Hardness in Water

Hard water is any water that forms a scum with soap. Hard water does not form a lather with soap. Soft water forms a lather with soap.

How is the Scum formed? Soap is sodium stearate All sodium salts are soluble in water so soap is soluble in water Calcium or magnesium ions in the water react with soap ions [stearate ions] to form the insoluble scum [calcium stearate or magnesium stearate]

How do calcium ions get into the water?

Carbon dioxide in the air reacts with water to form carbonic acid. CO2 + H2O = H2CO3 This is a weak acid and it reacts with limestone or marble [both are forms of calcium carbonate] to form calcium hydrogencarbonate which is soluble and dissolves in the water. CaCO3 + H2CO3 = Ca(HCO3)2

Ca(HCO3)2(aq) = CaCO3(s) + CO2(g)+ H2O(l) Temporary Hardness Caused by dissolved calcium [or magnesium] hydrogencarbonate When the water is heated this breaks down to form calcium carbonate called limescale of furring. Ca(HCO3)2(aq) = CaCO3(s) + CO2(g)+ H2O(l) To show that limescale is CaCO3 Add HCl and CO2 produced – turns lime-water milky

Problems caused by Temporary Hardness Scum in laundries – specks on dark wool Limescale or ‘furring’ in kettles & boilers. Limescale is a bad conductor of heat so it also makes the boilers inefficient Blocking pipes in boilers

Permanent Hardness Hardness NOT removed by boiling Caused by any or all of the following Calcium sulphate (CaSO4) Magnesium sulphate (MgSO4) Calcium chloride (CaCl2) Magnesium chloride (MgCl2) Dissolved in the water from soil It does not cause limescale but it does cause scum.

Removal of Hardness

(2) Ion exchange removes both Temporary and Permanent Boiling only removes temporary hardness Ca(HCO3)2(aq) = CaCO3(s) + CO2(g) + H2O(l) (2) Ion exchange removes both Temporary and Permanent Ca2+ and Mg2+ are exchanged for H+ Cl-, SO42-, HCO3- are exchanged for OH- then H+ and OH- combine to form H2O. Result is deionised water

Mixture of anion and cation exchange resins SO42- Cation Exchange Resin Anion Exchange Resin Water formed Water is neutral H+ and OH- cancel out No ions present Deionised water Sugar, alcohol and other non-ionic compounds pass straight through

Deionised v. Distilled Water Deionised has No ions Can have dirt, bacteria, dissolved gases and covalent solutes e.g. alcohol and sugar Distilled water is completely pure It is more expensive to prepare and it is often not worth the bother

(3) Washing Soda [Calgon] Na2CO3 Sodium carbonate [Na2CO3] is soluble like all sodium salts Dissolve it in hard water [Temporary or Permanent] CO32- Reacts with Ca2+ [and Mg2+] ions to form insoluble CaCO3 as a very fine powder which cannot be seen Ca2+(aq) + CO32-(aq) = CaCO3(s) Mg2+(aq) + CO32-(aq) = MgCO3(s)

Making water safe to drink Water Treatment Making water safe to drink

Water is one of the most dangerous substances on earth Carries diseases e.g. Cholera , typhus, schistasomiasis, polio, malaria [ mosquitoes breed in it], dysentery and lots of other parasites Disease far more prevalent in the Third World Because of unclean drinking water Here water is free of disease and pollution We treat our water very carefully before allowing it to be consumed.

Steps involved in purifying our water

Water Purification Aluminium Sulfate Reservoir Added Flocculation (Dam or Lake) Water Purification Aluminium Sulfate Added Flocculation Sedimentation Clumped particles sink to bottom Screening Removes large debris Sand Filtration Removes very fine particles Safe Clean Water Chlorination kills bacteria Fluoridation strengthens teeth Too acid add NaOH To alkaline add CO2 Underground storage Prevents contamination

1. Screening Water is collected in reservoirs People can dispose of all sorts of rubbish in the reservoirs either accidentally or on purpose A large mesh is used to filter out dead sheep, goats and elephants It also filters out babies nappies, coke bottles, sticks etc.

