earth's surface is dependent on a process known as the water cycle or hydrologic cycle.

        In this cycle, heat energy from the sun causes liquid water and ice to evaporate or sublimate and escape to the atmosphere as vapour. This vapour is then transported within the atmosphere until such time as changing conditions cause some of it to precipitate or fall back to the earth's surface as rain, snow, or dew. This water, once returned to the surface, can resume its downhill movement until it reaches the oceans or is vaporized once more. This process of ____________ is extremely important as a means of cleansing water of its dissolved substances, which are left behind when the water evaporates.

As human civilizations have developed, many additional uses of water have become important. Some of these uses have resulted in problems both for ourselves and for the earth as a whole. Our challenge it to use water in sustainable ways, such that its use by future generations of humans and other living organisms is not compromised. With its abundance and its properties as a solvent, water has long been used for cleaning and for dissolving waste products. We wash in it; we wash our possessions and products with it; we use it to dissolve our unwanted materials and flush them away. Unfortunately, when we have completed our cleaning and waste removal activities, the resulting water is dirty or "polluted". When we return water to nature in this state, we are relying on natural processes to cleanse it. We are also passing our __________ on to other users or to other living organisms. These wastes may be toxic to these other living things. The more concentrated these wastes become, the more toxic we can expect them to be. Water treatment facilities must be used to remove these wastes before the water is returned to nature. We must then find ways to safely dispose of, or reuse, these so-called "waste" materials, such that they do not cause adverse ecosystem effects now or in the future.

Humans rely on water to sustain their agricultural crops. Usually, the natural precipitation will be sufficient to water these crops. In other situations, artificial watering systems, usually referred to as irrigation, are used. These irrigation schemes involve modification of the natural water distribution patterns. Sometimes, extra water is drawn from natural groundwater reservoirs far below the soil surface. Other times, rivers are dammed and their waters are diverted through canals to be used on agricultural fields. In the short term, these irrigated croplands can be highly productive. However, any of these diversion schemes have potential long-term consequences for the ecosystems being altered. Often, we have failed to anticipate these problems. Over time, irrigated lands may become saturated with salts from this water, resulting in serious loss of __________. The runoff water from agricultural fields may be contaminated by fertilizers or pesticides used to grow and protect the crops. These contaminants may harm downstream ecosystems in a variety of ways.

Water running downhill is an important source of __________. By damming rivers and streams, we have harnessed some of this energy to generate electricity, power machinery, and provide light and heat for our homes and businesses. Done properly, this energy generation should have little polluting effect on the water being used.

Dams may also be beneficial to some users by retaining water longer, minimizing extremes of flow, and reducing downstream flooding. However, changing the flow patterns in rivers by damming can have adverse effects on ____________ and wildlife which rely on those rivers for their life support. It can also affect the downstream uses of those rivers by other people, and reduce the fertility and productivity of downstream ecosystems which rely on flooding for nutrient input. Creating new reservoirs can flood large areas and may, under certain conditions, lead to pollution of the water. The long term consequences of all these possible effects should be considered when making decisions about the use of water for energy generation.

 Water is also used industrially and domestically for transferring heat during various cooling or heating processes. Again, if done properly, this should not result in contamination of the water with pollutants. However, the very act of changing the water temperature can have adverse effects on downstream life when this water is discharged to nature. Care must be taken to ensure that such effects are avoided, or at least minimized.         Water, in its liquid form, is a primary medium for heavy ____________. Giant ships ply the oceans and large lakes of the world, carrying raw materials and manufactured products. In northwestern Ontario, the waterways were the only practical means of travel for native peoples and for the first Europeans who traded with them. These uses of water should be sustainable, provided ships are careful to avoid dumping waste materials, and provided accidents do not result in the spillage of toxic pollutants such as oil. Unfortunately, both of these problems continue to occur all too frequently.

Water is so fundamental to our very existence that we must seek to learn how it affects our lives, and how our lives affect it and the other living things that rely on water for their existence. One way to do this is to study the role of water in ecosystems, particularly in aquatic ecosystems where water is so abundant. In northwestern Ontario, lake ecosystems are everywhere. A better understanding of these systems can give us a better appreciation of the importance of water and the roles it plays in linking us to the world around us. The diagram, below, presents a simplified view of a typical lake ____________.Energy from the sun drives this ecosystem, but all the communities shown, the plants on shore, and those humans who use this ecosystem, are linked by the common element, water. Anything affecting the quality or availability of this water will have an impact on the entire ecosystem. However, ecosystems are complex and our understanding of them is incomplete. Often, We are not able to predict in advance exactly what that effect might be.

 Water is an ____________ human need. It is also an invaluable resource to human societies; for cleaning and waste removal, for energy generation, for food production, for cooling and heating, for transportation, and for recreation. 

Wise use of water, with a view to the long-term needs of other users, both human and otherwise, can be ____________. We must continue to study the role of water in our lives and learn more about the functioning of our ecosystems. Only with better knowledge and understanding can we make wise decisions for water use and management.