Chapter 3
The ocean
3.1 Composition of sea water
3.2 Extraction of common salt from sea water
3.3 Isolation of pure water from sea water
3.4 Test for the presence of sodium and
chloride in common salt
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3.5 Test for the presence of water in a sample
3.6 Electrolysis of sea water and uses of
products
Key terms
Progress check
Summary
Concept map
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3.1 Composition of sea water
The Earth is sometimes called a ‘water planet’.
70% of the Earth’s surface is covered by water.
About 97% of the water is found in the oceans.
Sea water contains about 3.5% by mass of
dissolved substances.
There are 3.5 g of dissolved substances in
100 g of sea water.
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Most of the dissolved substances in sea water are
salts.
The main one is ‘common salt’ — sodium
chloride.
Figure 3.1 The oceans are an important source of food and salt.
3.1 Composition of sea water
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common salt
(sodium chloride)
68%
magnesium
chloride
14.6%
sodium sulphate
11.4%
calcium chloride
3.1%
other salts
2.9%
Figure 3.2 The percentage by mass of different salts in a typical sea
water sample.
3.1 Composition of sea water
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Solute, solvent and solution
Sea water can also be regarded as a solution.
A solution is a mixture formed when one or more
substances (the solute(s)) dissolves in another
substance (the solvent).
Key point
solution
solute + solvent = __________
3.1 Composition of sea water
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salt
salt solution
water
salt
+ water
(solute) (solvent)
=
salt solution
(solution)
Figure 3.3 A salt solution forms when salt is added to water.
A substance that dissolves in a solvent is said to
be soluble in the solvent.
A substance that does not dissolve in a solvent is
said to be insoluble in the solvent.
3.1 Composition of sea water
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Dilute solution, concentrated solution and saturated
solution
A dilute solution contains a small amount of
solute in a given amount of solvent.
A concentrated solution contains a large
amount of solute in a given amount of solvent.
A saturated solution is a solution in which the
solvent has dissolved the maximum amount of the
solute it can at a particular temperature.
3.1 Composition of sea water
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add more copper(II)
sulphate crystals
dilute copper(II)
sulphate solution
add more copper(II)
sulphate crystals
concentrated copper(II)
sulphate solution
saturated copper(II)
sulphate solution
Figure 3.4 Copper(II) sulphate solution of different concentrations.
Class practice 3.1
3.1 Composition of sea water
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3.2 Extraction of common salt from sea water
Filtration
Insoluble substances can be removed from sea
water by filtration.
Filtration is a method that separates an insoluble
solid from a liquid or a solution.
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glass rod
glass
rod
a mixture of sand
and sea water
filter paper
filter funnel
residue
filter
funnel
a mixture of
sand and
sea water
filtrate
filtrate
Figure 3.5 Experimental set-up for filtration of sea water (with sand).
SBA note
A glass rod is used to guide the flow of sea water
down the filter funnel.
3.2 Extraction of common salt from sea water
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Sea water (with sand) is poured onto a piece of
folded filter paper in a filter funnel.
The filter paper acts as a sieve in filtration.
Insoluble substances, like sand, remain on the
filter paper as residue.
The sea water that passes through the filter paper
is collected as filtrate.
3.2 Extraction of common salt from sea water
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one
layer
fold
three
layers
fold
filter funnel
filter paper
Figure 3.6 A piece of filter paper is folded into a conical shape and placed in
a funnel.
Learning tip
The filter paper is folded in order to increase the
surface area that comes into contact with sea
water. This increases the efficiency of filtration.
3.2 Extraction of common salt from sea water
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Evaporation
Common salt can be separated from the filtered
sea water by evaporation.
Evaporation refers to the change of a liquid to a
vapour at a temperature below its boiling point.
sea water
evaporating dish
wire gauze
evaporating dish
steam
beaker
water
sea
water
tripod
heat
tripod
(a) Direct heating
heat
(b) Heating with a steam bath
Figure 3.7 Experimental set-ups for evaporating sea water.
3.2 Extraction of common salt from sea water
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In school laboratory, we can obtain common salt
from the filtered sea water quickly by heating it to
dryness.
If sea water is heated to dryness as above, the
solid left behind would be:
a mixture of salts containing sodium chloride
(common salt) and other salts like magnesium
chloride.
a powder, not crystals.
3.2 Extraction of common salt from sea water
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Crystallization
To obtain crystals of common salt, the method of
crystallization should be used.
Crystallization is a process of formation of solid
crystals from a saturated solution.
It can separate a solute from a solution.
Slow cooling of hot saturated sea water
Sea water is heated to remove some of the water.
