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  Salt: sodium chloride, extraction, uses of halogen
and halogen compounds
See also Group 7 The HALOGEN elements
of the Periodic Table - their physical properties and chemical reactions
Doc Brown's Chemistry KS4 science GCSE/IGCSE/O level Chemistry Revision Notes
Sub-index for this
page
1.
Sources
of sodium chloride salt - extracting it from natural resources
2. The
industrial electrolysis of sodium chloride solution (brine)
3.
The uses of halogens
and their compounds
4.
The Use of Salt in
the Food Industry and Health Issues
Salt deposits and sea water are
valuable sources of sodium chloride, from which many useful products are made in
the chlor-alkali chemical industry.
Sodium chloride solution (brine) is readily electrolysed to produce three
products directly - chlorine gas, hydrogen gas and sodium hydroxide solution
which can be converted to many other useful industrial and domestic consumer
products.
Revision notes on the
uses of fluorine, chlorine, iodine, extraction of salt, uses of salt sodium
chloride, help when revising for AQA
GCSE chemistry, Edexcel GCSE chemistry, OCR GCSE gateway science chemistry,
OCR GCSE 21st century science chemistry GCSE 9-1 chemistry examination
questions on the uses of halogens and halogen compounds.
Doc Brown's
chemistry revision notes: basic school chemistry science GCSE chemistry, IGCSE chemistry, O level
& ~US grades 8, 9 and 10 school science courses or equivalent for ~14-16 year old
science students for national examinations in chemistry
1. Sources
of sodium chloride salt - extracting it from natural resources
 In hot countries seawater is
evaporated in large open air tanks. As the water evaporates the salt crystals
form and be collected up. Seawater is allowed to flow into shallow pools and the
water left to evaporate. More seawater can be let in and the water evaporated
from the heat of the Sun's rays until a good layer of salt has built up, which
can be quite pure.
In the UK there are huge underground
deposits from dried up seas from hundreds of thousands or millions of years ago.
Rock salt is mined in large quantities from under Cheshire and formed the basis
of the Chlor-Alkali industry in the north-west of England. ICI has chemical
plants at Runcorn based on the salt extraction industry.
Rock salt solid is conventionally
mined by drilling, blasting and digging etc., though there are powerful machines
that can just cut away the rock directly with huge rotating blades. On
extraction, the solid rock salt is impure sodium chloride, containing silica
minerals and other chloride or sulfate salts.
Rock salt can also be extracted by
solution mining (diagram on the right). Hot water is pumped under
pressure down into the rock salt layers in an out pipe. The hot water dissolves
the salt to form a concentrated brine solution, which under pressure, is pumped
back to the surface up through the inner pipe. Brine is a name used for aqueous
sodium chloride solution. The brine is pumped to storage tanks and onto a
refining plant for purification. After impurities (like sand) are removed the brine is
boiled to crystallise the salt. A lot of salt, as brine solution, is processed
by electrolysis to give several important products (details in a section
below).
In areas such as Cheshire and County
Durham in the north-east of England there has been (and will be for some time to
come) land subsidence due to collapses of old mine workings underground. This
subsidence can affect roads and houses and ground collapses can happen without
warning! Wherever possible old mines should be filled in to minimise the risk
from subsidence. But, by using good mining techniques, such subsidence can
almost be prevented. What you do is leave good supporting pillars of rock salt
that prevent the rock salt layers and the ground above from collapsing. If
larger caverns are used, they should be spaced out and back-filled with waste
after use.
All forms of mining use lots of
energy, often from fossil fuels, which are finite resources and cause pollution.
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2.
The
industrial electrolysis of sodium chloride solution
(brine)
The chlor-alkali
industry process, the starting point for many useful products for
industrial and domestic use
|
THE ELECTROLYSIS CELL
Summary
of the ions involved and what happens to them at the two electrodes.
|
|
negative electrode
(cathode) product |
ions
from water |
ions
from salt |
positive
electrode (anode) product |
|
hydrogen gas formed
H2(g)
<==
|
hydrogen ion H+(aq)
changes to <==
|
chloride ion Cl–(aq)
changes to ==>
|
==> chlorine gas formed Cl2(g)
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|
ion
left in solution ==>
|
OH– |
Na+ |
<==
ion left in solution
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|
When
electricity is passed through the concentrated sodium chloride solution (brine)
there are three products,
(1) hydrogen is
formed at the negative electrode (–ve cathode),
(2) chlorine
at the positive electrode (+ve
anode) and (3) sodium
hydroxide is left in solution (Na+ plus OH–).
