Earth’s Biological Earth’s Biological MediumMedium

PolarityPolarity

• Slight charge on either end of the molecule due to the unequal electronegativities of hydrogen & oxygen

• This leads to many other properties…

CohesionCohesion• Water molecules stick

together with Hydrogen bonds

http://www.arroyoseco.org/conservation.htmwww.biology.arizona.edu

http://www.ccs.k12.in.us/chsBS/kons/kons/chemical_and_physical_propertiesanswer.htm

AdhesionAdhesion

• Water molecules stick (adhere) to other surfaces

en.wikivisual.com www.ccs.k12.in.us

Surface TensionSurface Tension

• Measure of how difficult it is to stretch or break the surface of a liquid

• Very high for waterhttp://faculty.vassar.edu/suter/1websites/bejohns/mateselection/files/female.htm

news.bbc.co.uk/.../photo_galleries/4670308.stm

www.ext.vt.edu

Specific HeatSpecific Heat• Amount of heat that must be absorbed or

lost for 1g of a substance to change by 1oC

• Water has high specific heat – it resists temperature change

• This keeps the earth within viable temperature limits.

www.michigan.org

Evaporative CoolingEvaporative Cooling

• A higher heat of vaporization allows a surface to be cooled as water sits on its surface www.doggonesafe.com/dog%20communication.htm

AP Photo/Darin Cummings

Density of IceDensity of Ice• Solid water is less dense than its

liquid because as hydrogen bonds freeze, they force molecules further apart

• In large bodies of water, a top layer of ice actually insulates the water below

• 4oC is when water is at its most dense

www.astrobiology.com/lter/album01/179.html

http://www.astrobiology.com/lter/album01/209.html

Universal SolventUniversal Solvent• Water dissolves many materials creating

aqueous solutions (water is solvent, what’s being dissolved is solute)

• This property is the direct result of water’s polar structure– HydrophilicHydrophilic – substances attracted to water– HydrophobicHydrophobic – repel water (or not attracted)

Water & pHWater & pH

• In pure water, even if dissociation occurs, the concentration of H+ and OH- ions is equal

• When acids or bases are added to water, these concentrations change quickly

• pH is a measure of hydrogen ion concentration on a scale between 0-14

AcidsAcids• The more acidic, the lower the pH

• pH < 7

• Taste sour

www.elmhurst.edu

BasesBases• The more basic, the higher its pH value

• Taste bitter

www.chemistryland.comcommunity.tvguide.com

NeutralizationNeutralization• When bases and acids of

similar strength are combined, the pH of the solution will approach neutral (pH 7)

• Water is often a product of a neutralization reaction.

BuffersBuffers

• Substances that resist changes in pH

• Many of these in the body since even minor changes can be life threatening– EX: blood ph is 7.4