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
(wrong) Table salt is dissolved in water
(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 suc
h ? To ensure smooth/even heating, avoid spu
rting of solution in flask To accurately record the temperature of th
e vapour to be condensed, i.e. the boiling point of the distillate
Distillation
Why is the direction of water-in & water-out 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 s
urface 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 f
rom Bunsen flame, i.e. excited Excess energy in metal ions can be released throug
h 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 th
e 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 compo
unds 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 sa
mple Add silver nitrate solution (AgNO3) to sam
ple The appearance of white precipitate (AgC
l) 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 CO3
2-) 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, s
ulphite) 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 spe
cified
Composition of Sea Water
Ppm : parts per million, used to measure very low concentration, e.g. pollutants in air
Chlorine 19 500
Sodium 10 770
Magnesium 1 290
Sulphur 905
Element Content (parts per million, ppm)
Electrolysis of Water
Electro : electricity Lysis : breakown (e.g. hydrolysis) Electrolysis :
Passing electricity through a substance to decompose it
A chemical process
Water hydrogen + oxygenWater hydrogen + oxygenelectricity
Electrolysis of Sea Water
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)
Sea water chlorine gas + hydrogen gas + sodium hydroxide solutionSea water chlorine gas + hydrogen gas + sodium hydroxide solutionelectricity
How are they produced ?
Sea water = sodium chloride in water Sea water (sodium chloride in water(H2O )
Chlorine gas OH & H
Hydrogen gas
Sodium hydroxide solution
Sea water chlorine gas + hydrogen gas + sodium hydroxide solutionSea water chlorine gas + hydrogen gas + sodium hydroxide solutionelectricity
Experimental Setup
hydrogengas
chlorinegas
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
Sea water chlorine gas + hydrogen gas + sodium hydroxide solutionSea water chlorine gas + hydrogen gas + sodium hydroxide solutionelectricity
The Salt Industry
Sodium chlorideSodium chloride
HydrogenHydrogen ChlorineChlorine Sodium hydroxideSodium
hydroxide
Manufacture of
ammonia
Manufacture of
ammonia
As rocket fuel
As rocket fuel
Manufacture of hydrochloric
acid
Manufacture of hydrochloric
acid
Sterilizing drinking
water
Sterilizing drinking
water
Sterilizing swimming
pool water
Sterilizing swimming
pool water
Manufacture of PVC and
organic solvents
Manufacture of PVC and
organic solvents
Manufacture of bleach
Manufacture of bleach
Manufacture of soaps
and detergents
Manufacture of soaps
and detergents
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)