Doc Brown's 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.

Potable water is water that is fit to drink

Drinking water should not contain anything that is potentially harmful.


Extra Aqueous Chemistry Index:

1. Water cycle, treatment, pollution (this page)

See also 2. Colloids - sols, foam and emulsions

3. Hard and soft water - causes and treatment 

4. Gas and salt solubility in water and solubility curves

5. Water of crystallisation calculation


1. What happens to water on the Earth's Surface?

The water on the Earth's surface is continually being re-cycled.

As it falls, rain water contains only dissolved gases but once it reaches the ground water becomes contaminated in various ways.

Potable water is water that is fit to drink

Drinking water should not contain anything that is potentially harmful ... read on ...


1a. The Water 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 condensation (exothermic).
    • 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.
  • Sources of water (all of course initially untreated for human use!)
    • River and reservoir water, regularly replenished by rain, can be treated to make it potable.
    • Ground water can be extracted from aquifers - this where water is trapped underground in particular rock and clay formations.
    • Waste water from domestic sewage or industry. Obviously quite a bit of processing is needed to make the water potable. Waste water contains more contaminants so needs more treatment than ground water
    • 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. It is very costly to extract potable water from seawater, it needs a lot of energy for distillation or reverse osmosis (the latter involving a semi-permeable membrane that allows water through but not the dissolved salts.
    • The water that comes into your home has been treated and tested to ensure it is safe to drink - potable!


1b. Water Treatment and pollution - domestic and industrial context

    • We need a good supply of water suitable for domestic consumption in homes, businesses, sewage systems etc.
    • 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 water.
      • 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 dissolved substances.
  • 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 country.
    • 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 treatment plant.
      • 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.
    • Sometimes the ground water is first aerated, that is air blown in it to increase the oxygen concentration. This displaces unwanted dissolved gases and removes some ions which react with the oxygen to form insoluble solid oxides which are later filtered out from the water later in the treatment process. The oxygen also encourages aerobic bacteria to grow and break down organic matter.
    • Screening (a 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 water source.
        • 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 alternative ).
        • The sewage plant sedimentation produces a sludge and above it the effluent water.
    • Clarification 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 below).
      • This stage is part of the clarification part of the water treatment process and is all about removing dirt and colour.
      • 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.
        • The lighter effluent floats above the sludge.
        • The 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.
          • Although not totally pure, thee water can be safely released back into the environment.
        • 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 larger particles.
      • 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 colourless.
      • However, for waste water containing toxic substances, additional special stages of treatment may needed to remove particular chemicals in the water e.g. to precipitate metals or using a membrane to filter out other harmful substances.
    • Final treatment 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) + H2SO4(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 is quite a toxic chemical so the amount of chlorine added is carefully controlled and the water tested to make it never reaches harmful levels in domestic water.
          • If water is not treated in this way there is a much greater risk of consumers coming into contact with harmful water-borne diseases. In developing countries (e.g. in Africa) thousands die each year from bacterial infections like cholera and dysentery which affect the intestine.
        • The chlorine will also prevent the growth of algae and removes bad tastes and odours and discolouration from organic compounds.
        • The 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.
        • This obviously reduces life-expectancy  and puts a strain on health services and the economics in general of poorer developing 3rd world countries.
        • 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.
          • Water 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).
          • But, 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 contaminated water.
          • Too 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 by-products.
      • 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 reservoirs.
      • 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.
        • If you are not satisfied with your local water you can buy various filters of carbon/silver to remove traces of organic molecules, the taste of chlorine and any residual microbes.
        • There are various types of water filters containing carbon, silver and ion exchange resins that can remove some dissolved substances from tap water to improve the taste and quality.

        • You can also get ion exchange resin filters to soften the water to minimise e.g. the furring of kettles (see 'Hard and Soft Water' page). They will also remove metal ions of lead and other metals.
        • To produce pure water you can use ion exchange resin to remove any ions but other filters would have to be used to remove traces of organic chemicals like pesticide residues.
        • Some of the purest water can be made by distillation.
          • Potable water can be made by distillation.
          • The water boils off leaving any dissolved substances behind as a solid residue (which have much higher boiling points than water because of the greater inter-particle forces).
          • The steam is condensed to give pretty pure water.
          • BUT, this is a very costly process involving the cost of a lot of energy to boil the water.
          • Distillation has been used in very 'dry' Middle Eastern countries where the oil revenue can support these desalination plants purifying seawater e.g. in Kuwait.
          • On a small scale distillation and ion exchange resins are used to provide scientific laboratories with pure water for experiments, analysis etc. and the production of pharmaceutical materials involving an aqueous based solution or cream etc.
        • Reverse osmosis can also be used to produce potable drinking water from seawater, but this again is a very expensive process requiring a lot of energy.
          • Osmosis usually involves the passage of water through a semi-membrane from a lower to a higher concentration of dissolved substances and this is the normal direction of flow.
          • The membrane only allows water to pass through, larger molecules and ions are trapped on one side of the membrane, thus separating reasonably pure water from the dissolved substance.
          • To reverse the process large pressures must be exerted to literally squeeze the water out of the salt solution through the semi-permeable membrane. This requires a lot of energy and therefore a rather costly process.
          • So, all desalination of seawater is costly and not practical for most countries, as is the case already described for distillation.
        • Distillation and reverse osmosis are less complicated processes compared to other water purification methods e.g. starting with ground water or sewage treatment but they are very costly and not always practicable.


  • Extra note on AQUIFER water source (e.g. borehole):
    • Aeration - water from aquifers contains dissolved carbon dioxide and is 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 described above.
  • Extra technical notes on aspects of water treatment
    • 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 below).
    • 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' taste.
      • Cooked vegetables turn brown (complex compounds with phenols).
    • Colloidal clay: A colloid consists of one substance (or mixture of substances) very finely dispersed in another substance (or a mixture of substances) without a new true solution forming.
      • 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 properties below)
    • Colloidal particles may be electrically charged.
    • 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 power:
        • positive cations: Al3+ > Mg2+ > Na+
        • negative anions: [Fe(CN)6]3- > SO42- > Cl- 
        • and this explains why aluminium sulphate Al2(SO4)3 is used to precipitate (coagulate) colloidal clay in water treatment for domestic water supplies.



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 southern England.


Extra Aqueous Chemistry Index: 1. Water cycle, treatment, pollution  *  2. Colloids - sols, foam and emulsions  *  3. Hard 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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