Water Properties PDF

1 Text Introduction: As far as human life is concerned, oxygen is the most important element. Water is the most important compound. In the human body, water is vital. Water is second only to oxygen in importance to the body. People cannot survive withou...

1 Text Introduction: As far as human life is concerned, oxygen is the most important element. Water is the most important compound. In the human body, water is vital. Water is second only to oxygen in importance to the body. People cannot survive without oxygen, and people cannot survive without water. A healthy adult may live for weeks without food but only a few days without water. A person can lose all reserve carbohydrate and fat and about half the body protein without real danger, but a loss of 10 of total body weight of water is serious, while a loss of 20 to 22 is fatal. Water makes up 67 to 75 of the total body weight. Water is involved in all body processes. Water is an efficient heat conductor and serves to maintain the uniform body temperature essential for health. As a protector of internal organs, water is indispensable; it serves as a cushion and prevents the transmission of shock from the outside. It also serves as nature's solvent for many chemical compounds and is the medium for many chemical reactions to occur. Water, in plant and animal foods and in the human body, transports other substances either in solution or suspension. Within the body cells, water migrates in and out. When the water is inside of the cells, it is part of the intracellular fluid (fluid contained within a cell). When water is outside of the cells, it is part of the extracellular fluid (fluid present outside the cells). Extracellular fluid is further divided into interstitial fluid (water between cells) and intervascular fluid (water in the bloodstream). Water shifts freely between one compartment to another. This aspect is very important. For example, if the blood volume falls, water can shift from the areas both inside and around cells to the bloodstream to increase blood volume. The opposite is also true if the blood volume becomes too high. Water also acts as a temperature regulator in the body. The body secretes fluids in the form of perspiration which evaporates though skin pores. This evaporation requires heat  2 energy and thus as the perspiration evaporates, heat energy is taken from the skin, cooling it in the process. The examples below are among the most important biochemical processes that occur in living things, but they are just two of many ways that water is involved in biochemical reactions. Photosynthesis—In this process, cells use the energy in sunlight to change carbon dioxide and water to glucose and oxygen. Water is a reactant in this process. The reactions of photosynthesis can be represented by the chemical equation 6CO2 + 6H2O + Energy → C6H12O6 + 6O2. Cellular respiration—In this process, cells break down glucose in the presence of oxygen and release carbon dioxide, water (a product), and energy. The reactions of cellular respiration can be represented by the chemical equation C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy. Water is also an important vehicle for removing body waste products. As part of the digestive juices, it helps to change consumed food into nutrients the body can use. Within the bloodstream, it also helps to carry those nutrients to the body cells in need, and carry away cellular waste products. Waste products are then excreted from the body. Most unwanted substances in the body can be removed from the body via the urine. Water helps to form the lubricants found in the joints of the body. It is also the basis for saliva, bile, and amniotic fluids (the important shock absorbing fluid which surrounds and unborn fetus). The recommended water intake for adults per day is 8 cups. If enough water is not consumed, the body first signals by making the person feel thirsty. But this mechanism is not always reliable, especially during illness, in elderly years, and when involved in  3 vigorous athletic events. Children who are ill, especially with a fever or diarrhea, are especially susceptible to dehydration. Polar water molecule Each molecule of water consists of one atom of oxygen and two atoms of hydrogen. The oxygen atom in a water molecule attracts negatively-charged electrons more strongly than the hydrogen atoms do. As a result, the oxygen atom has a slightly negative charge, and the hydrogen atoms have a slightly positive charge. A difference in electrical charge between different parts of the same molecule is called polarity, making water a polar molecule. Each hydrogen atom of a water molecule shares an electron pair with the central oxygen atom. The geometry of the molecule is dictated by the shapes of the outer electron orbitals of the oxygen atom, which are similar to the sp3 bonding orbitals of carbon. These orbitals describe a rough tetrahedron, with a hydrogen atom at each of two corners and unshared electron pairs at the other two corners The HOOOH bond angle is 104.5, slightly less than the 109.5 of a perfect tetrahedron because of crowding by the nonbonding orbitals of the oxygen atom. The oxygen nucleus attracts electrons more strongly than does the hydrogen nucleus (a proton); that is, oxygen is more electronegative. The sharing of electrons between H and O is therefore unequal; the electrons are more often in the vicinity of the oxygen atom than of the hydrogen. The result of this unequal electron sharing is two electric dipoles in the water molecule, one along each of the HOO bonds; each hydrogen bears a partial positive charge(+) and the oxygen atom bears a partial negative charge equal to the sum of the two partial positives As a result, there is an electrostatic attraction between the oxygen atom of one water molecule and the hydrogen of another (Fig. 2), called a hydrogen bond. Bonds between molecules are not as strong as bonds within molecules. There are just many more hydrogen bonds in water (between water molecules) than there are covalent bonds within a molecule. The hydrogen bonds may not be strong, but in water they are strong enough to hold together nearby molecules.  