Dangerous Chemicalshttp://www.dhmo.org/facts.html

Chapter 3Water & the Fitness of

the Environment

Water supports all life!

Properties of Water¾ of the Earth is covered in waterComposition of cells = 70-95% water3 billion years before life on landAll life on Earth requires water

Brainstorming What are the properties of water that

make is so suitable for supporting life?

Polarity of water molecules results in hydrogen bonding

Water molecule=polar moleculeWater molecules form hydrogen bondsThese H-bonds are what gives water its unique

properties

LE 3-2

What are the 4 emergent properties of water that contribute to Earth’s fitness for life?

I. Cohesion; adhesion; surface tension II. High specific heat III. Expansion upon freezing IV. Versatility as a solvent

CohesionCohesion: Water molecules stick togetherAdhesion: Water molecules stick to other

substances

LE 3-3

Water-conducting cells

100 µm

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

Surface tension is related to cohesion

Moderation of TemperatureH2O absorbs heat from warmer air H2O releases heat from cooler airWater absorbs or releases a large amount

of heat with only a slight change in its own temperature

Heat and TemperatureHeat- measure of the total amount of kinetic

energy due to molecular motionTemperature measures intensity of heat due to

the average kinetic energy of molecules

Which has more heat?

An ocean (30°C) vs Blue whale (38°C)

A boiling kettle of water (100°C) vs an Iceberg (-20°C)

Water’s High Specific HeatThe specific heat of a substance is the

amount of heat that must be absorbed or lost for 1 g of that substance to change its temperature by 1ºC

Specific heat of water – 1 cal/g/°C

Water has an unusually high specific heat◦Heat is absorbed when H-bonds break◦Heat is released when H-bonds form

Brainstorm

Take a few minutes to write a list of reasons for why the specific heat of water is important for biological processes. Be ready to explain your answer

Evaporative CoolingEvaporation- liquid to gasHeat of vaporization- heat a liquid must

absorb for 1 g to be converted to gasLiquid evaporates Remaining liquid

becomes coolerEvaporative cooling…

◦Stabilizes temperature of bodies of water◦Helps organisms maintain body heat

Insulation of Bodies of Water by Floating IceFrozen water is less dense than liquid

water (it floats)◦As the temperature of water decreases, h-

bonds are not broken as fast More hydrogen bonds greater spacing The density of ice < liquid water

What would happen if ice sank?

Why is Water the Solvent of Life?Solution-liquid homogeneous mix of

substancesSolvent- dissolving agent of a solutionSolute- substance dissolvedWater is a versatile solvent due to its

polarityAn aqueous solution is one in which water

is the solvent

Water forms hydrogen bonds with ionic solutes

Hydration shell

Water can also dissolve:Polar moleculesSome proteins

(a) Lysozyme molecule

in a nonaqueous environment.(b) – (c) Lysozyme molecule in an aqueous environment.

What substances will NOT dissolve in water?Hydrophobic Substances

Hydrophobic substances are nonpolarThey do not dissolve in water (which is polar)

Hydrophilic substances are polarThey easily dissolve in water

“Like dissolves like”

Solute Concentration in Aqueous SolutionsChemical reactions depend on collisions of

molecules and therefore on the concentration of solutes in an aqueous solution.

What is a mole?

ConcentrationMolarity (M)- # of moles of solute per L of solution

Molecular mass - sum of all masses of all atoms in a molecule

Avogadro’s Number – the number of atoms/molecules per mole.◦6.02 x 10^23

How would you make 1 liter of a 0.5 M solution of NaCl?How many moles of NaCl do you need?What is the molecular mass of NaCl?

◦Na - ~23 g/mol◦Cl - ~35.5 g/mol

How many grams of NaCl do you need?

Extra PracticeHow many molecules are in 60 grams of

NaCl?Determine the molarity of the solution:

100 g KCl dissolved into 0.75 L of solution.How many grams of KCl are necessary to

get 0.5 L of a 0.10M solution?

Water acts as both an acid and a base. Water dissociates to form H+ (or H3O+)

and OH- ions.At equilibrium – 10^-7M Hydronium (H3O+) and hydroxide (OH-)

ions are extremely reactive.