3.2 Extraction of common salt from sea water
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As water boils away, sea water becomes more
and more concentrated, and finally saturated.
When hot saturated sea water cools slowly, the sea
water cannot hold all the dissolved common salt.
Hence some common salt crystals will separate out.
The crystals obtained in this way are smaller in
size.
cool saturated
slow cooling
sea water
hot saturated
sea water
common salt
crystals form
Figure 3.8 Slow cooling of hot saturated sea water.
Think about
3.2 Extraction of common salt from sea water
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Slow evaporation of sea water at room temperature
When sea water is allowed to evaporate slowly at
room temperature, it becomes more and more
concentrated. Eventually, the sea water becomes
saturated.
Further evaporation of the sea water will cause
common salt crystals to separate out.
As evaporation continues, the solid crystals grow
slowly in size. After several days, the common salt
crystals can be filtered from the solution.
3.2 Extraction of common salt from sea water
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The crystals obtained are larger in size.
filter paper
sea
water
more
concentrated
sea water
sea water saturated
with respect to
sodium chloride
sodium chloride
crystals
water slowly evaporates at room temperature
Figure 3.9 Slow evaporation of sea water at room temperature.
Experiment 3.1
Experiment 3.1
Experiment 3.2
3.2 Extraction of common salt from sea water
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Experiment 3.2
Evaporation
(Heating sea
water to
dryness)
Method
•
•
Heating sea
water to
dryness
directly; or
Heating sea
water to
dryness with a
steam-bath
Crystallization
Crystallization
(Slow cooling
(Slow evaporation
of hot
of sea water at
saturated sea room temperature)
water)
•
Heating sea
water until it
becomes
saturated;
followed by
slow cooling
•
Evaporating sea
water slowly at
room temperature
until it becomes
saturated;
followed by
further
evaporation
Table 3.1 Comparisons between methods of evaporation and crystallization
in extracting common salt from sea water.
3.2 Extraction of common salt from sea water
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Crystallization Crystallization
Evaporation
(Slow
(Slow cooling
(Heating sea
evaporation of
of hot
sea water at
water to
saturated sea
room
dryness)
water)
temperature)
Form of
common salt
obtained
Purity of
common salt
obtained
Powder
Small crystals
Large crystals
Low
High
High
Table 3.1 Comparisons between methods of evaporation and crystallization
in extracting common salt from sea water.
Class practice 3.2
STSE connections 3.1
3.2 Extraction of common salt from sea water
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3.3 Isolation of pure water from sea water
Distillation
Using ‘Quickfit’ apparatus
During distillation, the sea water is boiled and the
water changes into water vapour.
Then the hot water vapour is cooled and condenses
into water again.
Figure 3.10 A set of ‘Quickfit’ apparatus.
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thermometer
water out
(to sink)
Liebig condenser
screw-cap
adaptor
clamp
round-bottomed flask
clamp
sea water
receiver adaptor
water in
wire
gauze (from tap)
anti-bumping
granules
tripod
heat
conical flask
(receiver)
pure water
Figure 3.11 Distillation of sea water using ‘Quickfit’ apparatus.
3.3 Isolation of pure water from sea water
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Before heating the sea water, we should add a few
anti-bumping granules to it.
The granules prevent bumping of sea water.
Cold running water is passed into the condenser
(or Liebig condenser) from the lower opening
and leaves from the upper opening.
This provides a better cooling effect for the
water vapour.
3.3 Isolation of pure water from sea water
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The water vapour is passed through the inner tube
of the condenser. It is cooled by the cold running
water flowing around the condenser.
Then the water vapour condenses into pure water
and is collected in the conical flask. The pure
water that distils over is called the distillate.
The solid left behind is called the residue.
The residue is a mixture of salts.
Skill corner 3.1
3.3 Isolation of pure water from sea water
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Using simple apparatus
In this set-up, the water vapour formed is passed
into a test tube, which is placed in a beaker of cold
water.
The water vapour then condenses into pure water.
Key point
Distillation involves boiling of a solution, followed by
condensation of the vapour formed.
______________
3.3 Isolation of pure water from sea water
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thermometer
clamp
boiling tube
delivery tube
receiver test tube
sea water
heat
anti-bumping granules
(to prevent ‘bumping’
water (cooling
of solution)
agent)
end of delivery tube should
be above the distillate
pure water
(distillate)
Figure 3.12 Distillation of sea water using simple apparatus.
Experiment 3.3
Think about
Experiment 3.3
STSE connections 3.2
3.3 Isolation of pure water from sea water
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3.4 Test for the presence of sodium and
chloride in common salt
Test for the presence of sodium in common salt
Some metals and metal compounds, when burnt
or heated strongly, produce a characteristic
coloured flame.
