Copyright © 2006 Cynthia Garrard publishing under Canyon Design Chapter 3 – Water Three-quarters...

Copyright © 2006 Cynthia Garrard publishing under Canyon Design

Chapter 3 – Water

• Three-quarters of the Earth’s surface is submerged in water

• The abundance of water is the main reason the Earth is habitable

Figure 3.1


Chapter 3 – Water

• Overview: 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


Polarity of Water Molecules

• Water is unique because of its structure and the interaction of its molecules

• Water is a polar molecule because oxygen is more electronegative than hydrogen


• The polarity of water molecules

– Allows them to form hydrogen bonds with each other

– Contributes to the various properties water exhibits

Hydrogenbonds

+

+

H

H+

+

–

–

– –

Figure 3.2


Polarity of Water

• The hydrogen bonds between water molecules give water 4 unique properties:

1) Water’s cohesive behavior

2) Water’s ability to moderate temperature

3) Water’s expansion on freezing

4) Water’s versatility as a solvent


Cohesion

Cohesion

– Is the bonding of a high percentage of the molecules to neighboring molecules

Water molecules pull each other along


Cohesion

• Cohesion

– Helps pull water up through the microscopic vessels of plants

Water conducting cells

100 µmFigure 3.3


Cohesion

• Adhesion is related to cohesion

• Adhesion – the clinging of one substance to another

Water sticks to the walls of cells, which helps it resist gravity


Cohesion

• Surface tension

– Is a measure of how hard it is to break the surface of a liquid

Figure 3.4


Moderation of Temperature

• Water moderates air temperature

– By absorbing heat from air that is warmer and releasing the stored heat to air that is cooler

This is one reason why Cape Cod has milder winters than Worcester County


Moderation of Temperature

• Water has high specific heat

– That is, it takes a lot of energy to raise the temperature of water

Why the ocean is still cold here in the summer


Moderation of Heat

Fast moving molecules can leave a liquid as gas

• Evaporative cooling – when the “hottest” or fastest moving molecules leave a liquid, thus cooling the remaining liquid


Moderation of Heat

•Water’s evaporative cooling property helps stabilize the temperature of lakes and ponds and allows for the mechanism of temperature regulation in living things too


Expansion of Water

• Water is less dense as a solid than as a liquid

– This is very rare!

• Water expands as it solidifies (freezes)

• As a result, ice floats

This plays an important role in Life


Expansion of Water

• The hydrogen bonds in ice

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

– The molecules are further apart in ice than liquid water

Liquid water

Hydrogen bonds constantly break and re-form

Ice

Hydrogen bonds are stable

Hydrogen bond

Figure 3.5


Expansion of Water

Since ice floats in water

– Life can exist under the frozen surfaces of lakes and polar seas


At what temperature is water the densest?


The Solvent of Life

Solution – a liquid that is completely homogenous mixture of two or more substances

Solvent – the dissolving agent of a solution

Solute – the substance dissolved

Aqueous solution – solution in which water is the solvent


The Solvent of Life

• Water is a versatile solvent due to its polarity

• If an ionic compound (NaCl) is placed in water, the ions exposed at the surface are attracted to the polarity of the water molecules

Result:

Water molecules separate individual ions and surround them, effectively “breaking up” the salt


The Solvent of Life

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

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–

Figure 3.6


The Solvent of Life

• Compounds don’t have to be ionic to be affected

• Water can also interact with polar molecules such as proteins or sugars

This oxygen is

attracted to a slight

positive charge on

the lysozyme

molecule.This oxygen is attracted to a slight

negative charge on the lysozyme

molecule.(a) Lysozyme molecule

in a nonaqueous

environment

(b) Lysozyme molecule (purple)

in an aqueous environment

such as tears or saliva

(c) Ionic and polar regions on the protein’s

Surface attract water molecules.

+

–

Figure 3.7

These compounds are still water soluble


The Solvent of Life

• A hydrophilic substance

– Has an affinity for water

• Anything ionic

• Polar molecule

• Substance doesn’t have to dissolve in water


The Solvent of Life

• A hydrophobic substance

– Does not have an affinity for water

– Non-ionic and non-polar substances

– Seem to repel water


Acids, Bases and pH

Polar water molecules don’t just break up ionic and polar compounds

• Water can also disassociate itself

H

Hydroniumion (H3O+)

H

Hydroxideion (OH–)

H

H

H

H

H

H

+ –

+

Figure on p. 53 of water dissociating


Acids, Bases and pH

• Water can dissociate into

– Hydrogen ions – single protons with a charge of 1+

– Hydroxide ions – charged with a 1-

• This reaction is reversible and will reach a state of dynamic equilibrium

– When the water dissociates at the same rate that it is being re-formed

H

Hydroniumion (H3O+)

H

Hydroxideion (OH–)

H

H

H

H

H

H

+ –

+

Figure on p. 53 of water dissociating


Acids, Bases and pH

In pure water there is 1 molecule disassociated for every 554 million molecules of water.

Despite its rarity, disassociated water molecules are extremely important to life


Acids, Bases and pH

Certain solutes can alter the concentration of these ions

• An acid

– Is any substance that increases the hydrogen ion concentration of a solution

• A base

– Is any substance that reduces the hydrogen ion concentration of a solution


Acids, Bases and pH

• Acids and bases that dissolve completely in water are considered strong

• Weak acids and bases bind reversibly to H+ or OH- and will eventually reach a state of equilibrium


Acids, Bases and pH

• The pH of a solution

– Is determined by the relative concentration of hydrogen ions

– Is low in an acid

– Is high in a base


• pH scale ranges from 0 – 14

– Less than 7 is acidic

• The lower the number the stronger the acid

– 7 is neutral

– Greater than 7 is basic (alkaline)

• The higher the number the stronger the base


Acids, Bases and pH

• The pH scale and pH values of various aqueous solutions

Incr

easi

ngly

Aci

dic

[H+]

> [

OH

–]

Incr

easi

ngly

Bas

ic[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

Figure 3.8


Acids, Bases and pH

• The pH scale is a logarithmic scale

• Therefore, a solution with a pH of 3 isn’t twice as acidic as one with a pH of 6, it is 1000 times more acidic!

If a small change in the number of H+ has such a big change on the pH level, how can

concentrations be maintained?


Acids, Bases and pH

• Buffers

– Are substances that minimize changes in the concentrations of hydrogen and hydroxide ions in a solution

– Consist of an acid-base pair that reversibly combines with hydrogen ions

– Works by accepting H+ from the solution when they are in excess and donating H+ to the solution when they have been depleted

Copyright © 2006 Cynthia Garrard publishing under Canyon Design Chapter 3 – Water Three-quarters...

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Transcript of Copyright © 2006 Cynthia Garrard publishing under Canyon Design Chapter 3 – Water Three-quarters...