Water is both an acid and a base.Bronsted Acid – A substance that donates

a proton in an aqueous solution.Bronsted Base – Proton acceptor.Amphoteric – A substance that can act as

either an acid or a base.Amphiprotic – A substance that can either

donate or accept a proton.

Concentrations of acids and bases.Concentrations of H+ and OH- are equal in

pure waterProduct is constant: [H+][OH-] = 10^-14Dissolving acids in aqueous solutions

increases the concentration of H+ ions.Bases [OH-] > [H+]

◦NaHO Na+ and OH-◦NH3 NH4+ and OH-

PracticeWhat is [H+] if [OH-] = 10^-8M?

◦[H+][OH-] = 10^-14◦10^-14/[OH-] = [H+]◦10^-14/10^-8 = [H+] = 10^-6M

pH is a measure of aciditypH + pOH = 14pH = -log [H+]

◦Acid- pH < 7◦Base- pH > 7

pOH = -log [OH-]◦Acid – pOH > 7◦Base – pOH < 7

pH practice What is the pH of a 0.10M solution of

NaOH?◦NaOH Na+ + OH-; determine the ratio of

moles.◦pOH = -log [OH-] ◦pH = 14 - pOH

pH practiceWhat is the concentration of a solution of

HCl if the pH = 3?

What makes an acid strong?Strong acids/bases dissociate completelyWeak acids/bases dissociate until

equilibrium is reachedExamples:

◦Strong base – NaOH◦Weak base – NH3

LE 3-8

pH Scale0

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

–]

Effect of pH on living cellsMost biological liquids – 6 ≤ pH ≤ 8Internal pH of most cells ~7

Brainstorming: What would happen to a cell if the internal pH was decreased by 1? Why?

BuffersThe internal pH of most living cells must

remain close to pH 7Buffers help to minimize changes in

concentrations of H+ & OH- in a solutionBuffer: a solution composed of a weak

acid and its conjugate base (or a weak base and its conjugate acid)

The Threat of Acid PrecipitationAcid precipitation (ppt)- rain, snow, or fog

w/ a pH < 5.6; caused by mixing pollutants w/ water in the air

Acid precipitation can damage life in lakes and streams

Effects of acid precipitation on soil chemistry are contributing to the decline of some forests

How do buffers work?NH3(g)+H2O(l)→NH4+(aq)+OH−(aq)NH3 = weak baseNH4+ = conjugate acidWhen an acid is added to a buffer

solution, the buffer is ready to accept protons (H+).

When a base is added, the buffer is ready to donate protons.

Think about it…

Why can’t buffers be made with strong acids/bases?

Le Chatelier’s PricipleFor a system that is at equilibrium,

changes in (concentration, temperature, pressure, volume) will cause the reaction to shift either forward or toward the reverse reaction in order to counteract the change.

LE 3-9

0123456789

1011121314

Morebasic

Normalrain

Moreacidic

Acidrain

Exam practice Buffers are substances that help resist

shifts in pH by A) releasing H+ in acidic solutions. B) donating H+ to a solution when they

have been depleted. C) releasing OH- in basic solutions. D) accepting H+ when the are in excess. E) both B and D

Exam practiceBuffers are substances that minimize

changes in the concentrations of H+ and OH- ions in a solution

The answer is E

Exam practiceWhich of the following is responsible for the

cohesive property of water?◦A. H-bonds b/w the oxygen atoms of 2 adjacent

water molecules◦B. covalent bonds b/w Hydrogen atoms of 2 adjacent

water molecules◦C. H-bonds b/w the oxygen atom of 1 water molecule

and the hydrogen atom of another◦D. covalent bonds b/w O atom of 1 water molecule &

H atom of another◦E. H-bonds b/w water molecules and other types of

molecules

Exam practiceThe cohesive property of water is due to

the hydrogen bonding between water molecules, specifically the hydrogen bonds b/w the oxygen atom of one water molecule and the hydrogen atom of another water molecule

The answer is C

Your pH investigation lab outcome should look like this