Water.ppt

Water and the Fitness of the

Environment

Water - Polarity

• Water - simple, common, but exceptional– H2O - two Hydrogen and one Oxygen

– covers 75% of earth’s surface

Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings

Water - Polarity

Polar bonds between O and two H atoms– Oxygen is more electronegative

• in water, –O (slightly negative charge)–H (slightly positive charge)


Fig 2.12 Polar covalent bonds in a water molecule

Hydrogen Bonds

• Partial charges on atoms in water allow bonds to form between molecules – Hydrogen bonding

• results when H from one molecule is attracted to O of a different molecule

• partial positive attracted to partial negative

Fig 3.2 Hydrogen bonds between water molecules

1

4

32

hybrid s+3p orbitals


Fig 2.15 A Hydrogen bond between water and ammonia


partial pos.

partial neg.

Water - Polarity

• Hydrogen Bonding – between water molecules– gives water its special properties

Cohesion

Adhesion

Surface Tension

Temperature Moderation

Water Properties - Cohesion

• Cohesion– Binding together of like molecules by H bonds– High in water– H-bonds constantly breaking and reforming– most water molecules are bonded to

neighboring molecules at any instant– Contributes to water transport in plants

Water Properties - Adhesion

• Adhesion– Clinging of one substance to another unlike

substance– also involves H-bonds– also contributes to water transport in plants– water adheres to molecules of the walls of the

xylem vessels in plant stems (trunks) helps counter the effects of gravity

Cohesion and adhesion move water through plants

Water Properties - Surface Tension

• Surface Tension– Measure of how

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

– Higher in water than other liquids

– Ordered arrangement of water molecules at air-water interface


Fig. 3.4

Water - Temperature Moderation

• Heat is the measure of the total amount of kinetic energy (energy of motion) in a body of matter

• Temperature is a measure of the intensity of heat due to the average kinetic energy of the molecules

• How does water moderate temperature?


• Water can absorb a lot of heat with only a small change in temperature– has a high Specific Heat

Water - Specific Heat

• Significance– A large body of water can absorb a large

amount of heat from the sun in daytime and during the summer, while warming only a few degrees.

– At night and during the winter, the warm water will warm cooler air.

– Stabilizes temperatures


• Liquids absorb heat as they evaporate– For water - 580 cal for each gram evaporated

• high compared with other liquids

– Heat of vaporization• Amount of heat a liquid must absorb for 1g

to be converted from liquid to gaseous state

Water - Heat of Vaporization

• Water’s high heat of vaporization – due to H bonding– results in “evaporative cooling”

• Hottest molecules evaporate, cooler molecules remain behind

• Cools oceans in tropics• Evaporation of sweat cools body

Fig 3.5 Hydrogen bonds in ice

• Are more “ordered” than in liquid water, making ice less dense

• Insulates the water it floats on

Ice WaterCopyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings

Water - Solvent of Life

• Solution – Liquid that is a completely homogeneous

mixture of two or more substances

*** Sugar in water ***


• Solvent - dissolving agent (water)

• Solute - substance that is dissolved (sugar)

• Aqueous solution - water is the solvent



• What happens when a solute dissolves in a solvent?

• Partial negative (O) and positive (H) charges on water molecule have affinity for positive and negative parts of solute

Fig 3.6 A crystal of table salt dissolving in water

Pos. charged Na+ ionscling to partial neg. charged O atomsof water molecules

Neg. charged Cl- ionscling to partial pos. charged H atomsof water molecules


Na+

Cl-


• Other compounds besides ionic ones can also be solubilized

• Most will have regions that are polar– Partial pos. and neg. regions (like water)– May have some ionic regions, too– Proteins, DNA, etc.


• Two categories of substances:– Hydrophilic (water-loving)

• compound with an affinity for water• polar or ionic compounds• can form H-bonds with water

– Hydrophobic (water-fearing)

• compounds which lack an affinity for water• non-polar compounds• lipids of the cell membrane• no H-bonding with water

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Water.ppt