A simple test, called flame test, is based on this
principle.
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Flame test
Moisten a clean platinum wire with concentrated
hydrochloric acid.
platinum wire
concentrated
hydrochloric acid
(a)
Figure 3.13 Procedure for a flame test.
3.4 Test for the presence of sodium and chloride in common salt
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Then dip the wire into a crushed sample (or
solution) of the substance to be tested.
After that, heat the end of the wire strongly in a
non-luminous flame.
non-luminous
flame
flame colour due
to metal ions
sample of
the salt to
be tested
(b)
(c)
Figure 3.13 Procedure for a flame test.
3.4 Test for the presence of sodium and chloride in common salt
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We can identify the metallic element in a compound
by observing the flame colour.
Compound of
potassium
sodium
calcium
copper
Lilac
Golden yellow
Brick-red
Bluish green
Flame
colour
Table 3.2 Flame colours of some metal compounds.
Experiment 3.4
3.4 Test for the presence of sodium and chloride in common salt
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Experiment 3.4
Test for the presence of chloride in common salt
To show that chloride is present in common salt,
we can use the silver nitrate test.
Silver nitrate test
First dissolve the common salt sample in deionized
water.
Then add excess dilute nitric acid to the solution of
the sample.
Afterwards, add silver nitrate solution to the sample.
3.4 Test for the presence of sodium and chloride in common salt
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Observations
The appearance of a white precipitate (silver
chloride which is insoluble in acid) indicates the
presence of chloride in the sample.
add excess dilute
nitric acid, followed by
solution of
common salt
silver nitrate solution
white
precipitate
of silver
chloride
Figure 3.14 Silver nitrate test can be used to show the presence of chloride
in common salt.
3.4 Test for the presence of sodium and chloride in common salt
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SBA note
The use of excess dilute nitric acid is to prevent the
formation of other precipitates (e.g. silver carbonate
and silver sulphite) which are soluble in dilute nitric
acid.
According to the results of flame test and silver
nitrate test on common salt, it contains sodium and
chloride.
Experiment 3.5
Experiment 3.5
3.4 Test for the presence of sodium and chloride in common salt
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Class practice 3.3
3.5 Test for the presence of water in a sample
Test for water using anhydrous copper(II) sulphate
Anhydrous copper(II) sulphate is a white powder.
When a sample containing water is added to
anhydrous copper(II) sulphate, it turns from white
to blue.
anhydrous copper(II) sulphate
Figure 3.15 Water turns white anhydrous
copper(II) sulphate blue.
Learning tip
‘Anhydrous’ means ‘without water’.
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Test for water using dry cobalt(II) chloride paper
A piece of dry cobalt(II) chloride paper is dipped
into a sample containing water.
The paper turns from blue to pink.
dry cobalt(II)
chloride paper
liquid
sample
Figure 3.16 Water turns blue dry cobalt(II) chloride paper pink.
3.5 Test for the presence of water in a sample
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Neither of the above tests can show that a sample
under test is pure water.
To prove that it is pure water, we should carry out
the boiling point test.
If the liquid boils at 100°C (at 1 atmospheric
pressure), it should be pure water.
Experiment 3.6
Concept check
Experiment 3.6
3.5 Test for the presence of water in a sample
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3.6 Electrolysis of sea water and uses of
products
Electrolysis of sea water
Electrolysis means ‘decomposition by
electricity’.
Sea water is a mixture of two compounds —
sodium chloride and water.
Results
Hydrogen gas and chlorine gas form at the
negative electrode and the positive electrode
respectively.
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Finally, the sea water left behind becomes sodium
hydroxide solution.
hydrogen gas
graphite electrode (–)
chlorine gas
sea water
graphite electrode (+)
direction of
electron flow
Figure 3.17 Electrolysis of sea water
3.6 Electrolysis of sea water and uses of products
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Key point
electricity
sea water
hydrogen gas + chlorine gas
sodium hydroxide solution
+ ________________
Uses of the products
Electrolysis of sea water is the basic principle
applied in an industry called chloroalkali industry.
3.6 Electrolysis of sea water and uses of products
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Hydrogen
To make margarine
As rocket fuel
To make ammonia,
which is one of the
raw materials to
produce fertilizers
Table 3.3 Some uses of the products from electrolysis of sea water.
3.6 Electrolysis of sea water and uses of products
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Chlorine
To sterilize
swimming pool
water
To make polyvinyl
chloride (PVC)
To make solvent
such as the thinner
used in correction
fluids
Table 3.3 Some uses of the products from electrolysis of sea water.
