Water and the fitness of the Environment

Water contributes to the fitness of the environment to support life:

--Life probably evolved in water

--Living cells 70-95% water

--Water covers 75% of Earth

--Water exists naturally in all 3 matter states

Water’s extraordinary properties are emergent properties resulting from water’s structure and molecular interactions (important)

* Water’s Polarity

--Results in hydrogen bonding

--Polar: asymmetrical

--H bonding orders water into a higher level of structural organization

--+ charged H of 1 molecule attracted to -O of another water molecule

--Can form a maximum of 4 H bonds with neighbors

Water has extraordinary properties:

--Cohesive behavior

--Resists changes in temperature

--Has a high heat of vaporization & cools surfaces as it evaporates

--Expands when freezes

--Is a versatile solvent

Cohesion = when a substance is held together by H bonds

--H bonds transient

--Contributes to upward water transport in plants by holding the water column together

--Adhesion counteracts gravity

Surface tension = measure of how difficult it is to stretch or break the surface of a liquid

Water has a greater surface tension than most liquids

At the air/water interface, surface water molecles are H bonded to each other and those below

--Causes water to bead

Water moderates temperatures on earth

Heat and Temperature

High Specific heat

Evaporative Cooling

Kinetic energy - energy of motion

Heat - total kinetic energy due to molecular motion in a body of matter

Temperature - measure of heat intensity Due to average kinetic energy of molecules

Calorie - amt. of heat needed to raise temp. of 1 g of water 1o Celsius (also amt. Released when cooled)

Kilocalorie (kcal/Cal) food 1000cal. Specific heat - amt. Of heat absorbed or lost for 1 g of a substance to temp by 1 degree Celsius

Specific heat of water = 1 cal/g/oC

Large heat loss or gain due to H bonding

Takes much energy to break bond before molecules can move faster

Heat Sink - large bodies of water

Vaporization (evaporation) = liquid to gas

Heat of vaporization - quantity of heat a liquid must absorb for 1 g to convert to a gas

High heat of vaporization at boiling pt. (540 cal/g or 2260 J/g) 1 J=.239 cal Cools organisms, moderates aquatic temps, dissipates solar heat gain

Water less dense as a solid than a liquid - due to H bonds

Water densest at 4oC

Water cools from 4oC - 0 degrees C =expands,less dense

As freezes, molecules lack kinetic energy to break H bonds

Crystalline lattice (4 H bonds) keeps molecules further apart

Prevents deep bodies of water from freezing

Ice forms on surface (less dense) releases heat to the water below, insulating it

Transitions between seasons =As freezes H bonds form, as melts, H bonds beak

Solution - a liquid that is a completely homogeneous mix of 2 or more substances

Solvent - dissolving agent

Solute - substance dissolved

Aqueous solution - water is solvent

Hydrophilic

--Ionic compounds dissolve in H2O

--Charged regions of polar molecule has attraction to charged ions

--H2O surrounds individual ions

--Polar compounds soluble (+charged to -charged)

Hydrophobic

Nonpolar compounds are NOT water-soluble

Have symmetric distribution of charges

Most Biochemical reactions involve solutes dissolved in water

Molecular weight - sum of the weight of all atoms in a molecule (in daltons)

Mole - amt. of a substance that has a mass in g numerically equivalent to its moleclar weight in daltons

Molarity - number of moles of solute per liter of solution

Mole of C12H22O11

C = 12 dal (x12)

H = 1 dal (x22)

O = 16 dal (x11)

Total+ 342 dal

To make a 1 M solution = take 342 g of sucrose and add water to make 1Liter

A mole of any substance has the same # of molecules as a mole of another- Avogadro’s #6.02 x 1023

Dissociation of water

--H+ can transfer from the shared O atom to the unshared orbital of the O atom to which it is H bonded

--Creates a hydronium (H3O+) ion

--Leaves behind a hydroxide ion (OH-)

--Reaction reversible

--At equilibrium, most water NOT ionized

At equilibrium, in pure H2O, H+ ions = OH- ions

M = 10-7 M

Only 1 out of every 554,000,000 molecules dissociates

Acid

Substance that increases the relative H+ of a solution Removes OH- because it tends to combine with H+ to form H2O

Base

Substance that reduces the relative H+ of a solution Alternately increases OH-

Neutral solution = H+ = OH-

Acidic solution H+ > OH-

Basic solution H+ < OH-

Strong acids and bases dissociate completely in water

Weak ones dissociate partially & reversibly

pH Scale

In any aqueous solution:

[H+][OH-]=1.0x10-14

pH Scale - used to measure degree of acidity 0-14

Each pH unit represents a tenfold difference (logarithmic), small * pH =large * H+

Buffer - substance that minimizes large sudden changes in pH

Combination of H+ donor and H+ acceptor forms in a solution of weak acids/bases

Accept H+ from solution when they are in excess

Donate H+ to solution when they are depleted

Acid Precipitation threatens the fitness of the environment

-- rain, snow or fog more strongly acidic than pH 5.6

Sulfur oxides & nitrogen oxides in the air react with H2O in air to form acids

Source of oxides = fossil fuels

Low pH affects mineral solubility, aquatic life