Chapter 3:

Water & Life

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

A view of earth from space, showing our planet’s abundance of water

Concept 3.1: Polar covalent bonds in water molecules result in hydrogen bonding

• The water molecule is a polar molecule: the opposite ends have opposite charges

• Polarity allows water molecules to form hydrogen bonds with each other

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

Figure 3.2 Hydrogen bonds between water molecules

Hydrogenbonds

+

+

H

H+

+

–

–

–

–

Concept 3.2: Four emergent properties of water contribute to Earth’s suitability for life

• Four of water’s properties that facilitate an environment for life are

– Cohesive behavior– Ability to moderate temperature– Expansion upon freezing– Versatility as a solvent

© 2011 Pearson Education, Inc.

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

Water transport in plants (cohesion)

Water conducting cells

100 µm

• Surface tension is a measure of how hard it is to break the surface of a liquid

• Surface tension is related to cohesion

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Figure 3.4

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

• Water needs to give away energy in form of heat when it goes from gas to liquid, and from liquid to solid. The opposite is also true: water needs to take energy in form of heat when going from solid to liquid or liquid to gas.

• Water is special in its need to absorb or expend a LARGE AMOUNT of heat to change form.

• The latter is the reason we perspire: evaporating water cools down our skin.

Water’s Ability to Moderate Temperature

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

Ice: crystalline structure and floating barrier

Liquid water

Hydrogen bonds constantly break and re-form

Ice

Hydrogen bonds are stable

Hydrogen bond

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

A crystal of table salt dissolving in water (use as a solvent)

Negative Oxygen regions

of polar water molecules are

attracted to sodium cations (Na+).

+

+

+

+Cl –

–

–

–

–

Na+Positive hydrogen regions

of water molecules cling to chloride

anions (Cl–).

+

+

+

+

–

–

–

–

–

–

Na+

Cl–

Water: The Solvent of Life

• A solution is a liquid that is a homogeneous mixture of substances

• A solvent is the dissolving agent of a solution• The solute is the substance that is dissolved• An aqueous solution is one in which water is

the solvent

© 2011 Pearson Education, Inc.

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

Reaction producing hydroxide and hydronium ions

H

Hydroniumion (H3O+)

H

Hydroxideion (OH–)

H

H

H

H

H

H

+ –

+

Figure 3.10pH Scale

Battery acid

Gastric juice, lemon juice

Vinegar, wine,cola

BeerTomato juice

Black coffee

Rainwater

Urine

SalivaPure water

Human blood, tears

Seawater

Inside of small intestine

Milk of magnesia

Household ammonia

Householdbleach

Oven cleaner

Basicsolution

Neutralsolution

Acidicsolution

0

1

2

3

4

5

6

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9

10

Neutral

[H+] = [OH]

Incr

easi

ng

ly B

asic

[H+]

< [

OH

]

Incr

easi

ng

ly A

cid

ic

[H+]

> [

OH

]H+H+

H+

H+H+

H+

H+

H+

OH

OH

H+

OH

H+

OH

OH

OHOH

H+H+

H+

H+

OHOH

OH

OH OHOH

OHH+

11

12

13

14

Acidification: A Threat to Water Quality

• Human activities such as burning fossil fuels threaten water quality

• CO2 is the main product of fossil fuel combustion

• About 25% of human-generated CO2 is absorbed by the oceans

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• The burning of fossil fuels is also a major source of sulfur oxides and nitrogen oxides

• These compounds react with water in the air to form strong acids that fall in rain or snow

• Acid precipitation is rain, fog, or snow with a ph lower than 5.2

• Acid precipitation damages life in lakes and streams and changes soil chemistry on land

© 2011 Pearson Education, Inc.

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

Figure 3.9 Acid precipitation and its effects on a forest

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Moreacidic

Acidrain

Normalrain

Morebasic