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)