1 Water-General Introduction

Hardeep Rai Sharma

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Objectives

  1. To explain the structure of water molecule
  2. To understand the different properties of water
  3. To explain the role and importance of water
  4. To understand different issues related with water

2. Concept Map

 

3.        Introduction

 

Water is necessity for survival of life on earth. About 70% of Earth (often called as Blue Planet) is covered with water and as far as we know, it is the only planet in our solar system containing liquid water. The percentage of water in human body having 70 kg of weight is 67.85 % (Mitchell et al. 1945). Many philosophers, scientists, saints from different sects including holy books quote the importance of water. Some of them like:

 

Aapo Hi Sstthaa Mayo-Bhuvasthaa Na Uurje Dadhatan |

 

Mahe Rannaath Caksshase ||from Aaph Suktam, Rig Veda (10.9)||

 

Which means “O water, because of your presence, the atmosphere is so refreshing, and imparts us with vigour and strength. We respect you who gladden us by your pure essence”.

 

The role of water in evolution and philosophical context can be understood as:

“Although our ancestors have been terrestrial for so long, each person spends the first nine months of life as aquatic as fish. Within the womb the fetus is buoyed up by a warm sea that is almost identical with bold plasma, and hence brackish-a third as rich in salts as the oceans of the earth” (Milne and Milne, 1965).

 

There are different terms and definitions related to water such as drinking water which can be defined as water for ingestion, basic personal and domestic hygiene and cooking [World Health Organization (WHO), 1993]. It excludes water for clothes washing, an activity that frequently happens at the water source, water point, in rivers or streams. Whereas, safe drinking water means that the water meets does not represent any significant risk to health over a lifetime of consumption, including different sensitivities that may occur between life stages (WHO, 2006). It should be colourless, odourless, pleasant taste and free from toxic chemicals and disease causing microorganisms. The United Nations Millennium Development Goals (MDG) Target 7C called on countries to “Halve, by 2015, the proportion of population without sustainable access to safe drinking-water and basic sanitation”. An improved drinking water source is defined as a type of drinking water facility or water delivery point that by the nature of its design protects the drinking water source from external contamination, particularly of fecal origin (UNICEF, 2009). Piped water into dwelling, plot or yard, public tap/standpipe, tubewell/borehole, protected dug well, protected spring and rainwater are included under improved whereas unprotected dug well and spring, tanker truck, small cart with tank/drum, and surface water (river, dam, lake, pond, stream, channel, irrigation channel) are considered as unimproved drinking water sources. Over 90 % of the world’s population now has access to improved sources of drinking water (UNICEF and WHO, 2015). Accessibility to water as defined in the “Right to Water” includes a continuous supply of a minimum amount of water sufficient for drinking, personal and domestic hygiene, for an affordable price, within a reasonable distance (UN, 2010). According to WHO basic access can be defined as the availability of at least 20 litres of drinking water per person per day within a distance of not more than one km of the dwelling, corresponding to a maximum water hauling round trip of 30 minutes (Howard and Bartram, 2003). This definition is more appropriate for rural areas as distance to a source is not a problem in urban areas. The United Nations General Assembly on 28 July 2010, through Resolution 64/292 unambiguously recognized the human right to water and sanitation and acknowledged that clean drinking water and sanitation are essential to the realization of all human rights. The resolution calls upon states and international organizations to provide financial resources, help capacity-building and technology transfer to help countries, in particular developing countries, to provide safe, clean, accessible and affordable drinking water and sanitation for all (United Nations, 2010). The study of water is known as hydrology and is divided into a number of subcategories. Limnology is the branch of the science dealing with the characteristics of fresh water including biological, chemical and physical properties, whereas oceanography is the science of ocean including its various characteristics.