The electrode equations are given and explained below. All three
products can be tapped, stored and further processed. |
Summary equation for the electrolysis of brine (sodium
chloride)
- sodium chloride + water
==> sodium hydroxide + hydrogen + chlorine
- 2NaCl (aq) + 2H2O
(l) + elec. energy ==> 2NaOH (aq) + H2 (g) + Cl2
(g)
|
- The
industrial electrodes must be made of an inert material like
titanium which is not attacked by chlorine or alkali.
- However a
simple cell using carbon electrodes can be used to demonstrate the
industrial process in the laboratory.
- The (–) cathode gas gives
a squeaky pop with a lit splint – hydrogen. The (+) anode
gas turns damp blue litmus red and then bleaches it white – chlorine.
|
The
electrode equation theory and details
-
The (–) cathode
attracts the Na+ and H+ ions. The hydrogen ions are reduced by
electron (e–) gain to form hydrogen molecules:
-
The (+) anode
attracts the OH– and Cl– ions. The
chloride ions are oxidised by electron loss to give chlorine
molecules:
-
The ions remaining in solution (Na+ plus OH–)
form the 3rd product, sodium hydroxide solution.
-
For more details laboratory
experiments and theoretical explanation see
Electrolysis of sodium chloride solution (brine)
and molten sodium chloride
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|
The environmental impact of
industrial processes - electrolysis of brine
Large scale electrolysis uses large
amounts of electrical energy, much of which is derived from fossil
fuelled power stations.
Fossil fuels are a finite resource
and their use releases carbon dioxide into the atmosphere
potentially leading to global warming - climate change, increased
acidity in the oceans with threats to ecological systems.
At one time, (hopefully not now?)
some kinds of electrolysis cells used large quantities of the toxic
metal mercury. If mercury 'escapes' via an accident into the
environment and enters river systems fish are poisoned and people
who eat the fish poisoned too.
Electrolysis plants have used
asbestos insulation (hopefully not now?), asbestos is toxic mineral
that causes lung damage leading to lung cancer. |
Revision notes on the
uses of fluorine, chlorine, iodine, extraction of salt, uses of salt sodium
chloride, help when revising for AQA
GCSE chemistry, Edexcel GCSE chemistry, OCR GCSE gateway science chemistry,
OCR GCSE 21st century science chemistry GCSE 9-1 chemistry examination
questions on the uses of halogens and halogen compounds.
TOP OF PAGE
and sub-index for this page
3. The uses of
halogens and their compounds
The Uses of Chlorine,
the brine electrolysis products and other halogens and
their compounds
|
Uses of
Sodium Chloride NaCl
'salt' & 'rock salt' |
- sodium chloride is an important
raw material found as
'rock salt' or in seawater.
- Purified sodium chloride, 'salt' is used directly for food
flavouring
and food preservation.
- Table salt is produced by
evaporating purified brine solution.
- Rock salt is used for
de–icing roads. Raw rock salt can be used directly on roads
to melt ice and preferably, stop ice forming in the first place.
Dissolved salts lower the freezing point of water. The rock salt
is mixed with grit to give better tyre grip on icy roads.
- BUT sodium chloride is mainly
converted into other products by electrolysis (see above for the production of chlorine,
hydrogen and sodium hydroxide by electrolysis of brine (aqueous
sodium chloride solution).
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|
 
Uses of
CHLORINE from brine electrolysis
Cl2
 
|
- All the Halogens are potentially
harmful substances and chlorine in particular is highly
toxic. It is dangerous to ingest halogens or breathe in any
halogen gas or vapour.
- Chlorine, a disinfectant,
is used to kill bacteria and so sterilise water for
domestic supply or in in swimming pools. Chlorine
kills most harmful microorganisms (all of them?).
-
The sodium hydroxide
and chlorine can be chemically combined at room temperature to
make the household bleach, sodium chlorate(I) NaClO. This is used
in some domestic cleaning agents, it chemically 'scours' and
chemically 'kills' germs!
- Organic phenolic chlorine
compounds are used
as
antiseptics and disinfectants like 'Dettol' or
'TCP'
- Organic chlorine compounds are
used as pesticides, including the now mainly banned DDT.
- Chlorine is used in making
CFC refrigerant gases/liquids but their production and use
are being reduced. They break down in the upper atmosphere and
the chlorine atoms catalyse the destruction of ozone O3
which absorbs harmful uv radiation.
