Properties of Water. Water: 2 atoms of hydrogen, one of oxygen Held together by strong, covalent...
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Transcript of Properties of Water. Water: 2 atoms of hydrogen, one of oxygen Held together by strong, covalent...
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
Fig. 3.12Cohesion of Water
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 BasesPure water dissociates to yield equal
amounts of H+ and OH- solution:
H2O OH- + H+
hydroxide hydrogen ion 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 waterTo form carbonic
Acidcarbonic 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/images/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