Chemistry KS4 science GCSE/IGCSE/O Level Chemistry Revision Notes
1. Water cycle - water as
a resource - potable water - water treatment and water purification
1. The components of the
water cycle are described and explained and the use of water as an important
resource, its treatment to make it safe to drink, pollution problems. The
importance of clean water to developing nations.
Extra Aqueous Chemistry
cycle, treatment, pollution (this page)
See also 2.
Colloids - sols, foam and emulsions
and soft water - causes and treatment
Gas and salt solubility
in water and solubility curves
5. Water of crystallisation
What happens to water on the Earth's Surface?
The water on the Earth's surface is continually
As it falls, rain water contains only
dissolved gases but once it reaches the ground water becomes contaminated in
Cycle and Water as a Resource
- Water is the most abundant substance on the
surface of our planet and is essential for all life.
- Water in rivers,
lakes and the oceans is evaporated by the heat of the Sun (endothermic).
- The water
vapour formed rises into the atmosphere, cools and forms clouds of
- Eventually this gives rain and snow
'precipitation' which on melting returns to the rivers, seas and oceans.
- This is known as the water cycle.
- Water is an important raw material and has
many uses. It is used as a solvent and as a coolant both in the home and
in industry. It is used in many important industrial processes including the manufacture of sulphuric acid.
- Domestically we use huge amounts for washing, toilets
and watering the garden.
- This of course produces lots of waste water products.
- Farming produces waste water including nutrient
run-off from agriculture methods involving artificial fertilisers and slurry
from animal farms.
- All of this waste water has to be treated.
- Seawater/brine is a valuable resource e.g. large scale evaporation in 'salt pans' (using fuel burning or solar
energy) to produce 'sea salt' sodium chloride NaCl, the water also
contains lots of other salts including bromides from which the element
bromine is extracted.
Treatment and pollution - domestic and industrial context
- We need a good supply of water suitable for domestic
consumption in homes, businesses, sewage systems
- It is also a valuable cheap
resource used in large quantities in industry and power stations.
- Water of a suitable quality is essential for life,
but most naturally occurring sources of water require processing to be made
fit for human consumption.
- For humans, drinking water must have sufficiently low
levels of dissolved salts and microbes to avoid harmful effects.
- Water that is safe to drink is called potable
- Potable water is NOT pure water in the chemical
sense because it still contains dissolved substances, but not in harmful
concentrations, but it must also be cleansed of potentially harmful
bacteria or other microbes (microorganisms).
- Potable water may have a pH from 6.5 to 8.5.
- Pure water has a pH of ~7 and contains no
- Different methods are used to produce potable
water depending on local supply of water and what it contains.
- In the United Kingdom (UK), rain, regarded as fresh water, provides water with low levels of dissolved substances
that collects in the ground (extracted from wells) and in lakes and
rivers feeding reservoirs or treatment plants directly.
- The most potable water is produced by: using an
appropriate source of fresh water, passing the water through filter
beds to remove any solids and sterilising to kill microbes.
- Sterilising agents used in producing potable
water include chlorine, ozone or ultraviolet light.
- If supplies of fresh water are limited,
desalination of salty water or sea water may be required which can be
done by distillation or by processes that use membranes such as
reverse osmosis, but both of these processes require large
amounts of energy.
- Distillation of salt solution can be used to
obtain very pure water.
- In general the ease of production and decreasing
cost of potable water is ground > waste > salt water.
- Around the world our increasingly urban lifestyles
and industrial processes produce large amounts of waste water that
require treatment before being released into the environment.
- Sewage and agricultural waste water always require
removal of organic matter and harmful microbes.
- Industrial waste water may require removal of organic
matter and harmful chemicals from the chemical industry AND from domestic
products we put down the sink.
- Sewage treatment includes screening, grit
removal, sedimentation to produce sewage sludge and effluent, anaerobic
digestion of sewage sludge and aerobic biological treatment of effluent.
- There are various undesirable materials that need
to be removed from water before it is fit for domestic consumption.
- They include colloidal clay, microscopic organisms,
chemicals which cause tastes or
odours and acidic substances.
- Before treatment most sources of water
contain at least one (or many) of the following, dissolved salts (some
beneficial & some harmful), minerals, microbes, insoluble materials,
pollutants nitrate and phosphate residues from fertilisers, lead compounds
from old lead pipes, pesticide residues.
- Nitrate residues are washed off farmland
into rivers and lakes and if too much nitrate is present it can affect the
blood transportation of oxygen in young babies blood.
- Lead from old lead piping can be dissolved
and lead is a toxic metal in solution (neurotoxin).
