Chapter 3: Water

Water: The Molecule That Supports All of Life

• Water is the biological medium here on Earth• All living organisms require water more than any other

substance• ¾ of the Earth’s surface is submerged in water• The abundance of water is the main reason the Earth is

habitable

Properties of Water

• The polarity of water molecules• Allows them to form hydrogen bonds with each other• Contributes to the various properties water exhibits that

make life as we know it possible

Hydrogenbonds

+

H

H+

+

–

–

– –

Properties of Water

• Cohesion• Is the bonding of like polar molecules to

neighboring like polar molecules• Is due to hydrogen bonding

Properties of Water

• Adhesion• Is the bonding of polar molecules to unlike

neighboring polar molecules• It is also due to hydrogen bonding

Cohesion and Adhesion

Capillary action

Cohesion and Adhesion

Heat and Temperature

• Kinetic Energy• Is the energy of motion• Heat is the measure of the total amount of

kinetic energy due to molecular motion• Temperature measures the intensity of heat

Specific Heat

• Specific Heat• Is the amount of heat that must be absorbed or

lost for 1 gram of that substance to change its temperature by 1ºC

• Water has a high specific heat, due to hydrogen bonding, which allows it to minimize temperature fluctuations• Heat is absorbed when hydrogen bonds break• Heat is released when hydrogen bonds form

Specific Heat

• Simply put:• If you add heat to water, it will be get

warmer slower compared to a liquid without hydrogen bonds

• If you take away heat from water, it will cool slower compared to other liquids

San Diego 72°

40 miles

Pacific Ocean70s (°F)

80s

90s

100s

Santa Barbara 73°Los Angeles (Airport) 75°

Burbank90°

San Bernardino100°Riverside 96°

Santa Ana 84° Palm Springs

106°

Specific Heat

Evaporative Cooling

• Evaporation• Is the transformation of a substance from a liquid to a gas• For water, H bonds BETWEEN water molecules must be broken for evaporation to occur

• Evaporative cooling• Is due to the molecules with the most kinetic energy entering the gas state, leaving behind those with less

energy • This phenomenon can cool biological organisms• Evaporation is impeded by high humidity however

Expression: It’s not the heat but the humidity!

Ice

• Solid water, or ice• Is less dense than liquid water• Floats in liquid water

• Since ice floats in water• Life can exist under the frozen surfaces of lakes and

polar seas

Ice

• The hydrogen bonds in ice are more “ordered” than in liquid water, making ice less dense

• In same space, fewer water molecules can fit when frozen compared to liquid water

Liquid water

Hydrogen bonds constantly break and re-form

Ice

Hydrogen bonds are stable

Hydrogen bond

Water: Solvent of Life

• Water is a versatile solvent due to its polarity

• It can form aqueous solutions

Water as a Solvent

The different regions of the polar water molecule can interact with ionic compounds called solutes and dissolve them

+

+

+

+Cl –

–

–

–

–

Na+

++

+

+

–

–

–

–

–

–Na+

Cl–

++

--

--

-

+

++ +

++

+ +

Water as a Solvent

• A solution is a liquid homogenous mixture of 2 or more substances

• The solute is the substance that is dissolved in a solution

• The solvent is the dissolving agent of a solution, often water (aqueous)

Water as a Solvent

• A hydrophilic substance

• Does have an affinity for water

• Typically contains polar covalent or ionic bonds

• A hydrophobic substance

• Does not have an affinity for water

• Typically contains non-polar covalent bonds (like fats/oil)

• Expression: Oil and water (don’t mix); Meaning incompatible things or people (can’t be made to work well together)

pH

• Water can break apart into hydrogen ions (H+) and hydroxide ions (OH-)

• Changes in the concentration of these ions can have a great affect on living organisms

pH

• The pH of a solution• Is determined by the concentration of hydrogen

ions, [H+] • [H+] high in an acid• [H+] low in a base

pH = -log[H+]

• e.g. A solution with [H+] = 1 x 10-

5M would have a pH value of 5

Acids and Bases

• An acid• Is any substance that increases the H+

concentration of a solution

• A base• Is any substance that reduces the H+

concentration of a solution

pH Scale

• pH scale is from 0-14• Solutions with

• pH values < 7 are acids• pH values = 7 are neutral• pH values > 7 are bases

Incr

easi

ng

ly A

cid

ic[H

+]

> [

OH

– ]In

creasi

ng

ly B

asic

[H+]

< [

OH

– ]

Neutral[H+] = [OH–]

Oven cleaner

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

pH Scale

Battery acid

Digestive (stomach) juice, lemon juiceVinegar, beer, wine,colaTomato juice

Black coffee RainwaterUrine

Pure waterHuman blood

Seawater

Milk of magnesia

Household ammonia

Household bleach

pH Scale

• For example, a testing kit indicates a pH level of 3 in a solution. This means:• The solution is an acid• The [H+] = 1 x 10-3M or 0.001M

• Another solution measures pH 9. This means:• The solution is a base• The [H+] = 1 x 10-9M or 0.000000001M

• pH3 has more H+ than pH 9 (0.001M > 0.000000001M)

Pretend iClicker ?

The concentration of H+ ions in a solution is 1 x 10-

8M. This tells you the pH of the solution is

A.pH 8, and it is an acid.

B.pH 8, and it is a base.

C.pH 6, and it is an acid.

D.pH 6, and it is a base

E.pH 14, and it is an acid.