Chapter 05 Water We Drink

Chapter 5:

Kevin Carter’s 1994 Pulitzer prize

winning photo of a vulture waiting for a child to die, so that it will eat it

epitomizes not only the hunger crises in Sudan but also in the whole of

Africa. (Photo source: Pulitzer)

“Water has never lost its mystery. After at least two and a half millennia of philosophical and scientific inquiry, the most vital of the world’s substances remains

surrounded by deep uncertainties. Without too much poetic license, we can reduce these questions to a single bare essential: What exactly is water?”

Philip Ball, in Life’s Matrix: A Biography of Water,

University of California Press, Berkeley, CA, 2001, p. 115

Do you know where your drinking water comes from?

Do you know where your drinking water comes from?

Do you know if your drinking water is safe to drink?

Do you know if your drinking water is safe to drink?

How would you know?

While normally free of pollutants, groundwater can be contaminated by a number of sources:

Abandoned mines _{Run off from fertilized fields}

Household chemicals poured down the drain or on the ground.

http://catharsiscorner.wordpress.com/2009/01/26/peta-air-tanah-dunia-sumber-kesejahteraan-dan-potensi-konflik/

The average water use in the world:

• _{70 % for agricultural needs,} • 8 % for domestic needs and

• 22 % for industry.

The average water use in the world:

• _{70 % for agricultural needs,} • _{8 % for domestic needs and} • _{22 % for industry.}

• Afghanistan and India >95% of water use for agriculture, • Britain and Canada > 70% for industry.

• Japan, Indonesia and Brazil 60% of water use for agriculture,

• the Americans use the 42 per cent for agriculture and 46 percent for industrial use.

5.3

Solution

• _{A solution is a homogeneous mixture of uniform}

composition.

• _{Solutions are made up of solvents and solutes.}

– _{Solvent = Substances capable of dissolving other}

substances- usually present in the greater amount.

– _{Solutes = Substances dissolved in a solvent- usually}

present in the lesser amount.

The

importance of water as a solvent in our bodies

The

Water in the Environment

5.4

Concentration Terms

Parts per hundred (percent)

Parts per million (ppm)

Parts per billion (ppb)

20 g of NaCl in 80 g of water is a 20% NaCl solution

2 ppb Hg_ 2 g Hg 1109 g H2O

 210

-6 _{g Hg}

1103 g H2O

_{1 L H}2 g Hg

Molarity (M) = moles solute

liter of solution

[ ] = “concentration of”

5.4 1.0 M NaCl solution

[NaCl] = 1.0 M = 1.0 mol NaCl/L solution Also – this solution is 1.0 M in Na+ _{and 1.0}

M in Cl

What is the concentration (

in M and mass

%)

of the resulting solution when you add 5

grams of NaOH to 95 mL of water?

What is the concentration (

in M and mass

%)

of the resulting solution when you add 5

grams of NaOH to 95 mL of water?

95 mL H₂O = 95 g H₂O mass % : 5 g NaOH/100 g solution

95 mL H₂O = .095 L = 5% NaOH

5 g NaOH = 0.125 moles NaOH 0.125 mole NaOH/0.095 L

= 1.3 M solution of NaOH

95 mL H₂O = 95 g H₂O mass % : 5 g NaOH/100 g solution

95 mL H₂O = .095 L = 5% NaOH

5 g NaOH = 0.125 moles NaOH 0.125 mole NaOH/0.095 L

= 1.3 M solution of NaOH

5.4

5.4

What is the molarity of glucose (C

₆

H

₁₂

O

₆

) in a

solution containing 126 mg glucose per 100.0 mL

solution?

What is the molarity of glucose (C

₆

H

₁₂

O

₆

) in a

solution containing 126 mg glucose per 100.0 mL

solution?

6.99 x 10

-3

M

5.4

How to prepare a 1.00 M NaCl solution:

How to prepare a 1.00 M NaCl solution:

Note- you do NOT add 58.5 g NaCl to 1.00 L of water.

The 58.5 g will take up some volume, resulting in slightly more than 1.00 L of solution- and the

molarity would be lower. mol solute

L of solution

5.5

Different Representations of Water

Different Representations of Water

Lewis structures Space-filling Charge-

density

Charge-density

Region of partial negative charge

5.5

EN Values assigned by Linus Pauling, winner of TWO Nobel Prizes.

EN Values assigned by Linus Pauling, winner of TWO Nobel Prizes.

Electronegativity

is a measure of an atom’s

attraction for the electrons it shares in a covalent

bond.

Electronegativity

is a measure of an atom’s

attraction for the electrons it shares in a covalent

bond.

5.5

H H

O

A difference in the

electronegativities of the atoms in a bond creates a polar bond.

A difference in the

electronegativities of the atoms in a bond creates a polar bond.

Partial charges result from bond polarization.

Partial charges result from bond polarization.

A polar covalent bond is a covalent bond in which the electrons are not equally

shared, but rather displaced toward the more

electronegative atom.

