“Water and Aqueous Systems”
“Aqua” Latin = water
Liquid Water and it’s Properties
 OBJECTIVES:
–Describe the hydrogen bonding
that occurs in water.
–Explain the high surface tension
and low vapor pressure of water
in terms of hydrogen bonding.
The Water Molecule
 Water
is a simple tri-atomic molecule.
 Each O-H bond is highly polar,
because of the high electronegativity of
the oxygen
 bond angle = 104,5 o
 due to the angular (bent) shape, the OH bond polarity vectors do not cancel
out. This means water as a whole is
polar.
The Water Molecule
 Water’s
bent shape and ability to
hydrogen bond gives water many
special properties.
 Water molecules are attracted to one
another.
 This gives water: high surface tension,
low vapor pressure, high specific heat,
high heat of vaporization, and high
boiling point
High Surface Tension
 liquid
water acts like it has a skin
–glass of water bulges over the top
 Water forms round drops
–spray water on greasy surface
 All because water hydrogen bonds
Surface Tension
water
molecule
hydrogen bonds
to another.
 Also, hydrogen
bonding occurs to
other molecules
all around.
d-
 One
H
d+
d+
d-
H
d+
d+
Surface Tension
 A water
molecule in
the middle of
solution is
pulled in all
directions.
Surface Tension
 Not
true at the
surface.
 Only pulled down
and to each side.
 Holds the molecules
together.
 Causes surface
tension.
Surface Tension
 Water
drops
are round,
because all
molecules on
the edge are
pulled to the
middle- not to
the air.
Surface Tension
Glass has polar
molecules.
 Glass can
hydrogen bond.
 Attracts the
water molecules.
 Some of the pull
is up a cylinder.

Meniscus
 Water
curves up
along the side of
glass.
 This makes the
meniscus, as in a
graduated cylinder
 Plastics are nonwetting; no attraction
Meniscus
In Glass
In Plastic
Surface tension
 All
liquids have surface tension
–water is higher than most others
 How to decrease surface tension?
–Use a surfactant - surface active
agent
–a wetting agent, like detergent or
soap
–interferes with hydrogen bonding
Low vapour pressure
 Hydrogen
bonding also explains
water’s unusually low vapor
pressure.
–Holds water molecules together,
so they do not escape
–good thing- lakes and oceans
would evaporate very quickly
Specific Heat Capacity
Water has a high heat capacity (also
called specific heat).
 It absorbs 4.18 J/gºC, while iron absorbs
only 0.447 J/gºC.

heat energy
mass x DT
 If we calculate the heat need to raise the
temperature of both iron and water by 75ºC water is almost 10 x more!

Remember: SHC =
Water Vapour and Ice
 OBJECTIVES:
–Account for the high heat of
vaporization and the high boiling
point of water, in terms of
hydrogen bonding.
Water Vapor and Ice
 OBJECTIVES:
–Explain why ice floats in water.
Evaporation and Condensation
Because of the strong hydrogen bonds, it
takes a large amount of energy to
change water from a liquid to a vapour.
 2260 J/g is the heat of vaporization.
– This much energy to boil 1 gram water
 You get this much energy back when it
condenses.
 Steam burns, but heats things well.

Ice
Most liquids contract (get smaller) as
they are cooled.
 They get more dense.
 When they change to solid, they are
more dense than the liquid.
 Solid metals sink in liquid metal.
– But, ice floats in water.
 Why?

Ice
 Water
becomes more dense as it
cools until it reaches 4ºC.
 Then it becomes less dense.
 As the molecules slow down, they
arrange themselves into
honeycomb shaped crystals.
 These are held together by
hydrogen bonds.
Liquid
Solid
O
Ice
 10%
greater volume than water.
 Water freezes from the top down.
–The layer of ice on a pond acts
as an insulator for water below
 It takes a great deal of energy to
turn solid water to liquid water.
 Heat of fusion is: 334 J/g.
Aqueous Solutions
 OBJECTIVES:
–Explain the significance of the
statement “like dissolves like”.
Aqueous Solutions
 OBJECTIVES:
–Distinguish among strong
electrolytes, weak electrolytes,
and non-electrolytes, giving
examples of each.
Solvents and Solutes
 Solution
- a homogenous mixture,
that is mixed molecule by molecule.
 Solvent - the dissolving medium
 Solute -the dissolved particles
 Aqueous solution- a solution with
water as the solvent.
 Particle size about 1 nm; cannot be
separated by filtration
Aqueous Solutions
Water dissolves ionic compounds and
polar covalent molecules best.
 The rule is: “like dissolves like”
 Polar dissolves polar.
 Non-polar dissolves non-polar.
 Oil is non-polar.
– Oil and water don’t mix.
 Salt is ionic- makes salt water.

How Ionic solids dissolve
 Called
solvation.
 Water breaks the + and - charged
pieces apart and surrounds them.
 In some ionic compounds, the
attraction between ions is greater
than the attraction exerted by water
– Barium sulphate and calcium
carbonate
How Ionic solids dissolve
H
H
H
H
H
Solids will dissolve if the attractive force
of the water molecules is stronger than
the attractive force of the crystal.
 If not, the solids are insoluble.
 Water doesn’t dissolve non-polar
molecules because the water
molecules can’t hold onto them.
 The water molecules hold onto each
other, and separate from the non-polar
molecules.
 Non-polars? No repulsion between
them

Electrolytes and Non-electrolytes
 Electrolytes-
compounds that
conduct an electric current in
aqueous solution, or in the molten
state
–all ionic compounds are
electrolytes (they are also salts)
 barium sulphate- will conduct when
molten, but is insoluble in water!
Electrolytes and Non-electrolytes
 Do
not conduct? Non-electrolytes.
–Many molecular materials,
because they do not have ions
 Not all electrolytes conduct to the
same degree
–there are weak electrolytes, and
strong electrolytes
–depends on: degree of ionization
Electrolyte Summary
Substances that conduct electricity
when dissolved in water, or molten.
 Must have charged particles that can
move.
 Ionic compounds break into charged
ions:
NaCl Na+ and Cl These ions can conduct electricity.

Non-electrolytes do not conduct
electricity when dissolved in water or
molten
 Polar covalent molecules such as
methanol (CH3OH) don’t fall apart into
ions when they dissolve.
 Weak electrolytes don’t fall completely
apart into ions.
 Strong electrolytes do ionise
completely.

Water of Hydration
(or Water of Crystallization)

Water molecules chemically bonded to solid
salt molecules (not in solution)
These compounds have fixed amounts of
water.
The water can be driven off by heating:

CuSO4 5H2O

Called copper(II)sulphate
- heat pentahydrate.


.
CuSO4 + 5H2O
+ heat
Hydrates
 Since
heat can drive off the water,
the forces holding it are weak
 If a hydrate has a vapour pressure
higher than that of water vapor in
air, the hydrate will effloresce by
losing the water of hydration
Hydrates
 Some
hydrates that have a low
vapour pressure remove water from
the air to form higher hydratescalled hygroscopic
–used as drying agents, or
dessicants
–packaged with products to absorb
moisture
Hydrates
 Some
compounds are so hygroscopic,
they become wet when exposed to
normally moist air- called deliquescent
–remove sufficient water to dissolve
completely and form solutions
Heterogeneous Aqueous Systems
 OBJECTIVES:
–Explain how colloids and
suspensions differ from solutions.
Heterogeneous Aqueous Systems
 OBJECTIVES:
–Describe the Tyndall effect.
Mixtures that are NOT
Solutions
 Suspensions:
mixtures that slowly settle
upon standing.
–Particles of a suspension are greater
in diameter than 100 nm.
–Can be separated by filtering
 Colloids: heterogeneous mixtures with
particles between size of suspensions
and true solutions (1-100 nm)
Mixtures that are NOT
Solutions
 The
small particles are the dispersed
phase, and are spread throughout the
dispersion medium
 The first colloids were glues. Others
include mixtures such as gelatin, paint,
aerosol sprays, and smoke
Mixtures that are NOT
Solutions
 Many
colloids are cloudy or milky in
appearance when concentrated, but
almost clear when dilute
–do not settle out
–cannot be filtered out
 Colloids exhibit the Tyndall effect- the
scattering of visible light in all directions.
–suspensions also show Tyndall effect
Mixtures that are NOT
Solutions
 Flashes
of light are seen when colloids
are studied under a microscope- light is
reflecting- called Brownian motion to
describe the chaotic movement of the
particles
Mixtures that are NOT
Solutions
 Emulsions-
colloids dispersions of
liquids in liquids
–an emulsifying agent is essential for
maintaining stability
–oil and water not soluble; but with
soap or detergent, they will be.
 Oil and vinegar dressing?
–Mayonnaise? Margarine?