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