4 The polarity of water is responsible for effectively dissolving other polar molecules, such as sugars and ionic compounds such as salt. Ionic compounds dissolve in water to form ions. This is important to remember because for most biological reactions to occur, the reactants must be dissolved in water. Properties of water: Polarity Polarity simply means that the molecule has both a positively and negatively charged end. More important, the polarity of water is responsible for effectively dissolving other polar molecules, such as sugars and ionic compounds such as salt. Ionic compounds dissolve in water to form ions. This is important to remember because for most biological reactions to occur, the reactants must be dissolved in water. Because water is able to dissolve so many common substances, it is known as the universal solvent. Substances that cannot be dissolved by water (such as oils) are called fat soluble and are non polar, nonionic compounds that are strongly covalently bonded. Insoluble substances make excellent containers of water, such as cell membranes and cell walls. Hydrogen Bonding When water molecules align with each other, a weak bond is established between the negatively charged oxygen atom of one water molecule and the positively charged hydrogen atoms of a neighboring water molecule. The weak bond that often forms between hydrogen atoms and neighboring atoms is the hydrogen bond. Hydrogen bonds are very common in living organisms; for example, hydrogen bonds form between the bases of DNA to help hold the DNA chain together. Hydrogen bonds give water molecules two additional characteristics: cohesion and surface tension. Cohesion Because of the extensive hydrogen bonding in water, the molecules tend to stick to each other in a regular pattern. This phenomenon, called cohesion, is easily observed as you carefully overfill a glass with water and observe the water molecules holding together above the rim until gravity overtakes the hydrogen bonding and the water  5 molecules spill down the side of the glass. Likewise, the cohesive property of water allows tall trees to bring water to their highest leaves from sources below ground. Surface Tension:A special type of cohesion is surface tension. The tension on the surface of water occurs when water molecules on the outside of the system align and are held together by hydrogen bonding to create an effect similar to a net made of atoms. For example, the surface tension of water allows water spiders to literally walk on water. Specific heat Water has a high specific heat index—it absorbs a lot of heat before it begins to get hot. This is why water is valuable to industries and in your car's radiator as a coolant. The high specific heat index of water also helps regulate the rate at which air changes temperature, which is why the temperature change between seasons is gradual rather than sudden, especially near the oceans. Heat of Vaporization—Water has a high 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. A leaf can keep cool in the bright sunlight by evaporating water from its surface. Water’s high heat conductivity makes possible the even distribution of heat throughout the body. Boiling and Freezing—Pure water at sea level boils at 100oC (212o F) and freezes at 0oC(32o F), 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. At higher elevations (lower atmospheric pressure) water’s boiling temperature decreases. This is why it takes longer to boil an egg at higher altitudes. The temperature does not get high enough to cook the egg properly. If a substance is dissolved in water, then the freezing point is lowered. That is why we spread salt on streets in winter to prevent ice formation. Energy is lost when water freezes. A great deal of heat is released into the environment when liquid water changes to ice. It is lost when the high energy phase of liquid water moves to the low energy phase of ice. Nights when ice freezes often feel warmer than nights when ice melts.  6 Water density--Water is most dense at 4оC and then begins to expand again (becoming less dense) as the temperature decreases further. This expansion occurs because its hydrogen bonds become more rigid and ordered. As a result, frozen water (ice floats) upon the denser cold water. The expansion of water takes place even before it actually freezes. This explains why a pond freezes from the surface down, rather than from the bottom up. As water temperature drops, the colder water (0-40C) where it is less dense— rises to the pond surface. It freezes to form a lid of ice. This ice insulates the water below from the wintry chill so that it is less likely to freeze. Organisms that inhabit the pond are able to survive the frigid winter below the icy surface. Solid Expansion--For most substances, solids are denser than liquids. But the special properties of water make it less dense as a solid. Ice floats on water! Strong hydrogen bonds formed at freezing 00C (320 F) lock water molecules away from each other. When ice melts, the structure collapses and molecules move closer together. Liquid water at 40 C (39.20 F) is about 9% denser than ice. This property plays an important role in lake and ocean ecosystems. Water molecules have a tendency to ionize. They dissociate into ions (charged particles) hydrogen ions (H+) and hydroxide ions (OH-). In pure water a very small number of water molecules form ions in this way. The tendency of water to dissociate is balanced by the tendency of hydrogen ions and hydroxide ions to reunite to form water. A neutral solution contains an equal number of hydroxide ions and hydrogen ions. A solution with a greater concentration of hydrogen ions (H+) is said to be acidic. A solution with a greater concentration of hydroxide (OH-) ions is said to be alkaline or basic. Ionization of water The self-ionization of water (also autoionization of water, and auto dissociation of water) is an ionization reaction in pure water or an aqueous solution, in which a water molecule, H2O, deprotonates (loses the nucleus of one of its hydrogen atoms) to become a hydroxide ion, OH−. The hydrogen nucleus, H+,  7 immediately protonates another water molecule to form hydronium, H3O+. It is an example of autoprotolysis, and exemplifies the amphoteric nature of water The ionization of water can be represented as H2O H+ + OH- (1). The law of mass action can be applied, defining the equilibrium constant for the reversible ionization of water :Keq Keq = [ H+][OH-] [H2O] The concentration of each moiety is expressed in moles per liter (M). The degree of ionization of water is very low and the number of water molecules dissociated into ions is minuscule. At any given time, the amount of hydronium ions and hydroxide ions present in water is extremely small and consequently the concentration of undissociated water molecules is virtually unchanged by this minute ionization and may be considered a constant Keq x [H2O] = [H+] [OH-] The product of Keq x [H2O] is a constant termed the ionic product of water. Therefore, the ionic product for water is (kw) is Kw= [H+] [OH-]  8 In pure water, the concentration of water is 55.5 M and the value for the equilibrium constant, , determined by electrical conductivity measurements, is M at 25°C (298 K) of temperature. Substituting these values in the equilibrium constant expression, Kw = ( 1.8x 10-16) x55.5 = 99.6 x 10-16 ⸗ 10-14 M2 The constant ionic product for water is equal to at 10-14 25°C. Protons (H+) initially hydrate as hydroxonium ions, H3O+ (also called oxonium or hydronium ions) and do not exist as naked protons in liquid or solid water, where they interact extremely strongly with electron clouds. All three hydrogen atoms in the hydroxonium ion are held by strong covalent bonds and are equivalent (that is, C3v symmetry in a vacuum. The proton is never found unhydrated in aqueous solution and the hydroxonium ions, H3O+, also has negligible independent existence in an aqueous environment. All the hydroxonium ion protons are predominantly hydrogen bonded + causing further hydration to H3O (H2O)n, where n depends on the conditions such as temperature, solutes, pressure and method of determination. When an acid or a base is added to water, the ionic concentration product, [H+][OH-], remains constant, i.e., equal to Kw but concentrations of H+ and OH- ions do not remain equal. The addition of acid increases the hydrogen ion concentration while that of hydroxyl ion concentration decreases, i.e., [H+] > [OH- ]; (Acidic solution) Similarly, when a base is added, the OH- ion concentration increases while H+ ion concentration decreases, i.e. , [OH- ] > [H+]; (Alkaline or basic solution) In neutral solution, [H+] = [OH- ] = 1 x 10-7 M  9 In acidic solution, [H+] > [OH- ] or [H+] > 1 x 10-7 M and [OH- ] < 1 x 10-7 M In alkaline solution, [OH- ] > [H+] or [OH- ] > 1 × 10-7 M and [H+] < 1 x 10-7 M Thus, if the hydrogen ion concentration is more than 1 x 10-7 M, the solution will be acidic in nature and if less than 1 x 10-7 M, the solution will be alkaline. [H+] = 10-0 10-1 10-2 10-3 10-4 10-5 10-6 (Acidic) [H+] = 10-7 (Neutral) [H+] = 10-14 10-13 10-12 10-11 10-10 10-9 10-8 (Alkaline) We shall have the following table if OH- ion concentration is taken into account. [OH- ] = 10-14 10-13 10-12 10-11 10-10 10-9 10-8 (Acidic) [OH- ] = 10-7 (Neutral) [OH- ] = 10-0 10-1 10-2 10-3 10-4 10-5 10-6 (Alkaline) It is, thus, concluded that every aqueous solution, whether acidic, neutral or alkaline contains both H+ and OH- ions. The product of their concentrations is always constant, equal to 1 × 10-14 at 25°C. If one increases, the other decreases accordingly so that the product remains 1×10-14 at 25o C. The concept of buffer: A buffer solution is an aqueous solution consisting of a mixture of a weak acid and its conjugate base, or vice versa. Its pH changes very little when a small or moderate amount of strong acid or base is added to it and thus it is used to prevent changes in the pH of a solution. Buffer solutions are used as a means of keeping pH at a nearly  10 constant value in a wide variety of chemical applications. Many life forms thrive only in a relatively small pH range so they utilize a buffer solution to maintain a constant pH. Buffer solutions achieve their resistance to pH change because of the presence of an equilibrium between the acid HA and its conjugate base A−. HA H + + A− Each acid has a tendency to lose its proton in aqueous solution. The stronger the acid, The greater the tendency. The titration curve of a weak acid and its conjugate base shows that these compounds can be used to buffer solutions. Buffering is a consequence of two reversible reactions taking place simultaneously and reaching the equilibrium points as governed by their equilibrium constants kw and ka. When some strong acid is added to an equilibrium mixture of the weak acid and its conjugate base, the equilibrium is shifted to the left, in accordance with Le Châtelier's principle. Because of this, the hydrogen ion concentration increases by less than the amount expected for the quantity of strong acid added. Similarly, if strong alkali is added to the mixture the hydrogen ion concentration decreases by less than the amount expected for the quantity of alkali added. The effect is illustrated by the simulated titration of a weak acid with pKa = 4.7. The relative concentration of undissociated acid is shown in blue and of its conjugate base in red. The pH changes relatively slowly in the buffer region, pH = pKa ± 1, centered at pH = 4.7 where [HA] = [A−]. The hydrogen ion concentration decreases by less than the amount expected because most of the added hydroxide ion is consumed in the reaction.