Water Water: An absolute necessity for life  ¾ of the planet is covered by it  Life existed in water 2 Billion years prior to adapting to land  60% of human weight from water  83% of human blood composition  Where water exists, life may be found  Water is critical to cell biochemistry  Water is critical for photosynthesis  Water is the ‘universal solvent’

A generalized water cycle for Earth’s systems

Water’s structure Simple molecular structure with Oxygen bound to 2 Hydrogen atoms by single covalent bonds Electronegativity of O is much greater than H – so the bonds between these atoms are “polar”

Hydrogen bonding: outstanding chemical property of water “The polarity of water impacts water’s chemistry and the chemistry of life…”

Properties of water Cohesion = Adhesion = High specific heat = High heat of vaporization = Solid water less dense than liquid = Has solvent properties =

Fig. 2.14

Water can form ions The covalent bonds of water can break spontaneously to produce H+ and OH- H2O  OH - (hydroxide ion) + H + (hydrogen ion) moleAt 25° C, one liter of water contains one ten-millionth (10 -7 ) mole of H+ ions MoleA Mole is equal to the weight of a substance (in grams) that corresponds to the atomic mass of the atoms forming the substance H+ has an atomic mass of 1, so a mole of H+ would weigh 1 gram

Water and pH pH = -log [H+] Therefore, pure water, having moles/liter concentration of H +, has a pH of 7.0 A pH of 7.0 is determined to be “neutral” due to equal concentrations of H + and OH - ions

pH and logarithms Note that the pH scale is logarithmic, meaning that a difference of 1 of pH means a 10-fold change in H+ ion concentration Therefore, a solution having a pH of 3 has 10X more H+ ions than a solution with a pH of 4; 100X more H+ than a solution with a pH of 5

Fig. 2.16

Acids and bases Acids Any solution with a pH less than 7.0 The stronger an acid is, the more H+ ions it releases to solution causticHigh amounts of H+ ions are caustic = capable of burning, corroding, or destroying living tissueBases Any solution with a pH greater than 7.0 The stronger a base is, the more OH- or similar ions it releases to solution causticHigh amounts of OH- ions are caustic

Buffers help stabilize pH A buffer is a substance that resists changes in pH Within organisms, buffers consist of acid-base pairings In human blood, carbonic acid (H 2 CO 3 ) serves as a buffer which can split to form bicarbonate ion (HCO 3 - ) and a Hydrogen ion (H + ) to aid in stabilizing the pH

Articles on Reserve for Article Review #2 Taylor, D Employment preferences and salary expectations of students in science and engineering. Bioscience. 57: Ostfeld, R.S. et al Controlling ticks and tick-borne zoonoses with biological and chemical agents. Bioscience. 56: DeLong, E.F A plentitude of ocean life. Natural History. 112: Schmid-Hempel, P Fight of the bumblebee. Natural History. 112: