Properties of Water
Water:
•2 atoms of hydrogen, one of oxygen
•Held together by strong, covalent bond - electrons are
‘shared’
•Water molecules interact – attracted to each other due
to electrically charged regions – weak bonds (Hydrogen
bonds) formed
Bonds formed between
water molecules – break
and reform – like velcro
Properties of Water – Due to interactions
between molecules
• Liquid at room temperature
• Solvent for polar + ionic molecules – due to its
own polar nature
• Water is cohesive
• Has high specific heat – requires a lot of heat
to change its temperature
• High heat of vaporization
• Solid water (ice) floats
• Properties altered by dissolved substances
Cohesion of
Water
Fig. 3.12
Life in water influenced by:
• Dissolved nutrients
• Dissolved salts – salinity
• Dissolved oxygen
• pH
• Light
• Temperature
pH and water – acids and bases
• Due to dissociation of water molecules into
Hydrogen and hydroxyl ions
• pH is a measure of hydrogen ion
concentration
• Impacted by dissolved substances –
organic materials, gasses, salts
Acids and Bases
Pure water dissociates to yield equal
amounts of H+ and OH- solution:
H2O  OH- + H+
hydroxide
ion
hydrogen
ion
Acids and Bases
• Acid: excess of H+ ions
• Base: excess of OH- ions
pH is a measure of H+ ion
concentration on a log scale:
pH = -log [H+]
• lower number indicates a higher
hydrogen ion concentration or a
more acidic condition
Buffers
• A buffer is a substance that when
dissolved in water contributes H or OH
ions as needed to resist large changes
in pH
– sort of like a chemical shock absorber
• Important in living systems – pH is
critical to maintenance of life
processes
• Carbon Dioxide acts as an important How carbon
dioxide acts as an important natural buffer - how it
works:
• water absorbs CO2 from atmosphere – how it works:
Carbon Dioxide
Dissolves in
water
To form carbonic
Acid
carbonic Acid
dissolves to
yield H+ ions
All reactions are reversible – if more H+ ions are added
reaction pushed to left, more OH- ions reaction pushed
to right
• CO2 is absorbed from atmosphere
• Enters rain water and diffuses directly into
surface waters
– Creates moderately acidic condition but also
some buffering capacity
• Other atmospheric gasses may increase
acidity of rain water: = acid rain
– Sulfur oxides  sulfuric acid
– Nitrogen oxides  nitric acid
• Strong acids, overcome buffering capacity, create
acidic bodies of water
• Particular problem for areas with granite substrate
Penetration of Water by Light
• % of surface light at various depths:
•
Depth
% of surface light
1 cm
73
1 meter
44,5
10 meters
22.2
100 meters
0.53
•varies with turbidity – assume clear water
•Different wavelengths penetrate water to
different degrees – blue penetrates the furthest
http://staffwww.fullcoll.edu/tmorris/elements_of_ecology/image
s/light_spectral_absorption_water.jpg
• Estimation of
turbidity of water
using senchi disc
• Turbidity is a
function of
suspended
plankton growth
and amount
particulate matter in
water
Viscosity of Water
• 100X that of air
• Means movement through water meets
considerable resistance
• Adaptations of reverse streamlining
– Short, blunt anterior
– Rapidly tapering body – minimizes
replacement of water immediately behind
moving animal
• Water has a high specific heat
• Large amounts of heat exchange required
for change in temperature
• Temperature slow to change – relative to
atmosphere
– Cells/organisms composed largely of water,
slower to change temp. than atmosphere
• Acts as effective heat sink
– High heat loss by organisms to surrounding
environment
• Large amounts of heat required to change
state – eg. liquid to solid
• Changes in density with temperature
• Greatest density at 4C
• Ice floats – expands due to intermolecular
interactions
• Develops layers of stratification
– Surface waters warmed (in summer)
– Deeper waters cool
– Thermocline – region of rapid change in temp.
with depth
Oxygen in Water
• Dissolves in water from atmosphere
• Enters and moves by diffusion
• solubility function of
– Temperature – greater at lower temperatures
– Salinity – more soluble in fresh water
– Atmospheric pressure
• Oxygen and Depth
• Dissolves at surface
• Reaches minimal concentration between
surface and ~ 1000 meters depth
• Produced by photosynthetic activity
• Absorbed by metabolic processes
• Anoxic or Anaerobic – without oxygen
– Certain deep waters
– Consequence of metabolic activity
• Summary
• Life on earth depends on water and its
properties
• Water is a polar compound
– Ends of each molecule have different charges
• Water is a solvent for ionic solids – salts which
dissociate into positively and negatively charged
ions
• pH is a measure of H+ ion concentration
– Lower pH means higher H+ ion concentration
• Light is quickly absorbed by water meaning is in
only available at the surface of bodies of water
• Water is much more viscous than air