WaterWater: An absolute necessity for life
¾ of the planet is covered by itLife existed in water 2 Billion years prior to adapting to
land60% of human weight from water83% of human blood composition
Where water exists, life may be foundWater is critical to cell biochemistryWater 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)
• At 25° C, one liter of water contains one ten-millionth (10-7) molemole of H+ ions
• A MoleMole 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 10-7 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
AcidsAcids• Any solution with a pH
less than 7.0• The stronger an acid is,
the more H+ ions it releases to solution
• High amounts of H+ ions are causticcaustic = capable of burning, corroding, or destroying living tissue
BasesBases• Any solution with a pH
greater than 7.0• The stronger a base is,
the more OH- or similar ions it releases to solution
• High amounts of OH- ions are causticcaustic
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 (H2CO3) serves as a
buffer which can split to form bicarbonate ion (HCO3-)
and a Hydrogen ion (H+) to aid in stabilizing the pH
Articles on Reserve for Article Review #2
• Taylor, D. 2007. Employment preferences and salary expectations of students in science and engineering. Bioscience. 57: 175-185.
• Ostfeld, R.S. et al. 2006. Controlling ticks and tick-borne zoonoses with biological and chemical agents. Bioscience. 56: 383-394.
• DeLong, E.F. 2003. A plentitude of ocean life. Natural History. 112: 40-46.
• Schmid-Hempel, P. 2003. Fight of the bumblebee. Natural History. 112: 52-59.
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