3.6 Electrolysis of sea water and uses of products
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Sodium hydroxide
To make soaps
To make drain
cleaner
To neutralize
acidic effluents
from factories
Table 3.3 Some uses of the products from electrolysis of sea water.
3.6 Electrolysis of sea water and uses of products
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Hydrogen and
chlorine
To make
hydrochloric acid
Chlorine and
sodium hydroxide
To make
chlorine bleach
Table 3.3 Some uses of the products from electrolysis of sea water.
3.6 Electrolysis of sea water and uses of products
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Key terms
1.
2.
3.
4.
5.
6.
7.
8.
9.
anti-bumping granule 防漰沸小粒
chloroalkali industry 氯鹼工業
concentrated solution 濃溶液
condenser/Liebig condenser
冷凝器/利氏冷凝器
crystallization 結晶法
dilute solution 稀溶液
distillate 餾液
distillation 蒸餾法
electrolysis 電解
P. 45 / 57
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
evaporation 蒸發
filtrate 濾液
filtration 過濾法
flame test 焰色試驗
residue 殘餘物
saturated solution 飽和溶液
silver nitrate test 硝酸銀試驗
solute 溶質
solution 溶液
solvent 溶劑
Key terms
P. 46 / 57
Progress check
1.
2.
3.
4.
5.
6.
What is the composition of sea water?
What is a solution?
What is a saturated solution?
How can we obtain common salt from sea water?
How can we obtain pure water from sea water?
How can we test for the presence of sodium in
common salt?
7. How can we test for the presence of chloride in
common salt?
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8. How can we test for the presence of water in a
sample?
9. What are the products of electrolysis of sea
water?
10.What are the uses of the products of electrolysis
of sea water?
Progress check
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Summary
3.1 Composition of sea water
1.
Sea water contains about 3.5% by mass of
dissolved substances. The main solute in sea
water is common salt (sodium chloride).
2.
A solution is a mixture formed when one or
more substances (the solute(s)) dissolves in
another substance (the solvent).
3.
A saturated solution is a solution in which the
solvent has dissolved the maximum amount of
solute it can at a particular temperature.
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3.2
Extraction of common salt from sea water
4.
Common salt of low purity can be extracted
from sea water by filtration and then
evaporation.
5.
Common salt of high purity can be extracted
from sea water by filtration and then
crystallization.
3.3
Isolation of pure water from sea water
6.
Pure water can be extracted from sea water by
distillation.
Summary
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7.
The table below summarizes some common
separation methods.
Separation
method
Filtration
Evaporation or
crystallization
Distillation
What are separated?
Separating an
insoluble solid from a
liquid or a solution
Separating a
dissolved solute from
a solution
Separating a solvent
from a solution
Example
Separating sand
from sea water
Extracting
common salt from
sea water
Isolating pure
water from sea
water
Summary
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3.4 Test for the presence of sodium and chloride
in common salt
8.
Some metals in a compound can be identified
by flame test. Potassium: lilac flame colour,
sodium: golden yellow flame colour, calcium:
brick-red flame colour, copper: bluish green
flame colour.
9.
To test for the presence of chloride in common
salt, we add excess dilute nitric acid, followed
by silver nitrate solution to a sample of
common salt solution. A white precipitate forms
if chloride is present.
Summary
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3.5 Test for the presence of water in a sample
10.
Water turns anhydrous copper(II) sulphate from
white to blue. It also turns dry cobalt(II) chloride
paper from blue to pink.
3.6 Electrolysis of sea water and uses of
products
11.
Electrolysis of sea water produces useful
chemicals: hydrogen, chlorine and sodium
hydroxide. Refer to Table 3.3 on p.17 for their
uses.
Summary
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Concept map
Uses:
• to make
margarine
• as rocket fuel
Hydrogen
Uses:
• to sterilize
swimming
pool water
• to make PVC
Uses:
• to make
soaps
• to make drain
cleaner
Chlorine
Sodium
hydroxide
products of electrolysis
SEA WATER
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SEA WATER
evaporation
1. filtration
__________
crystallization
2. _____________
Purer sodium
chloride
Impure sodium
chloride
distillation
_________
Water
Concept map
P. 55 / 57
Purer sodium chloride
contains
Sodium
flame test
Golden yellow
_____________
flame
Chloride
__________
silver
__________
nitrate test
White
precipitate
Concept map
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Water
tested with
Anhydrous
copper(II)
sulphate
which changes
from
White to blue
Dry cobalt(II)
chloride paper
which changes
from
Blue to
pink
_________
Concept map
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