- Pesticide residue can get into water courses
if the spraying is not carefully controlled near lakes and rivers.
- The water for a domestic water supply
doesn't have to be absolutely pure, in fact it can contain traces of nutrient minerals
like calcium, iron and iodine compounds, BUT it must not contain harmful substances
like bacteria or poisonous metal compounds.
- There is no unique way of treating water, it
depends on the water source and the technology and chemicals available to a
- The diagram and notes below give you some
ideas of how naturally occurring water can be treated to give a safe supply
of cooking and drinking water..
- In the developing countries of e.g. Africa,
waterborne diseases claim many lives and ill-health exacerbates the effects
of the pre-existing poverty of many people.
- Via TV you see many examples of famine and disaster in the world
and the misery caused by lack of clean water (as well as food) in various disaster
zones e.g. famine struck areas.
- Water may be contaminated with microbes
(microorganisms), iron/manganese compounds, phosphates and nitrates
(poisonous) from overuse of fertilisers, pesticide residues, contaminants
from the chemical industry, all of which must be dealt with at a water
- Untreated biologically contaminated
water in poor countries is responsible potentially debilitating and
fatal diseases such as cholera and dysentery.
- In developed countries the water
gathered from groundwater, rain, rivers, lakes etc. is closely monitored
for pollution by government agencies as well as the water companies
themselves who must design and operate systems to treat the water to
make it fit for use.
- Drinking water is made fit for
domestic home consumption by many processes.
- The overall method for treating water for
domestic use can be complicated and dependant on the original water source.
- Some of the methods of water purification
and their sequence are outlined below with the help of the diagram above
(repeated at the end).
- I've added and highlighted an extra section on the
treatment of sewage
from the domestic uses of water.
In the UK water sources from
surface water e.g. rainfall collected in lakes, rivers or reservoirs.
- Groundwater is obtained from aquifers
where water is trapped underground by particular rock and clay
formations. Boreholes can be drilled down to extract and pump the water
to the surface (quite common in southern England).
- This initial stage is called abstraction
and all these water resources are limited by annual rainfall, so
extended periods of drought can cause water shortages if the reservoir
levels or groundwater levels fall appreciably.
- Water from
aquifers is already of high quality because it has filtered through
many layers of chalk or sand and may need only disinfection with chlorine.
- However, water from rivers, lakes and reservoirs contains a
wide range of substances (dissolved or floating/carried) debris which have to be removed through several processes.
It is good to be able to
store water in a
reservoir for two reasons ...
- storing the water in reservoirs starts the
natural clean-up process, as heavier particles settle to the bottom, so
water companies donít always have to take them out,
- and, reservoirs (usually) ensure a supply of water
is always available in periods of drought.
- Even in developed countries like the UK
conservation is important as the weather has become more erratic with
climate change and even the UK with its temperate climate suffers from
periods of serious droughts in summer.
crude filtering) involves removing material like
branches, twigs and cleaves and other plant material that might clog up the pipes
in the water treatment plant.
- Even at this early stage in the water
treatment process, sometimes pre-ozonation is carried out with the very chemically active form of oxygen, ozone
(O3), which is
passed through the water to destroy micro-organisms and oxidise metals
(helps to remove them in the clarification stage). Pre-chlorination may be
done to reduce growth of algae and other biological growth and in
conjunction with aeration (blowing air through) which helps in the
precipitation and removal of dissolved iron and manganese compounds.
- If the
water source is sewage the process gets more complicated and usually
carried out in a specialised sewage treatment plant that takes in
directly waste domestic water and not from some surface water or ground
- In a
sewage treatment plant the waste water from domestic houses enters the
process at this stage and after screening the waste is fed into a
sedimentation tank and then processed in a different way (see below
sewage plant sedimentation produces a sludge and above it the
via coagulation-flocculation-sedimentation .
- Safe chemicals called coagulants may be added to the
water to act as a binding agent for particles to coagulate them (flocculate
them) together to give larger 'lumps' (called 'floc' or 'flocs') that
sediment ('settle') out faster under gravity.
- This also helps to precipitate out dissolved
metals and to remove organic matter.
- Aluminium sulphate (alum), iron sulfate
or lime is added to
coagulate colloidal clay (see colloids
- This stage is part of the clarification
part of the water treatment process and is all about removing dirt and
- Once the 'sludge' has settled out (sedimentation) it is removed
and the clarified effluent water pumped on to the filtration units.