 

According to Goncharuk (2006), there are three available sources of water. First the groundwater from springs and wells which people mainly used for drinking in the first half of the 20th century. Fresh surface water is the second source of drinking water supply. As this water contains plenty of biological life from lower to higher organisms, including fish and frogs therefore considered as “live water” in contrast to “dead water”, that is, tap water treated with chlorine, or distilled water. However, due to industrialization, agricultural complexes, methods of mass transport, utilities, communal infrastructures; the growth of cities and settlements all resulted in the mass pollution of surface water. Due to pollution load, the self-purification capacity of water bodies by the natural process is either reduced or lost. The third source of drinking water supply can be seas and oceans, where the average salinity of about 30 g/l (30,000 ppm) makes it unfit for direct use. Through main desalination technologies of distillation, membrane technology, and electrochemical the sea water is desalinized. The desalinated water is not drinking water. It is necessary to make adjustments to modify the salt composition. Such waters should be “conditioned”. This could be one of the best option where there is no alternative source of fresh water.

 

4. Structure of water molecule

 

Water’s properties can best be understood by considering the structure and bonding of the water molecule. The water molecule is made up of two hydrogen atoms bonded to an oxygen atom. The bonds which hold the hydrogen and oxygen together are called covalent bonds and are very strong. The three atoms are not in a straight line; instead, as shown in figure 1, they form an angle of 104.5°. The water molecule maintains a bent shape (bent at 107.50 actually) because of two reasons. First the tetrahedral arrangement around the oxygen and second the presence of lone pair electrons on the oxygen. These are the electrons that are not involved in the covalent bonds and are left alone. These lone pairs are very negative – containing two negative electrons each – and want to stay away from each other as much as possible. These repulsive forces act to push the hydrogen’s closer together. The importance of the bent structure of water is that it provides water with two distinct “sides”: one side of the water molecule has two negative lone pairs, while the other side presents two hydrogen’s. The hydrogen’s are slightly positive because of the electro-negativity of oxygen. Electro-negativity is a measure of how much one atom wants to have electrons, and oxygen wants to have electrons more than hydrogen does. Oxygen has a higher electro-negativity. Due to the difference in electro-negativity, the electrons in the covalent bonds between oxygen and hydrogen get pulled slightly toward the oxygen. This leaves the hydrogen’s a little bit electron-deficient and thus slightly positive (Figure 2). Because water has a slightly negative end and a slightly positive end, it can interact with itself and form a highly organized ‘inter-molecular’ network. The positive hydrogen end of one molecule can interact favourably with the negative lone pair of another water molecule. This interaction is call as “Hydrogen bonding” which is a type of weak electrostatic attraction (positive to negative). Because each and every water molecules can form four Hydrogen Bonds, an elaborate network of molecules is formed. The polarity also allows water interacts with an electric field and to interact with other polar molecules – which is how substances get dissolved in water. Because of water’s bent structure and the fact that the oxygen atom attracts the negative electrons more strongly than do the hydrogen atoms, the water molecule behaves like a dipole having opposite electrical charges at either end. The water dipole may be attracted to either positively or negatively charged ions.

 

5.    Unique properties of water

 

No other substance on the globe can exist simultaneously in all three phase states i.e. liquid, solid, and vapour. Although water is an apparently simple molecule, it has a highly complex and anomalous character due to its intra-molecular hydrogen bonding. As a gas, water is one of lightest known, as a liquid it is much denser than expected and as a solid it is much lighter than expected when compared with its liquid form. The continual movement of water around the globe is known as the hydrologic cycle. Due to several processes of the cycle i.e. evaporation, transpiration, condensation, precipitation, surface runoff, percolation and collection, water will change from one phase to another, and is explained in third module. Liquid water is also characterized by an amazing variety of states from super-cooled to –70°C, fresh, brines, hydrates, fog, clouds, water bound in cells of an organism, and pseudo liquefied water. Also one must remember that water is an “universal solvent” capable of dissolving many kinds of substances due to which natural water is never absolutely pure. Due to its polar character, water is a good solvent for salts, polar organic compounds and gases. Because of this property almost all biochemical reactions take place in aqueous solutions and water is a key reaction medium for all metabolic processes in living cells. Water dissolves a very large number of molecules in concentrations that are sometimes minute but sufficient to trigger biological or biochemical effects on organisms. As a solvent and a main principal participant of reactions and processes, water is a medium for chemical reactions, biological processes, and various physical phenomena.

 

Surface Tension: Next to mercury, water has the highest surface tension of all commonly occurring liquids. Water molecules at the surface (next to air) hold closely together, forming an invisible film. The surface tension is denoted by the Greek letter gamma (γ) and is the magnitude F of the force per unit length (L) over which it acts. .

 

γ= F/L (The SI unit of surface tension is N/m.)

 

Surface tension of water at 200C and 1000C are 0.073 and 0.059 N/m, respectively. Water’s surface tension can hold weight that would normally sink. Floating a paper piece on top of the water, walking of some aquatic insects e.g. water strider is due to water surface tension property. Surface tension is essential for the transfer of energy from wind to water to create waves. Waves are necessary for rapid oxygen diffusion in lakes and seas.

 

Capillary action: Is defined as the movement of water within the spaces of a porous material due to the forces of adhesion, cohesion, and surface tension. This property of water is essential for the growth of plants and trees where it enters the plant’s roots and moves to other parts of the plant through tiny tube-like structures called xylem. By capillary action water reaches more than 90 m above the ground in tallest trees of redwood!

 

Adhesion: Water molecules stick to other substances e.g. sponge used to soak-up spilled water.

 

Cohesion: Water molecules stick to each other which is due to the hydrogen bonds among the molecules. At the surface water molecules has a much greater attraction for each other than for molecules in the air.

 

Thermal Properties: Water absorbs or releases more heat than many substances for each degree of temperature increase or decrease. Because of this, it is widely used for cooling and for transferring heat in thermal and chemical processes. Differences in temperature between lakes and rivers and the surrounding air may have a variety of effects. Large bodies of water, such as the oceans or big lakes, have a profound influence on climate. They are the world’s great heat reservoirs and heat exchangers and the source of much of the moisture that reaches as rain and snow over adjacent landmasses. This property of water is crucial for stabilizing temperatures on earth.

 

Specific Heat: Water has high specific heat of 1.0. The amount of energy required to raise the temperature of water by one degree celsius is quite large. Because so much heat loss or heat input is required to lower or raise the temperature of water, the oceans and other large bodies of water have relatively constant temperatures.

 

Heat of Vaporization: Water absorbs heat as it changes from a liquid to a gas; the human body can dissipate excess heat by the evaporation of its sweat. Water’s high heat conductivity makes possible the even distribution of heat throughout the body. Water has average Heat of vaporization of about 540 cal/gm, or about 0.4 eV/molecule.

 

Boiling and Freezing: At sea level pure water boils at 1000C and freezes at 00C, but extra energy is needed to push water molecules into the air. This is called latent heat—the heat required to change water from one phase to another. The high latent heat of evaporation gives resistance to dehydration and considerable evaporative cooling. If a substance is dissolved in water, then the freezing point is lowered. That is why salt is used for street spreading during winter to prevent ice formation. Energy is lost when water freezes. A great amount of heat is released into the environment when liquid water changes to ice. Nights when ice freezes often feel warmer than nights when ice melts.

 

Water density: Water is most dense at 40C and then begins to expand again (becoming less dense) as the temperature drops further. Liquid water at 40C is about 9% denser than ice. This expansion is due to hydrogen bonds in water become more rigid and ordered. As a result, ice floats (frozen water) upon the denser cold water. Water’s high surface tension plus its expansion on freezing encourages the erosion of rocks to give soil for our agriculture.

 

Solid Expansion: For most substances, solids are denser than liquids. But the special properties of water make it less dense as a solid. This property plays an important role in lake and ocean ecosystems. Floating ice often insulates and protects animals and plants living in the deep water below.

 

Specific Heat capacity: It means the resistance to temperature change. It takes a lot of energy to change the water temperature as it takes a lot of energy to break the hydrogen bonds. Water absorbs and stores a large amount of heat from the sun and, as it cools, it releases this heat into the air that moves towards land. During winter season, when the air is colder than the water, it literally picks up the water’s heat while passing over water onto land and warms it. Similarly, during summer when the air is hotter than the water, the air cools off as it travels past the ocean to land and cools the shore. Water’s high specific heat capacity also keeps ocean temperatures fairly moderate, which is important for marine life survival.

 

Isotopes of water: Hydrogen has two stable isotopes i.e. 1H and 2H (deuterium) and 3H (tritium) a radioactive isotope whereas Oxygen has three stable isotopes of 16O, 17O, and 18O. Distinguished by the mass numbers and characteristics of hydrogen and oxygen, there are eighteen isotopic kinds of water. However, because of the low abundance of the heavier isotopes, almost all water molecules are of three isotopic combinations of 1H216O, 2H216O and 1H218O (Hoefs, 1997). Stable isotopes in water (2H216O and 1H218O) provide the basis for hydrological, paleoclimatic, ecological, archaeological and forensic studies.

 

6. Importance of water in life

 

Water is second after oxygen as being vital for life. It is a vital link in the development of mankind, from the earliest times, people settled along rivers, lakes, seas, and oceans. Availability of ample water was one of the main factor behind the establishment of early river-valley civilizations like the Indus Valley Civilization, Ancient Egypt (the Nile), Mesopotamian (along the Tigris and Euphrates rivers), and Chinese civilization (along the Yellow river). People in this world require water for domestic (drinking, cleaning and washing), agricultural (irrigation), industrial, hydro-power generation etc. Humans may feel thirst after a fluid loss of only 1 % of bodily fluid and be in danger of death when fluid loss nears 10 % (ICRP Report, 1975). Minimum water requirements for fluid replacement have been estimated at about three liters/day under average temperate climate conditions which can vary according to climate and activity level. According to different organizations and researchers, the average daily water requirement for human survival is between 1.5‒5.0 liters per capita per day (Gleick and IWRA, 1996). Agriculture is the largest consumer of water used by humans worldwide. The standard of living has always been determined to a great extent by the quality of water available. The quality of fresh drinking water depends on the purification level of wastewater.

 

7. Use of water

 

Water is used for many different purposes throughout the world for domestic (drinking, cooking, personal cleaning, cloth washing, home cleaning, irrigation), industrial and for waste disposal. Globally, industry uses about twice as much water as used in households, mostly for cooling in the production of electricity. Water uses can be classified into consumptive and non-consumptive use. Consumptive use denotes that water which partially or totally used up, through evapotranspiration, transformation, contamination or other processes. This categorization is limited in extent as water never disappears from the water cycle and the amount of water cannot be increased or decreased on the planet. Rather, consumptive uses remove this water from the terrestrial part of the water cycle and return it to a vapour state. Examples of this uses are water for domestic and municipal needs, irrigation and industry. Nearly 85% of total human consumptive use of water worldwide is for irrigation in agriculture. About 70% of the total water used in industry is for cooling purpose only. Non-consumptive uses do not reduce the volume of water available to that stage of the water cycle and it includes hydropower generation, recreation and water sports, fisheries, inland navigation, and ecosystem maintenance.

 

8. Scarcity of water

 

Water scarcity involves water stress, water shortage or deficits, and water crisis. The relatively new concept of water stress is the difficulty of obtaining sources of fresh water for use during a period of time and may result in further depletion and deterioration of available water resources. Water scarcity is the lack of sufficient available water resources to meet the demands of water usage within a region. It already affects every continent and around 2.8 billion people around the world at least one month out of every year. More than 1.2 billion people lack access to clean drinking water (ScienceDaily, 2016). Water shortages may be caused by climate change, such as changed weather patterns including droughts or floods, increased pollution and human demands and overuse of water. Water scarcity is being driven by two converging phenomena’s of growing freshwater use and depletion of usable freshwater resources. Water scarcity can be a consequence of two mechanisms where the first physical (absolute) water scarcity is a result of inadequate natural water resources to supply a region’s demand, and second economic water scarcity is a result of poor management of the sufficient available water resources. A water crisis is a situation where the available potable, unpolluted water within a region is less than that region’s demand. Population growth, industrialization, urbanization and rising affluence in the 20th century resulted in a substantial increase in water consumption. The demand on water resources will continue to increase in coming years. The problem is further aggravated by the uneven water distribution on earth. According to the United Nations Development Programme (UNDP), economic water scarcity is comparatively more contributing in countries or regions experiencing water scarcity, as most countries or regions have enough water to meet domestic, industrial, agricultural, and environmental needs, but lack the means to provide it in an accessible manner. The UN through it MDG Target 7C recognizes the importance of reducing the number of people without sustainable access to clean water and sanitation.

 

9. Water Pollution

 

Water pollution is defined as the addition of any substance to water or changing of water’s physical and chemical characteristics in any way which interfere with its use for genuine purposes. In other words we can also define water pollution as excessive concentration of particular substances for sufficient periods of time to cause identifiable effects. The polluted water is unfit for drinking, bathing or washing, agricultural and industrial use, for aquatic life, recreation and aesthetic point of view. Different types of pathogens, pollutants in water along with poor hygiene practices may result into major illness and mortality. About 3.4 million people die each year from diseases associated with pathogens in water, like cholera, typhoid, infectious hepatitis, polio, cryptosporidiosis, ascariasis and diarrheal diseases. Many of these diseases are due to the presence of human waste in water (UNEP, 2016). Access to quality water is essential for human health and human development. Both are at risk if we fail to stop the pollution. Population growth, increased economic activity, expansion and agriculture intensification, and discharge of untreated sewage into rivers and lakes are the main reasons behind the troubling rise in surface water pollution in many countries including India. Many big drains passing through cities discharge their wastes [including municipal solid wastes (MSW)] directly into rivers (Fig. 3 a & b.)

 

In conclusion water is a vital resource necessary for our survival and proper functioning of the environment. The unique properties and characteristics of it make water use in various activities, which in turn resulted into water pollution, water stress and water scarcity. One should realize the importance of this precious resource and use in such a manner to ensure its sustainability for present as well as for future generations.

Summary

 

In this module we learnt about:

Definitions and types of water

The importance of water

The structure and different unique properties of water molecule

Problems related with water

you can view video on Water-General Introduction

References

  • Gleick, P. H. and IWRA, M. (1996). Basic Water Requirements for Human Activities: Meeting Basic Needs. Water International, 21: 83-92.
  • Goncharuk, V. (2006). Water: Problems of Stable Progress of Civilization in XXI Century in Chemistry for water, CHEMRAWN XV, Plenary Lectures and Perspectives, from the International Conference on Chemistry for Water, Paris, France, 21-23 June 2004, Edited by Association Chimie et Eau, Paris, pp: 40-49.
  • Howard,          G.        and      Bartram,          J.          (2003). Domestic         Water  Quantity,         Service Level  and    Health.
  • WHO/SDE/WSH/03.02, WHO Document Production Services, Geneva, Switzerland, pp:3. Hoefs, J., Stable Isotope Geochemistry, 4th ed., Springer-Verlag, Berlin, pp: 4, 1997.
  • International Commission on Radiological Protection (ICRP), Report on the Task Group on Reference Man, ICRP No. 23, Pergamon Press, New York, NY, U.S.A., 1975.
  • Milne, L. J. and Milne, M. (1965). Water and life. Atheneum Press, New York.
  • Mitchell, H. H., Hamilton, T. S., Steggerda F. R. and Bean, H. W. (1945). The chemical composition of the adult human body and its bearing on the biochemistry of growth. J. Biol. Chem., 158:625-637.
  • ScienceDaily (2016).  Water   scarcity,    available   at: https://www.sciencedaily.com/terms/water_scarcity.htm, accessed on 30.11.16.
  • UNEP (2016). Hundreds of Millions Face Health Risk as Water Pollution Rises Across Three Continents. Available at: http://www.worldwaterweek.org/wp-content/uploads/2016/08/13.00-Water-Quality-PR.pdf (accessed on 30.11.2016).
  • UNICEF (2009). Status and trends. Drinking water and sanitation in East Asia and the Pacific. A regional perspective based on the 2008 Report of the WHO/UNICEF Joint Monitoring Programme for Water Supply and Sanitation, pp:1-28.
  • UNICEF and WHO (2015). Progress on sanitation and drinking water – 2015 update and MDG assessment, Geneva, Switzerland.
  • United Nations (2010). The right to water. Fact Sheet No 35. Office of the United Nations High Commissioner for Human Rights, United Nations Office, Geneva, Switzerland.
  • United Nations (UN), Resolution A/RES/64/292, United Nations General Assembly, 64th Session, Agenda item 48, July 2010.
  • WHO (1993). Guidelines for drinking-water quality: Volume 1 Recommendations 2nd edition, WHO, Geneva, Switzerland.
  • WHO (2006). Guidelines for Drinking-water Quality First addendum to third edition, Volume 1 Recommendations, WHO, Geneva, Switzerland.