Sea Water : A Vast Solution





What is the meaning for each of the
following term ?
Solvent
Solute
Solution
Solute + solvent  solution
Solubility




Soluble
Insoluble
Table salt is melted in water
Table salt is dissolved in water
(wrong)
(correct)
Concentration







What is concentration ?
How to express concentration ?
Amount of solute / amount of solution
Usually g cm-3
Dilute
Concentrated
Saturated
Filtration
Filtration : Labels


Two classes of labels
Apparatus




Glass rod
Filter funnel
Filter paper (must be shown)
Chemicals



Mixture
Residue
Filtrate
Filtration : Questions




What is the function of glass rod ?
What if it is not used ?
To guide the flow of the mixture solution
If it is not used, direct flow of mixture
solution may be so strong to ruin/damage
the filter paper, and solution may just pass
through funnel without being filtered
Evaporation
Evaporation : Label

Apparatus





Evaporating dish
Wire gauze (must be shown)
Tripod
Bunsen burner (not bunsen burner)
Chemicals

solution
Crystallization
Crystallization




Why must the solution be saturated before
crystallization occurs ?
What is the relationship between size of
crystal & time of crystallization ?
If the solution was not yet saturated, the
solute would have been dissolved (not
melted) in the solution
The longer the time, the larger the crystals
Crystallization




Why is there a relationship between size of
crystal & time of crystallization?
Particles in crystals are arranged in a regular
pattern.
If particles are to come out from solution to be
solidified in a short time, they don’t have time to
arrange regularly, there will only be small
crystals
The longer the time allowed, the higher the
chance for particles to arrange themselves
regularly, larger crystals result
Distillation
Distillation : Label

Apparatus








Heat
Thermometer
Distillation flask
Anti-bumping granule
Condenser
Water-in, water-out (must be shown)
Conical flask
Chemicals


Mixture (sea water)
Distillate (pure water)
Distillation




Why is anti-bumping granules required ?
Why is the thermometer positioned as
such ?
To ensure smooth/even heating, avoid
spurting of solution in flask
To accurately record the temperature of
the vapour to be condensed, i.e. the
boiling point of the distillate
Distillation


Why is the direction of water-in & waterout as such ?
To ensure the condenser is completely
filled with water at all time, such that there
is constant contact between the cooling
agent and the vapour in condenser for
effective cooling.
Distillation : Simple Set up
Distillation : Simple Set up




What is the function of the ice-water bath?
Why is the delivery tube above the liquid
surface of the distillate?
To condense the vapour coming out
To avoid sucking back
Sucking Back




As T , P 
During heating, if there is a T (due to removing
of heating/heating becomes weaker), P
If delivery tube was in the distillate, P leads to
atmospheric P to push cold distillate to be
sucked back to hot test tube  crack test tube
 dangerous
If delivery tube is not in distillate, P leads to
atmospheric P to push only air to be sucked
back to hot test tube  safe
Flame Test
Principle





Conc. HCl converts metal compounds to metal
chlorides
Metal chlorides can be vapourized by Bunsen
flame
Metal chlorides in gas phase can absorbed
energy from Bunsen flame, i.e. excited
Excess energy in metal ions can be released
through emission of light at different frequencies
(i.e. different colour)
The colour observed can be used to identify
metal
Flame Test



How to tell the nichrome wire is clean?
A clean wire will show no colour (except
the background blue colour) in the flame
It is necessary to dip the wire in conc. HCl
and heat in Bunsen flame for a few times,
until no characteristic flame colour can be
observed
Flame Test


What is the function of conc. HCl?
Conc. HCl can convert most metal
compounds to metal chlorides
Metal chlorides are relatively volatile (i.e.
low b.p.) and can be vaporized easily by
the heat of Bunsen flame
Flame Test Result





potassium : purple/lilac
sodium : golden yellow
calcium : brick-red
copper : bluish-green
Must be specifically stated!
Test for Chlorides




Dissolve (not melt) solid sample in water
Add excess dilute nitric acid (HNO3) to
sample
Add silver nitrate solution (AgNO3) to
sample
The appearance of white precipitate (AgCl)
indicates the presence of chloride ion (Cl-)
in sample
Chloride Test : Principle


