PAPER No.4: Environmental Chemistry MODULE No.5 : Properties of Water and hydrologic cycle

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Nathaniel Harmon
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1 Subject Chemistry Paper No and Title Module No and Title Module Tag Paper 4: Environmental Chemistry 5: Properties of Water and Hydrologic Cycle CHE_P4_M5

2 TABLE OF CONTENTS 1. Learning Outcomes 2. Introduction 3. Properties of water 3.1 Physical state of water 3.2 Structure of water molecule 3.3 Chemical bonding in water 3.4 Heat capacity 3.5 Surface Tension 3.6. Density 3.7 Water as Universal Solvent 3.8 Isotopes of Water 4. Biogeochemical Cycle 4.1 Hydrological Cycle 5. Summary

3 1. Learning Outcomes After studying this module, you shall be able to: Know the importance and various physico-chemical properties of water. Learn about the geological cycle. Identify various components of hydrological cycle. Learn about ground water and water table. 2. Introduction Water is essential to all forms of life. The essentiality of water is beyond question for organisms and, consequently, life processes that exist on the earth. It is common to assume that water is a typical liquid, much like any other liquid. But in fact, virtually every physical and chemical property of water is unusual when compared with other liquids, and these differences make water essential to life. To appreciate its significance in the living system it is worthwhile to briefly recapitulate the physical and chemical properties of water. 3. Properties of Water 3.1 Physical State of Water Water can take various forms; we are generally more familiar with the liquid state. The solid state, ice exists in many crystalline and amorphous forms, while the gaseous state, water vapor is characterized by water taking the form of a transparent cloud. Figure 1displays the phase diagram of water. The fourth state is that of a supercritical fluid and is much less common than the other three.the supercritical water is naturally occurring and found in the hottest parts where volcanic activitiesheat water to its critical temperature and the crushing weights bring about the critical pressure. Supercritical water is expected to be present in any region having volcanic activity; oceans produce it below a depth of 2.25 km. The triple point of water is point for temperature and pressure where solid, liquid and gaseous water is in equilibrium with each other. The triple point of water is K and Pa.

Figure 1: Phase diagram of water Water phase diagram: y-axis = Pressure in pascals (10 n ); x-axis = temperature in Kelvin; S = solid; L = liquid; V = vapor; CP = critical point; TP = triple point of

It is a polar molecule with extensive intermolecular hydrogen bonding which is responsible for its existence as a liquid at room temperature and atmospheric pressure.

4 Figure 1: Phase diagram of water Water phase diagram: y-axis = Pressure in pascals (10 n ); x-axis = temperature in Kelvin; S = solid; L = liquid; V = vapor; CP = critical point; TP = triple point of water 3.2 Structure of Water Molecule: Water has the chemical formula H 2 O. The IUPAC name is oxidane, though the simpler systematic name, dihydrogen monooxide is more widely used. It is a polar molecule with extensive intermolecular hydrogen bonding which is responsible for its existence as a liquid at room temperature and atmospheric pressure. A molecule of water has a bent structure with the more electronegative oxygen atoms acquiring partial negative charge and the hydrogen atoms acquiring partial positive charge as shown in figure 2. It is has a high dipole moment and high dielectric constant.

Figure2: Structure of water molecule (dihydrogen monoxide) 3.3 Chemical Bonding in Water: The atoms of oxygen and hydrogen are joined by a covalent bond.

Figure 3 illustrates the covalent and hydrogen bond in water molecule. The polarity in molecule results in attraction of water molecules and formation of strong molecular bonds takes place.

5 Figure2: Structure of water molecule (dihydrogen monoxide) 3.3 Chemical Bonding in Water: The atoms of oxygen and hydrogen are joined by a covalent bond. Due to electronegativity difference between the two elements there is charge separation and one side of the molecule has a negative charge and other side has a positive charge. Figure 3 illustrates the covalent and hydrogen bond in water molecule. The polarity in molecule results in attraction of water molecules and formation of strong molecular bonds takes place. Figure 3: Chemical bonds in water 3.4Heat Capacity: The amount of heat that must be absorbed or evolved to change the temperature of 1g of a substance by 1 o C is called heat capacity. The specific heat capacity and enthalpy of vaporization of water is high and results in the formation of intermolecular hydrogen bonding. These properties of water results in change in earth s climate by change in temperature. It is estimated that the oceans can absorb 1000 times more heat than the atmosphere and hold 80 to 90% of the heat of global warming. 3.5 Density The density of water and ice is of great interest. It has a nonlinear relationship with temperature and the maximum value is attained at about 277 K. The density decreases on further cooling and this unusual thermal expansion is due to strong orientation dependent intermolecular hydrogen bonding. Water at a temperature of 277 K will always accumulate at the bottom of freshwater lakes irrespective of the temperature in the atmosphere. Water and ice are both good thermal insulators and hence deep lakes do not freeze completely, allowing aquatic life to survive during extreme winter.

Ordered molecular structure Semi-ordered structure Random molecular structure Solid Ice Liquid water Gaseous water Figure 4: Different arrangement pattern of water molecules in different states of

the spherical shape of water droplets is attributed to surface tension. These water droplets can easily deformed and tend to form a spherical shape. 3.7.

this makes water to carry solvent nutrients in runoff, infiltration, groundwater flow as well as in living organisms. 3.8.

6 Ordered molecular structure Semi-ordered structure Random molecular structure Solid Ice Liquid water Gaseous water Figure 4: Different arrangement pattern of water molecules in different states of matter 3.6 Surface Tension Water has exceptionally high surface tension. This is due to the cohesive forces among liquid molecules.the spherical shape of water droplets is attributed to surface tension. These water droplets can easily deformed and tend to form a spherical shape Water as a Universal Solvent: Water can dissolve any ionic or polar substance to a certain extent and is therefore regarded as universal solvent.this makes water to carry solvent nutrients in runoff, infiltration, groundwater flow as well as in living organisms. 3.8.Isotopes of Water Several isotopes of hydrogen and oxygen exist giving rise to different isotopes of water The existence of 3 isotopes of hydrogen - 1H, 2H, 3H and 3 isotopes of oxygen 16O, 17O, 18O implies that water is a mixture of several isotopes each form differing from the other in hydrological, radioactive and biological properties. The most common water isotope contains 1H and 16O and this is a necessary condition for existence of life on earth. Some water isotopes do not support life, rather they may decelerate chemical responses, may be toxic or inert. Tables 1 and 2 summarize the different physical and chemical properties of water.

7 Table 1: Important physico-chemical properties of water Property Explanation Importance Density On cooling water Small solid things float on liquid water. contracts until it reaches 4 C (277 K) Icebergs float in the big water bodies and aquatic animals can live in the unfrozen Freezing Boiling Point & Heat capacity (Specific Heat) Heat of Evaporation Surface Tension Universal Solvent Pure water boils at 100 C (373 K) and freezes at 0 C (273 K) Water has unusually high specific heat (highest after NH 3 ) Water has high heat of vaporization Water has second highest surface tension, after mercury. Water is polar and has high dielectric constant. water. High boiling point makes water exist in liquid form on earth s surface Moderate temperature by preventing extreme fluctuations. Stabilization of organisms and geographic regions. Balances the difference in atmospheric temperature and humidity. Regulates nutrients movement in the body and helps in transport of water from roots to leaves in plants. Dissolves almost all ionic compounds and helps in transfer of dissolved substances in biological systems. Table 2: Physical parameters of water Property Value Boiling point 100 C (373 K) Freezing Point 0 C (273 K) Maximum Density at 3.98 C kg/m 3 Viscosity at 25 C mns/m 2 Surface Tension at 25 C 72 mn/m Heat capacity at 25 C kj/kg K Enthalpy of Evaporation kj/mol Enthalpy of fusion kj/mol Dielectric constant at 25 C Electrical conductivity at 25 C 8 µs/m Refractive index at 25 C Thermal conductivity W/mK

4. Biogeochemical cycles To maintain the biosphere and thus life, the chemical elements (like carbon, oxygen, nitrogen etc.) must undergo recycling or used again.

The cycling of these elements makes the supply of nutrients possible in different environmental segments.

8 4. Biogeochemical cycles To maintain the biosphere and thus life, the chemical elements (like carbon, oxygen, nitrogen etc.) must undergo recycling or used again. The distribution of elements in atmosphere, lithosphere and hydrosphere depends on the movement of chemical substances from one segment to other. The cycling of these elements makes the supply of nutrients possible in different environmental segments. These movements of nutrients are natural and hence it prevent the accumulation of various elements in forms, quantities and locations that are harmful to life. This transfer of elements between atmosphere, hydrosphere and lithosphere is called geochemical cycle. In these cycles interchange of nutrients occur among the abiotic (nonliving components) and biotic community (living component). In geochemical cycles the transfer of elements can be described as movements of elements form a reservoir (where a material is concentrated) to various other locations and back to the reservoir. The quantitative estimates of geochemical cycles are made from both the amount of elements in each reservoir and the flux between the reservoirs. The concentration of material passed from one pool to another at unit time and unit area or volume is known as is called flux rate. Each geochemical cycle is a model which explains the movement of a chemical or its compound in different segments of the environment. Many types of chemical reactions take place in these geochemical cycles and presence or absence of any component influences the chemical reactions that occur in geochemical cycles. Most of the reactions can also be affected by biological activities. If they are also considered then the geochemical processes are called biogeochemical processes or cycles. Geochemical cycles are element specific whereas a biogeochemical cycle is specific to those elements that are key participants in biotic process. Biogeochemical cycles can be defined as the cycling of matter involving biological, chemical and geological processes and phenomena. 4.1 The Hydrologic Cycle Most of the water of the world 97% is found in oceans but because of high salt content it cannot be used for municipal, agricultural or most industrial needs. Water serves as a pool for many dissolved and suspended materials. It is a vital component for all living cells and for various biochemical processes. In water cycle, water interconverts in all the three states i.e., liquid form, vapor and solid form. The water cycle is a key environmental dynamic for all types of processesphysical, chemical and biological and had been in existence ever since oceans came into being,

