Sea Water : A Vast Solution

1
Sea Water A Vast Solution

• What is the meaning for each of the following
  term ?
• Solvent
• Solute
• Solution
• Solute solvent ? solution

2
Solubility

• Soluble
• Insoluble
• Table salt is melted in water (wrong)
• Table salt is dissolved in water (correct)

3
Concentration

• What is concentration ?
• How to express concentration ?
• Amount of solute / amount of solution
• Usually g cm-3
• Dilute
• Concentrated
• Saturated

4
Filtration
5
Filtration Labels

• Two classes of labels
• Apparatus
• Glass rod
• Filter funnel
• Filter paper (must be shown)
• Chemicals
• Mixture
• Residue
• Filtrate

6
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

7
Evaporation
8
Evaporation Label

• Apparatus
• Evaporating dish
• Wire gauze (must be shown)
• Tripod
• Bunsen burner (not bunsen burner)
• Chemicals
• solution

9
Crystallization
10
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

11
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 dont 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

12
Distillation
13
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)

14
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

15
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.

16
Distillation Simple Set up
17
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

18
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

19
Flame Test
20
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

21
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

22
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

23
Flame Test Result

• potassium purple/lilac
• sodium golden yellow
• calcium brick-red
• copper bluish-green
• Must be specifically stated!

24
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

25
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

26
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

27
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

28
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

29
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

30
Composition of Sea Water

• Ppm parts per million, used to measure very low
  concentration, e.g. pollutants in air

31
Electrolysis of Water

• Electro electricity
• Lysis breakown (e.g. hydrolysis)
• Electrolysis
• Passing electricity through a substance to
  decompose it
• A chemical process

32
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)

33
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
34
Experimental Setup
35
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

36
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

37
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

38
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

39
The Salt Industry
40
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)