Chloride ions (Cl-) from sample reacts with
silver ions (Ag+) from silver nitrate solution
to form silver chloride (AgCl)
Silver chloride (AgCl) is insoluble in water
and it is observed as white precipitate
Question




Why is nitric acid (HNO3) added before adding
silver nitrate solution (AgNO3) ?
To eliminate other interfering ions which also
form white precipitate with silver nitrate solution
Some ions (like sulphite SO32- & carbonate
CO32-) also form white precipitate with AgNO3
Unknown + AgNO3  white precipitate
Unknown can be sulphite, carbonate, chloride 
test not conclusive
Question


Sulphite & carbonate ions react with and
are therefore removed by nitric acid
(details later)
Unknown + HNO3 then + AgNO3
 white precipitate
Unknown must be chloride  test
conclusive
Question


Why is nitric acid added to be in excess ?
To ensure all interfering ions (carbonate,
sulphite) to be completely removed in the
sample, otherwise the test is still not
conclusive
Test for Water



Test paper : cobalt(II) chloride paper
Positive result (when H2O is present) :
from blue to pink
Both initial color & final colour must be
specified
Composition of Sea Water

Ppm : parts per million, used to measure very
low concentration, e.g. pollutants in air
Element
Content (parts per million, ppm)
Chlorine
19 500
Sodium
10 770
Magnesium
1 290
Sulphur
905
Electrolysis of Water




Electro : electricity
Lysis : breakown (e.g. hydrolysis)
Electrolysis :
Passing electricity through a substance to
decompose it
A chemical process
electricity
Water
hydrogen + oxygen
Electrolysis of Sea Water
electricity
Sea water


chlorine gas + hydrogen gas + sodium hydroxide solution
Sea water mainly contains sodium
chloride dissolved in water
When sea water is electrolyzed,
Water is H2O, it is decomposed to
hydrogen (H) & hydroxide (OH)
How are they produced ?


Sea water = sodium chloride in water
Sea water (sodium chloride in water(H2O )
Chlorine gas
OH & H
Sodium hydroxide solution
Hydrogen gas
electricity
Sea water
chlorine gas + hydrogen gas + sodium hydroxide solution
Experimental Setup
hydrogen
gas
chlorine
gas
sea
water
carbon
electrode
(-)
carbon
electrode
(+)
Ions in Sea Water



In sea water, there are ions (charged
species) which can conduct electricity
Positive ions: sodium ions & hydrogen
ions
Negative ions: chloride ions & hydroxide
ions
At the positive electrode



Both hydroxide ions & chloride ions are
attracted due to +ve/-ve attraction
Chloride ions react to give chlorine gas
(observed in experiment)
Hydroxide ions do not react and stay in
solution
At the negative electrode



Both sodium ions & hydrogen ions are
attracted due to +ve/-ve attraction
Hydrogen ions react to give hydrogen gas
(observed in experiment)
Sodium ions do not react and stay in
solution
Products in electrolysis



At positive electrode, chlorine gas is
produced
At negative electrode, hydrogen gas is
produced
In solution, sodium ions & hydroxide ions
are present in water, i.e. sodium hydroxide
solution
electricity
Sea water
chlorine gas + hydrogen gas + sodium hydroxide solution
The Salt Industry
Sodium
chloride
Hydrogen
Manufacture of
hydrochloric acid
Manufacture
of ammonia
Sodium
hydroxide
Chlorine
As rocket
fuel
Manufacture
of bleach
Manufacture
of soaps and
detergents
Sterilizing
drinking
water
Sterilizing
swimming
pool water
Manufacture
of PVC and
organic
solvents
Physical & Chemical Changes




Physical change is a change that do not involve
change in composition of the substance, i.e. no
new substances are formed during the change
Usually involved small amount of energy
Chemical change is a change in which the
composition of the substance has changed, i.e.
new substances are formed
Usually involved much larger amount of energy
(in the form of heat, light or electricity)