9 i.e., more than two billion years ago. The movement of water on the earth s surface and through the atmosphere is known as hydrologic cycle. Figure 5 displays the water cycle and its various processes. The water cycle can be either a long cycle or various short cycles. Short Cycle: It involves only evaporation and immediate condensation and so on. The reservoir for short cycle could be any water body. Long Cycle: It involves various processes like evaporation, condensation, transpiration, infiltration and interception, run off, stream off and subsurface flow. Figure 5: Hydrological Cycle Water reaches the atmosphere from the earth s surface in vapor form through evaporation and these vapors get condensed and form clouds. The water then returns to the earth in the form of rain. The reservoirs of water cycle are atmosphere, ocean, lake and rivers etc Evaporation, Transpiration and Condensation Evaporation is the process where water from the sea, rivers and lakes is converted into water vapors in the atmosphere. The excess of water vapor present in the atmosphere is called as humidity. The water vapors condensed around particles that are small in size and called as nuclei which is then suspended in the atmosphere. The nuclei generally are dust particles, smoke, volcanic ash or any organic material. This transformation of water vapors to small droplets depends on how warm the water is and how much water vapor or moisture the air contains. Initially due to small size these droplets remained in the atmosphere as clouds. Cloud formation starts in the atmosphere when the moisture content of air gets close to maximum possible amount. Finally these vapors form precipitation. By the process of precipitation the water comes to the water bodies and thus comes back to the earth from the atmosphere. The phenomenon by which water evaporates from the surface of leaves is called transpiration.

10 4.1.2 Run off and Infiltration When rain falls on the earth a part of the rain water is caught by vegetation by the process of interception. Some of the water seeps into ground by the process of infiltration. Infiltration basically depends upon the type of vegetation and type of rock. As greater the openings in the surface in the form of cracks, pores or joints more is the infiltration. The water that is not infiltrated is called surface runoff, which flows over the surface and gets discharged into streams. The term runoff water is generally used for the sum of surface water plus the ground water that enters into stream. Figure 6: Groundwater and its different zones Subsurface flow It incorporates movement of water within the earth. During precipitation, the water reaches the earth s surface and some of it flow with the surface as runoff and rest will soaks into the soil.the saturated zoneis called an aquifer and the water present here is called groundwater. The upper surface of a zone of saturation is the water table. It is the first occurrence of groundwater above it there is zone of aeration. Once the water has entered the aquifer, it doesn t stop there. This movement of water underground is called groundwater flow. Figure 6 and 7 shows groundwater and water table and surface flow. There is some water in the zone of aeration, but it will not flow into a well. Therefore successful wells need to be deeper than the water table.

After years of underground movement, the groundwater reaches the area where it s enter lake or stream and contributes to surface water. The water once again be evaporated and form the cycle again.

Also all the physical forms are interchangeable. As water cools, it contracts until it reaches 4 C (277 K), then it expands until it freezes at 0 C (273 K). Water is regarded as universal solvent.

11 After years of underground movement, the groundwater reaches the area where it s enter lake or stream and contributes to surface water. The water once again be evaporated and form the cycle again. Figure 7: Subsurface flow 5. Summary Water has unique physical and chemical properties that make it essential for the existence of life on earth. All the three forms of water i.e. solid, liquid or gas is naturally occurring form of water. Also all the physical forms are interchangeable. As water cools, it contracts until it reaches 4 C (277 K), then it expands until it freezes at 0 C (273 K). Water is regarded as universal solvent. Hydrogen bonding and the polarity of water explain its unique solvent properties. The intermolecular forces between liquid molecules are responsible for surface tension. Water has second highest surface tension. Water has a high specific heat. It means that it will take more energy to raise the temperature of one gram of water by 1 C. The hydrological cycle or water cycle is a key environmental dynamic for all types of processes- physical, chemical and biological. The various stages of water cycle are evaporation, condensation, runoff, infiltration, subsurface flow.

The properties of water in all phases are determined by its structure.

The properties of water in all phases are determined by its structure. Section 5 Water Water is a familiar substance in all three physical states: solid, liquid, and gas. On Earth, water is by far the most abundant liquid. Oceans, rivers, and lakes cover about 75% of Earth