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 OHH2O  OH- (hydroxide ion) + H+ (hydrogen ion)
• At 25° C, one liter of water contains one ten-millionth
(10-7) mole of H+ ions
• A 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 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 OHions
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
• High amounts of H+ ions
are caustic = capable of
burning, corroding, or
destroying living tissue
Bases
• 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 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 (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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