- PVC plastic: Chlorine
(from electrolysis of NaCl)
and ethene (from
cracking oil fractions) are used to make a chemical
called chloro(ethene), which used to be called vinyl
chloride, this is then converted into the plastic–polymer
poly(chloroethene) or PVC, because it is shorthand
for the old name polyvinylchloride! (equation below)
-
- Poly(chloroethene), old names
PVC, from chloroethene (vinyl chloride) is much tougher than
poly(ethene) and very hard wearing with good heat stability. so
it is used for covering electrical wiring and plugs. It is also
replacing metals for use as gas and water drain pipes and has
found a use as artificial leather and readily dyed to bright
colours!
- PVC
is very tough hard wearing useful plastic and a good electrical
insulator and is used for water piping, window frames, part of
electrical fittings e.g. plug covers etc.
- (old names : polyvinyl chloride,
shortened to PVC)
- Liquid organic chlorine
compounds are used as dry cleaning or de–greasing
solvents.
- Chlorine
is used in the manufacture of potassium chlorate weed killer,
KClO3.
- Chlorinated organic compounds
are used as insecticides.
|
 
|
- Hydrogen chloride &
hydrochloric acid - a joint use of chlorine plus
hydrogen
-
HCl(g
=> aq)
As described above, some of the hydrogen and chlorine from the
electrolysis of sodium chloride solution are combined to form
hydrogen chloride gas.
- H2(g) + Cl2(g)
==> 2HCl(g)
- This gas is dissolved in water
to
manufacture hydrochloric acid.
- HCl(g) + aq ==>
HCl(aq)
or ==> H+(aq) + Cl–(aq)
- This is a very important acid
used in the chemical industry to make chloride salts.
- If there is an excess of
hydrogen chloride (for some reason?) it can be oxidised back to
chlorine.
- Hydrogen chloride and oxygen are
passed over a catalyst at high temperature
- hydrogen chloride + oxygen
==> chlorine + water
- 4HCl + O2
===> Cl2 + 2H2O
|
Uses of
silver salts Ag+X–
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-
Silver chloride
(AgCl), silver bromide (AgBr) and silver
iodide (AgI) are all sensitive to light ('photosensitive'),
and all three are used in the production of various types of
photographic film used to detect visible light and beta and
gamma radiation from radioactive materials.
-
Each silver halide
salt has a different sensitivity to light. When radiation hits
the film the silver ions in the salt are reduced by electron
gain to silver.
-
AgI is the
least sensitive and used in X–ray radiography,
AgCl is the most sensitive and used in 'fast' film for
cameras, and AgBr is used in most standard photographic
films – but much of their use is being superceded by digital
cameras!
|
|
Uses of
other halogens
FLUORINE F2
BROMINE Br2
IODINE I2
|
-
Fluorine
is used as
fluoride salts in toothpaste or added to
domestic water supplies to strengthen teeth enamel helping
to minimise tooth decay. (e.g. potassium fluoride).
-
Fluorine
is used in the manufacture of the tough non–stick plastic
PTFE
coating of cooking pans.
-
Fluorine
is used in manufacture of aerosol propellants and refrigerant
gases.
-
Apart from its
silver salt use in photography,
bromine is used to manufacture organic pesticides and
fungicides because of their poisonous nature
-
Organic bromine
compounds are used as
flame inhibitor chemicals (flame retardants) for plastic
products to reduce their flammability and as petrol additives
to reduce the build–up of lead in car engines (a use decreasing
as 'green' unleaded are becoming more popular).
-
Bromine
and iodine
are both used in 'halogen' car headlamps.
-
Iodine
is used in hospitals in the mild antiseptic solution 'tincture
of iodine'.
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Uses of
HYDROGEN H2 from brine
electrolysis |
-
Hydrogen is used
in the manufacture of
ammonia for fertilisers, nitric acid, domestic cleaning
products.
-
The hydrogen is used
to partially hydrogenate unsaturated vegetable oils/fats to make margarine
by adding hydrogen to the C=C double bonds in these natural
molecules.
-
The hydrogen can be
combined with chlorine to make hydrogen chloride, which
dissolved in water becomes the important industrial chemical hydrochloric acid.
-
Hydrogen is used in
hydrogen-oxygen fuel cells.
-
Hydrogen is used as
fuel in welding and metal cutting equipment.
-
Hydrogen isn't a halogen,
but it is made from the electrolysis of salt solution.
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Uses of
SODIUM HYDROXIDE
NaOH
from brine electrolysis |
- Sodium
hydroxide, a very strong alkali,
is used in the manufacture of soaps, detergents, paper pulp,
oven cleaner, ceramics
and to make soluble salts of organic acids with low solubility
in water (e.g. soluble Aspirin).
