NOTES: 2.2 – Properties of Water

Water Overview…• Water is perhaps the most important compound

in living organisms

• Because so many substances can dissolve in water:

molecules and ions are free to

move and collide with one

another in the reactions of life;water serves as a means of

material transportation

(i.e. blood, plant sap)

Water Overview…• Living cells are 70%-95%

water

• Water covers approx. ¾ of earth.

• In nature, water naturally exists in all 3 states of matter: solid, liquid and gas

Water is a POLAR molecule • the bonds that hold the hydrogen and oxygen

atoms together are POLAR COVALENT BONDS… this means that the electrons that make up the covalent bonds are shared UNEQUALLY …they spend more time near the oxygen atom than they do near the hydrogen atom

so, a water molecule has a positive end and a negative end

Water is POLAR…

• POLAR WATER MOLECULES attract one another, as well as ions and other polar molecules

Water Molecules…

• The polarity of water molecules

results in HYDROGEN BONDING

• a HYDROGEN BOND is the

attraction of opposite charges

between hydrogen and oxygen

• Each water molecule can form a maximum of 4 hydrogen bonds with neighboring water molecules

Properties of Water (as a result of

polarity and H-bonding):

► COHESION: molecules are held together by H bonds

• contributes to upward

movement of water in

plants from roots to leaves

Tension–Cohesion Model

Properties of Water

► ADHESION: water sticks to other surfaces (by polarity or H bonds); can counteract gravity in plant vessels

► SURFACE TENSION: measure of how difficult it is to stretch the surface of a liquid

-water has greater surface tension than most liquids because at the air/water interface the surface water molecules are H-bonded to each other and to the water molecules below

-causes water to “bead”

-creates a “skin” on the surface

► HIGH SPECIFIC HEAT (resists changes in temp)

• Specific Heat: amt. of heat that must be absorbed or lost for 1 gram of a substance to change its temp by 1°C

• Water’s high specific heat means that it resists temp. changes when it absorbs or releases heat

*Heat is absorbed to break H-bonds; and given off when they form

► HIGH HEAT OF VAPORIZATION

• Heat of vaporization = amt. of heat a liquid must absorb for 1 g to be converted to gas state

• for water molecules to evaporate, H bonds must be broken, which requires heat energy

► EVAPORATIVE COOLING: after high temp. molecules have evaporated, the remaining liquid is cooler (ex: sweating)

-stabilizes temp. in aquatic ecosystems

-protects organisms from overheating

*(as 1 g of water evaporates

from our skin, 539 g

of body cools by 1°C)

The air in the shower stall is at the same temperature as the air outside, but there's less water vapor outside to condense on the skin.

EVAPORATIVE COOLING

SPECIFIC HEAT

(WHILE WATER IS HEATING)

HEAT OF VAPORIZATION (WHILE CHANGING STATE)

► WATER EXPANDS WHEN IT FREEZES• because of H-bonding, water is less dense as

a solid than it is as a liquid...ice floats!• since ice is less dense, it forms on the surface

at first• as water freezes, it releases heat to the water

below and insulates it!• Oceans and lakes don’t freeze solid!!

► WATER IS THE SOLVENT OF LIFE

• due to its polarity, water is a versatile solvent

cell

Water forms Solutions

– All the components of a solution are evenly distributed throughout the solution.

– In a saltwater solution, table salt is the solute—the substance that is dissolved.

– Water is the solvent—the substance in which the solute dissolves.

Water forms Suspensions– Some materials do not dissolve in water,

but separate into small pieces. Such mixtures of water and nondissolved material are known as suspensions.

– Some of the most important biological fluids are both solutions and suspensions.

– Blood is mostly water. It contains many dissolved compounds, but also cells and other undissolved particles that remain in suspension as the blood moves through the body.

Acids, Bases, and pH• Water molecules sometimes split apart to form

hydrogen ions and hydroxide ions.

• This reaction can be summarized by a chemical equation in which double arrows are used to show that the reaction can occur in either direction.

• Chemists devised a measurement system called the pH scale to indicate the concentration of H+ ions in solution.

• The pH scale ranges from 0 to 14. • At a pH of 7, the concentration of H+ ions and OH– ions

is equal. Pure water has a pH of 7. – Acids have a pH less than 7. – Bases have a pH greater than 7.

pH Scale

Buffers – The pH of the fluids within most cells in the human

body must generally be kept between 6.5 and 7.5 in order to maintain homeostasis. If the pH is lower or higher, it will affect the chemical reactions that take place within the cells.

– One of the ways that organisms control pH is through dissolved compounds called buffers, which are weak acids or bases that can react with strong acids or bases to prevent sharp, sudden changes in pH.