Alternative after stage==
sewage treatment ==>
- In the
settlement tank the water and 'associated waste'! undergo sedimentation
so that the heavier solids settle out to form a sludge.
lighter effluent floats above the sludge.
effluent from the sedimentation-filtration tanks (stage 3) is drained
off to other tanks and undergoes biological aerobic digestion
aided by pumping air into it to encourage aerobic bacteria to break down
the organic material including other microbes in the effluent.
not totally pure, thee water can be safely released back into the
sludge is transferred to separate tanks and undergoes anaerobic
digestion i.e. the organic matter is broken down by bacteria to
produce fertiliser and methane gas which can be used as a
fuel energy resource.
- In areas
with little natural water, recycling sewage water is an option. Although
it requires more processes that is needed in treating fresh ground water
or surface water, it uses less energy than desalination using
distillation or reverse osmosis. So it is cheaper to recycle sewage
water than e.g. desalinating salty waters e.g. sea water or brackish
(salty) inland water.
(a) Filtration - gravity
filters to remove anything solid still floating/suspended in the water:
- The clarified water is then pumped onto the
filtering stage and passed through gravel and sand filter beds to remove
finer solid particles that hadn't previously settle out.
- Rapid gravity filters of course sand or
gravel traps the
- Then slow sand filters-large beds of fine sand trap
the finer particles before final treatment.
- (b) You can also use carbon pressure filters
containing 'Granular Activated Carbon'
to absorb and remove taste, smell/odours and very fine particle.
- Pesticide are also removal by
these granular activated carbon filters and may be used in association with ozone treatment.
- Ion-exchange resins can be used to further
purify the water from certain dissolve minerals that cannot be filtered out.
- After filtration the filtered water is clean and
with chlorine to kill bacteria (disinfection)
- Once the water has been through the treatment
process, the last stage is to add a
very small amount of chlorine to it.
- This kills any residual organisms or bacteria
(microbes-pathogens) and keeps the water
safe, right up to the point it reaches your tap.
- Small amounts of
sulphur dioxide may be added to remove excess toxic chlorine
- the molecular equation is SO2(aq)
+ Cl2(aq) + 2H2O(l) ==> 2HCl(aq)
- the ionic equation is SO2(aq)
+ Cl2(aq) + 2H2O(l) ==> 2Cl-(aq)
+ SO42-(aq) + 4H+(aq)
- Issues over the use of chlorine
in water treatment
- If the
right amount of chlorine is added to water, all the bacteria are killed
quickly, but there should be a little excess chlorine to kill any
microorganism that get into the water further on in the pipe delivery
system to homes and factories etc.
chlorine will also prevent the growth of algae and removes bad tastes
and odours and discolouration from organic compounds.
World Health Organisation (WHO) and the United Nations estimate that a
billion people don't have access to clean safe water and every year up
to 1.8 billion people die from waterborne diseases. These diseases
include cholera, dysentery and typhoid from bacterial contamination
often causing severe diarrhoea, dehydration, lack of nutrition intake,
hence often fatal in the end.
obviously reduces life-expectancy and puts a strain on health
services and the economics in general of poorer developing 3rd world
- It is
expensive to treat water and often poor countries cannot afford the
infra-structure to treat and deliver safe water for drinking and
cooking. Getting any water often involves travelling some distance to
bring it back from a well.
- Even if
the water is chlorinated, there can still be issues.
may contain traces of organic compounds e.g. from the decomposition of
plant materials and these may react with chlorine to produce harmful
chlorinated hydrocarbons that may cause cancer (carcinogenic).
this risk is far outweighed by the dangers from bacteria in the water
and the nasty diseases they cause.
Thousands of people died in Victorian times from cholera when using
much chlorine is itself unpleasant in terms of smell and taste, but we
shouldn't complain too much if the water company occasionally gets a bit
wrong, at least the water is safe to use.
Chlorine is delivered to water treatment plants as liquid in high
pressure cylinders. Therefore bearing in mind the harmful and toxic
effects of chlorine on your skin and lungs, you don't want any accidents
due to careless handling.
- Alternative ultra violet for
disinfection - the water is passed through a strong ultraviolet light for disinfection
to kill harmful bacteria.
- Ozone gas (O3) is a powerful
oxidising agent, and like chlorine will kill bacteria, but as with
chlorine, just the right amount must be used, since residual chlorine or
ozone would be harmful to us.
- The water's pH value may have to be altered with
chemicals to reduce corrosion and to make the water more stable, it must not
be too acid or too alkaline.
- Lime slurry can be added to
neutralise the water if it is too acid.
- Phosphate dosing - a
phosphate chemical can be added to reduce the dissolving (dissolution) of lead from old pipes.
- Fluoride is added to drinking
water (fluoridation) in some areas of the country to reduce tooth decay.
