Water - Its Properties and Its Role in the Fitness of Environment

• importance of water to life
• chemical characteristics of water
• polarity and properties associated with it

• dissociation of water molecules

• Importance of water to life
• covers ¾ of surface of earth
• is where life evolved
• essential to life on earth
• ~ 2/3 of mass of all organisms
• chemical structure of water (H₂O)
• 2 H atoms covalently bonded to 1 O atom
• resulting molecule is stable
• outer electron shells full
• no net charge
• no unpaired electrons
• Water is a polar molecule
• O atom is more electronegative than H atoms
• attracts electrons more strongly than do H atoms
• shared electrons in a water molecule more likely to be found near O nucleus than near the H nuclei,
• partial – charge on O atom
• partial + charge on each H atom
• water molecule as a whole is neutral
• but partial charges cause molecule to have "poles"
• negative pole
• O end due to partial – charge on O atom
• positive poles
• H ends due to partial + charge on each H atom

• Water’s polarity leads to unusual properties that make life possible
• hydrogen bonds
• cohesion
• surface tension
• temperature moderation
• less dense as solid than liquid
• versatile solvent
• role in acid/base conditions
• polarity of water molecules
• causes them interact with each other

• this attraction results in formation of weak bonds
• called hydrogen bonds
• hydrogen bonds
• result when polar molecules interact with one another
• partial – charge of one molecule is attracted to the partial + charge of another molecule
• individually weak
• cumulatively strong
• form between each water molecule and four of its neighboring molecules
• hydrogen bonds extremely important to biological systems
• Like no other common substance on Earth, water exists in nature in all three physical states (or phases of matter)
• solid (ice)
• liquid (water)
• gas (water vapor)

• cohesion
• attraction resulting from polar water molecules being attracted to each other
• water molecules have a strong tendency to stick together
• much stronger for water than for most other liquids
• of water is important in living world
• example, trees
• surface tension
• related to cohesion
• a measure of how difficult it is to stretch or break surface of a liquid
• at air-water interface, all hydrogen bonds in water face downward, causing molecules of water surface to cling together
• polar water molecules are "repelled" by nonpolar molecules in the air
• water has highest surface tension of any liquid except for liquid mercury
• adhesion
• attraction resulting from polar water molecules being attracted to other polar non-water molecules
• )
• capillary action
• tendency of water to rise in small tubes, as a result of cohesive and adhesive forces

• Water has greater ability to resist temperature change than most other substances
• due to its hydrogen bonds
• heat
• amount of energy associated with movement of atoms and molecules in a body of matter
• temperature
• intensity of heat
• average speed of molecules rather than total amount of heat in a body of matter
• Water resists temperature increases
• raising temperature of a substance involves
• adding heat energy to make its molecules move faster
• in water, some of H bonds must first be broken
• to allow the molecules to move more freely
• much of energy added to water is used up in breaking the H bonds
• only a portion of heat energy is available to speed movement of water molecules
• Water stores heat
• heat is absorbed as H bonds break
• water absorbs and stores a large amount of heat while warming up only a few degrees
• Water cools slowly
• as water cools, H bonds re-form
• heat energy is released as H bonds form, thus slowing the cooling process
• Water resists temperature change
• enables organisms to maintain relatively constant internal temperatures
• crucial in stabilizing temperatures on earth
• by storing heat from sun during warm periods, releasing heat during cooler times
• Water resists tendency to evaporate or vaporize
• liquids vaporize when some of their molecules move fast enough to overcome attractions that keep molecules close together
• heating a liquid increases vaporization by increasing energy of molecules
• providing some of molecules with enough energy to escape
• Water resists tendency to vaporize
• large amount of energy is required to change one gram of liquid water into a gas
• water’s resistance to vaporization results from the hydrogen bonding of its molecules
• transition of water from a liquid to a gas requires input of energy to break hydrogen bonds
• source of such energy can be surface of substance on which water is located
• evaporation of water from surfaces causing a cooling of that surface
• enables organisms to dispose of excess heat by evaporative cooling
• organism gives up some heat energy to break H bonds in the water molecules
• such molecules then have enough heat energy to escape
• they take that heat energy with them when they go

• Water is less dense as a solid than as a liquid
• most substances become more dense as temperature decreases
• water is most dense at 4° C then becomes less dense as temperature decreases below that point
• hydrogen bonds in liquid water are unstable
• they constantly break and re-form
• hydrogen bonds in ice are stable
• each molecule bonds to 4 neighbors forming a 3-D crystal
• liquid water expands (becomes less dense) as it freezes because
• H bonds joining water molecules in crystalline lattice keep molecules far enough apart to give ice a density about 10% less than density of water
• less dense frozen water (ice) floats on more dense cold, unfrozen water
• Ice is less dense than liquid water
• frozen water floats on liquid water
• extremely important factor in enabling life to appear, survive and evolve
• if ice were more dense than water it would sink
• all ponds, lakes, oceans would freeze solid from the bottom to surface making life impossible
• since ice floats on water instead of sinking
• a body of deep water freezes at top, becoming covered with floating ice
• ice insulates liquid water below it
• preventing water from freezing solid
• allowing certain animals and plants to survive below icy surface

• Solution
• liquid, that is uniform throughout (homogeneous), consisting of a mixture of two or more substances
• solvent
• substance in a solution that serves as dissolving agent
• usually a liquid, capable of dissolving one or more other substances
• solute
• substance which is dissolved by solvent
• solution that has water as its solvent is called an aqueous solution

• Water is a versatile solvent
• dissolves an enormous variety of solutes necessary for life
• water is solvent in all cells
• .
• results from polarity of its molecules
• solutes whose charges or polarity allow them to stick to water molecules will dissolve in water, forming an aqueous solution
• consider how a crystal salt dissolves in water
• Na⁺ and Cl^- ions at surface of salt crystal have affinities for different parts of water molecules
• Na⁺ ions attract - area of H₂O at O
• Cl^- ions attract + areas at H’s
• water molecules surround and separate Na⁺ and Cl^- ions (hydration shell)
• causing salt crystal to dissolve

• Most water molecules remain intact in aqueous solutions within living organisms
• but some water molecules actually break apart in a process called dissociation or ionization
• formation of ions when covalent bonds in a water molecule break spontaneously
• Two types of ions result from dissociation of water molecules (H₂O)
• hydrogen ions (H⁺) with + charge result
• when one of protons (from hydrogen atom nuclei) dissociate from the rest of the molecule
• hydroxide ions (OH^-) with - charge results
• from the rest of the dissociated water molecule
• which has retained shared electron from covalent bond, is negatively charged and forms a hydroxide ion, OH^-
• Hydrogen and hydroxide ions result from spontaneous dissociation of water molecules in aqueous solutions
• right balance of these two ions is required for proper functioning of chemical processes within organisms
• the balance between these two ions is described and measured in terms of acids, bases and pH scale
• acid
• any substance that dissociates in water to increase concentration of H⁺ ions
• base (or alkali)
• any substance that combines with H⁺ ions when dissolved in water
• neutral
• any substance in which concentrations of H⁺ ions and OH^- ions are equal
• pH scale
• used to measure acidity or alkalinity of a solution
• pH stands for" potential hydrogen"
• the negative logarithm of hydrogen ion ([H⁺]) concentration in solution
• (negative logarithm of 10^-7 equals 7, therefore pH of pure water is 7)
• acid
• any substance that dissociates in water to increase concentration of H⁺ ions
• stronger an acid is, the more H⁺ ions it produces
• acidic solutions have pH values below 7
• strong hydrochloric acid (HCl), abundant in your stomach, ionizes completely in water to H⁺ and Cl^- ions, has a pH of 1
• base
• any substance that combines with H⁺ ions when dissolved in water
• by combining with H⁺ ions, a base lowers H⁺ ion concentration in solution
• basic, or alkaline, solutions have pH values above 7
• strong bases, such as sodium hydroxide (NaOH), have pH values of 12 or more
• Neutral
• any substance in which concentrations of H⁺ ions and OH^- ions are equal
• neutral solutions have a pH value of 7
• at 25° C, a liter of pure water contains 1/10,000,000 (or 10^-7) mole of H⁺ ions
• negative logarithm of 10^-7 equals 7, and therefore the pH of pure water is 7
• pH inside almost all cells, and in fluid surrounding cells, is fairly close to 7
• even a slight change in pH can be harmful
• biological fluids contain buffers that resist changes in pH
• buffer
• substance that resists changes in pH by
• accepting H⁺ ions when they’re in excess
• donating H⁺ ions when they’re depleted
• acts as a reservoir for hydrogen (H⁺) ions
• takes H⁺ ions from solution when their concentration increases
• donates H⁺ ions to solution when their concentration falls
• buffers are not foolproof
• important that cell maintain a constant pH level
• pH of an organism is kept at relatively constant pH by buffers
• within organisms most buffers act as pairs of substances
• one an acid and one a base
• example

• changes in pH can harm living organisms
• changes in pH of environment can have drastic effects
• acid precipitation (rain, fog, snow) can cause changes in pH of environment
• pH changes can kill fish in lake, trees in forests, affect human health, erode buildings
• precipitation with a pH below 5.6
• rain with pH of 2-3, more acidic than vinegar, recorded in eastern US
• fog with pH1.7, nearly acidic as human stomach digestive juices, recorded downwind from LA
• results mainly from presence in air of sulfur oxides and nitrogen oxides
• which result mostly from the burning of fossil fuels in factories and automobiles
• coal, oil and gas are fossil fuels
• complex environmental problem with no easy solution

• chemical reactions lead to chemical changes in matter
• are essence of chemistry and life
• all chemical reactions involve
• shifting of atoms from one molecule or ionic compound to another
• via formation and breaking of chemical bonds
• without any change in number or identity of atoms
• reactants
• original molecules before a chemical reaction starts
• products
• molecules resulting from chemical reaction
• chemical reactions can be described by chemical equations
• reactants
• written on left side of equation
• products
• written on right side of equation
• arrow (instead of =)between "reactants" side and "products" side
• means "yields"
• indicates direction in which reaction tends to proceed
• chemical equations
• example: 2H₂ + O₂ ® 2H₂O
• reactants products
• same numbers of H and O atoms appear on both left and right hand side of arrow but are grouped differently
• (H-H) + (H-H) + (O-O) = (H-O-H) + (H-O-H)
• 4 H, 2 O = 4 H, 2 O
• 2 molecules of H plus 1 molecule of O yields 2 molecules of water
• can proceed in two directions
• forward = to the right ®
• reverse = to the left ¬
• when rates of forward and reverse reactions are equal, reaction has reached equilibrium
• organisms carry out a great number of chemical reactions, most involving carbon, that rearrange matter in significant ways
• examples
• photosynthesis
• 6CO₂ + 6H₂O + energy ® 6O₂ + C₆H₁₂O₆
• 6C, 12H, 18O ® 6C, 12H, 18O
• production of vitamin A in human cells
• C₄₀H₅₆ + O₂ + 4H ® 2C₂₀H₃₀O
• beta-carotene vitamin A
• 40C, 2O, 60H ® 40C, 2O, 60H

The End