- It isn't a halogen compound, but
it is made from the electrolysis of salt solution.
-
The sodium hydroxide
and chlorine can be chemically combined at room temperature to
make the household bleach, sodium chlorate(I) NaClO. This is used
in some domestic cleaning agents, it chemically 'scours' and
chemically 'kills' germs!
|
Revision notes on the
uses of fluorine, chlorine, iodine, extraction of salt, uses of salt sodium
chloride, help when revising for AQA
GCSE chemistry, Edexcel GCSE chemistry, OCR GCSE gateway science chemistry,
OCR GCSE 21st century science chemistry GCSE 9-1 chemistry examination
questions on the uses of halogens and halogen compounds.
TOP OF PAGE
and sub-index for this page
4. The Use of Salt in
the Food Industry and Health Issues
Purified (refined) salt (sodium
chloride) is extensively used in the food industry.
It is an ingredient in many
processed food, so we take in significant amounts of salt if you eat
appreciable amounts of processed food. Salt is used as a preservative to
give foodstuffs a longer shelf-life and prevent decay due to mould and
bacteria (stop food 'going off'). If meat is treated with salt it dries the
meat by absorbing water and this dehydrating effect kills bacteria. 'Cured
meat' is preserved in this way.
Although we all enjoy the taste
enhancing properties of salt there are health issues related to
'over-consumption' of salt. We do need salt in our diet, but not too much.
Consuming too much salt can
cause high blood pressure which in turn can cause heart attacks and strokes.
Taking in too much salt has been
linked to increased risk of osteoporosis, renal failure (kidney failure) and
stomach cancer.
Food products should be labelled
with the amount of salt in them, its not always clear. Food labels may give
you the amount of salt in the food in grams or the % of the maximum
recommended intake of salt.
If you don't eat much processed
food, the odd sprinkle on your meals shouldn't cause you any harm.
The Food Standards Agency (FSA)
is an independent food safety watchdog that advises the government and the
public about food safety and health and issues guidelines on the maximum
amount of salt you should take in a day. This is known as the 'Guideline
Daily Amount' (GDA).
Two government departments,
Department of Health and the Department for Environment, Food and Rural
Affairs, are both involved with food safety issues and do risk assessments
to check on the safety of chemicals used in food and advise the public on
how food, and food additives may affect their health.
We know that too much salt is
not good for us, but many food products still have significant amounts of
salt in them.
This may due to the cost of
producing a replacement 'recipe' that tastes as good, lasts as long too
(shelf-life) and sales maintained at previous levels.
This brand of baked beans claims to contain 25% less salt than its 'standard
baked beans' product. The product contains 0.4g/100g i.e. 0.4% sodium
chloride (as NaCl). A good helping (1/2 can) gives you 15% of the RDA
(recommended daily intake).
Sometimes the nutritional
information just quotes the mass of sodium ion in the food, but you can
do a simple calculation to work out the actual mass of sodium.
Suppose the mass of sodium
was quoted as 0.4g per 100g in a food product.
Atomic masses: Na = 23, Cl =
35.5, so formula mass of NaCl = 23 + 35.5 = 58.5
Scaling up gives 0.4 x 58.5
/ 23 = 1.0g NaCl/100g of food product, i.e. 1.0% salt in the food.
Note that this calculation
assumes all the sodium is associated with sodium chloride, but the
sodium may come from other food additives like flavour enhancer sodium
glutamate or the preservative sodium nitrate.
However, I can't imagine
many consumers doing this sort of 'shopping calculation' in the middle
of busy supermarket!
Salt is added to many food
products e.g. cereals (above), so it all adds up through the day! and the
more processed food you eat, the less control you have over your salt
intake.
Note that in cereal 2a they
have done the calculation for you, 0.18g Na x 58.5/35.5 = 0.46g of NaCl
salt
-
See also
Group 7 The HALOGEN elements
of the Periodic Table
Revision notes on the
uses of fluorine, chlorine, iodine, extraction of salt, uses of salt sodium
chloride, help when revising for AQA
GCSE chemistry, Edexcel GCSE chemistry, OCR GCSE gateway science chemistry,
OCR GCSE 21st century science chemistry GCSE 9-1 chemistry examination
questions on the uses of halogens and halogen compounds.
KS4 Science extraction of salt
sodium chloride uses of salt products from electrolysis of brine GCSE chemistry
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