- Adding fluoride is controversial
since (i) too much fluoride is actually harmful (cancer, bone
problems) and (ii) is this mass medication justified since you can
get the fluoride in toothpaste, which is your choice!
- Adding chlorine has its critics,
despite its clear disinfectant action, chlorine can react with
naturally organic compounds to produce harmful chloro-compound
- Some water supplies may have to undergo special
chemical treatment to precipitate harmful metals.
After the final treatment, the water
is pumped from the treatment works and stored in covered
- It is then pumped to the network of pipes
('water mains pipes') and pumping stations to provide the water
supply for homes, businesses, factories etc.
- Extra note on AQUIFER water source
- Aeration - water from aquifers contains dissolved carbon dioxide
remove by blowing air through the water and reduces the amount of lime needed
later on in the water treatment process.
- Aeration also converts soluble salts of iron and manganese, that
occur naturally in the water, into insoluble precipitates which are
more easily removed later.
- Softening may be required if the water
originates from limestone country.
- In the softening tank, lime, fine sand or a
coagulant are added.
- The lime reacts with the bicarbonates, which cause
the hardness, to form chalk which precipitates out onto the sand grains to form
pellets or the particles of chalk coagulate into a sludge, either will
settle on the bottom of the tank.
- The softened water is then filtered etc. as
- Extra technical notes on aspects of water
- The use of artificial fertilisers results
in many natural waters being contaminated with dissolved nitrate and
ammonium ions. Dissolved nitrate ions can have harmful effects on babies
and so the levels of nitrate are carefully monitored. Nitrates may be
carcinogenic. The ions from this pollution are not easy to remove on a
large cost-efficient scale.
- An ion-exchange filter can remove
these and other ions which can cause problems e.g. calcium and magnesium
which cause hardness in water and iron compounds (see
- Iron in water
is a non-harmful but an aesthetic nuisance impurity:
- readily soluble iron(II) when exposed
to air form rusty brown insoluble iron(III) hydroxide or hydrated
iron(III) oxide compounds. These stain yellow/orange/brown washing
laundry and white plumbing facilities!
- The iron(III) ions also form inky black
compounds with the tannic acids in tea and giving it a 'metallic'
- Cooked vegetables turn brown (complex
compounds with phenols).
- Colloidal clay: A
consists of one substance (or mixture of substances) very finely dispersed
another substance (or a mixture of substances) without a new true
- A colloid can be thought of as
intermediate between a true solution and a mixture of e.g. a liquid and
an insoluble solid.
- No filtration separation is possible with solutions
but filtration is easy and effective with an insoluble solid.
- However the
colloidal particles are big enough for their surface area properties
to be significant (see electrical
- Colloidal particles may be
- Colloids are destroyed when the
particles of the disperse phase join together and separate out from the
continuous phase. This process is called coagulation.
- Sols are also very sensitive to the
presence of ions, so any electrolyte ions present can affect the
electrical double layer (the theory is complex but just think of the
ions charge as affecting the stability of the double layer).
- The more
highly charged the ion, the greater the electrical field
force effect, so the greater its coagulating
power. The ions reduce the repulsion between the colloid
particles and allow coagulation to occur.
- Examples of coagulating
- positive cations: Al3+ > Mg2+ > Na+
- negative anions: [Fe(CN)6]3- > SO42-
- and this explains why aluminium
sulphate Al2(SO4)3 is used to
precipitate (coagulate) colloidal clay in water treatment for domestic
- Other onsite references to water
- TESTING FOR WATER PURITY
- It is possible to do some simple tests to
detect certain ions in water, though these tests described, suitable for
doing in schools and colleges, are not effective in detecting very low
- Never-the-less you can do simple
precipitation tests e.g.
Two diagrams illustrating
the way collected water may be treated and purified for domestic water use
(above & below) Some examples of water
companies do their water treatment to produce water fit for a harmless
domestic water supply
(Apologies in advance for any breach of
copyright but I don't know where I collected them from a few months ago?, but
they do illustrate the complexity of water treatment!)
The above diagram illustrates each stage in
a water treatment processes to treat both river and borehole water in
Extra Aqueous Chemistry
Index: 1. Water
cycle, treatment, pollution * 2.
Colloids - sols, foam and emulsions * 3.
and soft water - causes and treatment * 4.
Gas and salt solubility
in water and solubility curves * 5.
Calculation of water of crystallisation
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water cycle? why does fresh water have to be treated for domestic use?
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potable? what ways can we desalinate water? what is reverse osmosis? how
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screening filtering in water treatment, what is a desalination plant?
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