A polar covalent bond is a covalent bond in which the electrons are not equally

shared, but rather displaced toward the more

5.5

H

H

H₂ has a non-polar covalent bond.

NaCl

NaCl has an ionic bond-look at the

EN difference.

Na = 1.0 Cl = 2.9 DEN = 1.9

A water molecule is polar – due to polar covalent bonds and the

shape of the molecule.

A water molecule is polar – due to polar covalent bonds and the

5.6 Polarized bonds

allow hydrogen bonding to occur.

H–bonds are intermolecular bonds. Covalent bonds are

intramolecular bonds.

H–bonds are intermolecular bonds. Covalent bonds are

intramolecular bonds.

A hydrogen bond is an electrostatic attraction between an atom bearing a partial positive charge in one molecule and an atom bearing a partial negative charge in a neighboring molecule. The H atom must be bonded to an O, N, or F atom.

Hydrogen bonds typically are only about one-fifteenth as strong as the covalent bonds that connect atoms together

within molecules.

A hydrogen bond is an electrostatic attraction between an atom bearing a partial positive charge in one molecule and an atom bearing a partial negative charge in a neighboring molecule. The H atom must be bonded to an O, N, or F atom.

Hydrogen bonds typically are only about one-fifteenth as strong as the covalent bonds that connect atoms together

+ 1 e

-Na _Na

Na atom Na+ ion Forming ions

Forming ions

+ 1 e

-Cl atom

Cl- ion Cl

Cl

5.7

When

ions

(charged particles) are in aqueous

solutions, the solutions are able to conduct

electricity.

(a) Pure distilled water (non-conducting)

5.7

Substances that will dissociate in solution are called

electrolytes.

Dissolution of NaCl in Water

The polar water molecules stabilize the ions as they break apart (dissociate).

The polar water molecules stabilize the ions as they break apart (dissociate).

Ions are simply charged particles-atoms or groups of atoms.

They may be positively charged – cations.

Or negatively charged- anions.

Ions are simply charged particles-atoms or groups of atoms.

They may be positively charged – cations.

Or negatively charged-

anions.

Some atoms form more than one stable ion

Naming simple ionic compounds is easy -Name the metallic element (cation) first, followed by the non-metallic element (the anion) second, but with an –ide suffix.

5.7 MgO Mg is the metal, O is the non-metal

magnesium oxide

NaBr

5.7

Ions that are themselves made up of more than one atom or element are called polyatomic

ions.

Ions that are themselves made up of more than one atom or element are called polyatomic

ions.

NaSO₄ (sodium sulfate) dissociates in water to form:

Na+

Sodium ions

and

Sulfate ions

Naming polyatomic ionic compounds is also easy -Name the cation first, followed by the anion second.

5.7 MgOH Mg+ is the cation, OH- is the anion

magnesium hydroxide

NH₄Br NH₄+_{is the anion, Br}- _{is the anion}

5.8

Simple generalizations about ionic

compounds allow us to predict their water

solubility.

Ions Solubility of Compounds Solubility Exceptions Examples

sodium, potassium,

and ammonium All soluble None NaNO_{KBr is soluble}3 is soluble

nitrates All soluble None LiNO₃ is soluble

Mg(NO₃)₂ is soluble chlorides Most soluble Silver, some mercury, and

lead chlorides MgCl_PbCl 2 is soluble

2 is insoluble

sulfates Most soluble Strontium, barium, and lead

sulfate K_BaSO2SO4 is soluble

4 is insoluble

carbonates Mostly insoluble* Group IA and NH₄1

carbonates are soluble

Na₂CO₃ is soluble CaCO₃ is insoluble

hydroxides and

sulfides Mostly insoluble* Group IA and NH4

1

hydroxides and sulfides are soluble

KOH is soluble Al(OH)₃ is insoluble

5.9

Covalent molecules in solution

A sucrose molecule – when dissolved in water, sugar molecules interact with and become

surrounded by water molecules, but the sucrose molecules do not dissociate like ionic

compounds do; covalent molecules remain intact when dissolved in solution.

Like dissolves like

5.10

Maximum Contaminant Level Goal (MCLG) and Maximum Contaminant Level (MCL)

A pipe with hard-water scale build up

Hard water contains high concentrations of dissolved calcium and magnesium ions.

Soft water contains few of these dissolved ions.

Because calcium ions, Ca2+, are generally the largest

contributors to hard water, hardness is usually expressed in parts per million of calcium carbonate (CaCO₃) by mass.

It specifies the mass of solid CaCO₃ that could be formed from the Ca2+ in solution, provided sufficient CO

32- ions were

also present:

Ca2+_{(aq) + CO}

32–(aq)  CaCO3(s)

A hardness of 10 ppm indicates that 10 mg of CaCO₃ could be formed from the Ca2+ _{ions present in 1 L of water.}

5.11

5.12

Getting the lead out:

Schematic of a typical spectrophotometer

Using a plot of

absorbance vs. concentration

AAS = Atomic

5.14

5.14

Two water purification techniques: