Figure 3-01

Water is Central to Life

• Water has several properties that life is dependent upon including:

• Cohesion• High heat of vaporization• High specific heat• Less dense as a solid than a liquid • A “universal solvent”• All of these properties are due to the polar

nature of water molecules and hydrogen bonding

LE 3-3

Water-conducting cells

100 µm

Figure 3-04

LE 3-6

Na+

Na+

Cl–Cl–

+

+

+

+

+

+

++

–

–

–

–

–

–

–

–

–

–

–

Dissociation

• Many molecules will dissociate when the are placed in water

• Ions are separated and “released” into the water environment

• Table salts dissolves in water (dissociates into sodium and chlorine ions)

Acids

• When hydrogen chloride is placed in water it dissociates into Hydrogen ions (H+) and chlorine ions (Cl-)

• HCl is an acid

• Acids are substance that release H+ ions (protons) when they dissociate in water

• The more acidic a solution, the higher concentration of H+ ions (protons) it has.

Bases

• When sodium hydroxide is placed in water it dissociates into sodium ions (Na+) and hydroxide ions (OH-)

• NaOH is a base

• Bases are substance that release OH- ions when they dissociate in water

• The more acidic a solution, the higher concentration of OH- ions it has.

Water Dissociates

• Water molecules are not completely stable

• They spontaneously dissociate (at a very slow rate) into H+ and OH- ions.

• At any given moment only 1/554,000,000 water molecules in pure water dissociates

LE 3-UN53

Hydroniumion (H3O+)

Hydroxideion (OH–)

Spontaneous Dissociation of Water

• Is water and acid? Is it a base?

• Water is amphoteric

• At equilibrium, the [H+] concentration is 1X10-7 M.

• At equilibrium, the [OH-] concentration is 1X10-7 M.

pH Scale

• pH is the –log [H+]

• The pH scale measure the acidity (or basicity) of an aqueous solution

• The pH scale range from 0-14

• The lower the pH, the higher the [H+]

• The higher the pH the lower the [H+]

• A pH of 7 is considered to be neutral

Ph Scale

LE 3-8pH Scale

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14Oven cleaner

Household bleach

Household ammonia

Milk of magnesia

Seawater

Pure waterHuman blood

UrineRainwater

Black coffee

Tomato juice

Vinegar, beer, wine,cola

Digestive (stomach)juice, lemon juice

Battery acid

Neutral[H+] = [OH–]

Inc

rea

sin

gly

Ac

idic

[H+]

> [

OH

–]

Inc

rea

sin

gly

Ba

sic

[H+]

< [

OH

– ]

The importance of pH to life

• Living things typically exist is an aqueous environment

• The pH of the environment influences the charge of molecules present in that environment

• The charge of molecules influences their shape (structure)

• The shape of molecules determines their function

Case study-pH and Blood

• The normal pH of blood is about 7.4

• What are the consequences of a change in that pH?

• Effect on hemoglobin?

• Why is hemoglobin affected by a change in pH?

• What homeostatic mechanisms does the blood have to keep its pH constant?

Buffering Systems