2. Flocculation Al2SO4 [ a Flocculating Agent] is added to the water. There are tiny particles of dirt suspended in the water which make it cloudy. They are too small to sink to the bottom The Al2SO4 makes them clump together and sink to the bottom [Flocculation]

3. Sedimentation The mixture passes through a series of tanks It moves very slowly This allows particles time to sink Plan View

4. Filtration Water now passed through a filter bed of sand on top of gravel Removes any very small particles left Rubbish Water in Fine sand Coarse sand Fine gravel Coarse gravel Cleaned by Back-washing Mesh to keep in coarse gravel Water out

5. Additives Chlorine added to kill bacteria - enough to keep it safe till it reaches houses. Fluoride added for strong teeth - no cavities pH balance if water is too acid NaOH added if water is too alkaline H2SO4 or CO2 6. Storage Stored underground to prevent contamination

Sewage Treatment 1. No Treatment Needed to control disease Various degrees of treatment 1. No Treatment Straight into rivers Your problem washed downstream. You get it from upstream

2. Primary Treatment (i) Screening – removing lumpy bits (ii) Sedimentation – let solids settle Discharge of liquid Composting of solids left behind kills pathogenic bacteria Better than nothing but not great

3. Secondary Treatment Follows primary and is a big improvement (i) Biological Oxidation [E.g. Activated Sludge Process] Stir the sludge vigorously [or pour over large surface area] to oxygenate Encourages growth of aerobic micro- organisms These digest the sewage and destroy pathogens

(ii) Sedimentation Liquid then discharged into rivers etc. Biologically safe i.e. you won’t get disease from it Problem Eutrophication: Excess plant growth caused by excess nutrients. When these run out the plants die, rot and use up the oxygen which kills almost all the animals and Destroys the habitat.

4. Tertiary Treatment Follows Secondary Treatment Not common - it is expensive Removes ions by precipitation PO43- [phosphate] and NO31- [nitrate] Removal of these ions prevents eutrophication Ions also poisonous Water now completely safe.

Eutrophication Excess plant growth [algae] caused by excess nutrients in water. Mainly caused by excess nitrates [NO31-] and phosphates[PO43-] Sources of nutrients Fertilisers washed off land Silage effluent Pig slurry Sewage etc.

Results of Eutrophication When nutrients run out Cannot sustain plant life. [algae] Plants [algae] die Bacteria rot plant remains Aerobic bacteria use up oxygen turn place anaerobic Then anaerobic bacteria take over producing H2S which sours [poisons] the environment Only a few specialised animals can survive Death of pond, river etc.

Other Concerns High Nitrate content in water may cause Stomach cancer Death in babies [when powdered milk and nitrate rich water is used to make their food]

Heavy Metal Pollution

Introduction Release of toxic metal ions into water Lead Pb2+, mercury Hg2+, cadmium Cd2+ Ions Called Heavy Metals due to high RAM Cumulative poisons Concentrations build up in body on continuous exposure [over time] Sources (i) Industrial effluent (ii) From batteries of various kinds if not recycled (iii) Pb2+ from the water pipes of old houses - especially if water is acid

Case Study Mercury metal Poisonous inhaled into lungs Not poisonous if swallowed Passes out in a few days Inorganic mercury salts very dangerous at high concentrations. E.g. HgCl2, Hg(NO3)2 etc. Organic at low levels Damage kidneys and intestine Makes one “poco loco”. “Mad as a Hatter”

Minimata Bay Japan - Late 1950’s Industrial waste discharged into bay Hg salts got into food chain High concentrations built up in fish People ate a lot of fish Caused birth defects and death Called Minimata Disease

E U Limits For levels of nitrate, phosphate and specific metal ions [2 examples] Maximum admissible Concentration (aq) Substance : Nitrates : 50 mg / L Phosphates : 2.2 mg / L Lead : Mercury : 1 µg / L

Removal Usually removed before discharge (i) Removed by precipitation Pb2+(aq) + 2 Cl-(aq) = PbCl2(s) (ii) Ion exchange

Water Analysis Instrumental Methods

pH Meter Used to check river and lake water Use Buffer Solutions* to calibrate pH meter [*keeps pH constant] Monitors pH constantly Adjust pH of water to keep it within limits 7-9

Colorimeter White light passed through a coloured solution The colour of the solution is then compared to the colour of solutions of known concentration

Social and Applied Aspects Principle on which it works Colour Intensity is directly proportional to concentration Social and Applied Aspects Fertilisers in water Chlorine in pools

Atomic Absorption Spectrometer (AAS) Principles Ground state atoms of an element absorb light characteristic of that element Absorbance is directly proportional to concentration

Processes Dissolving of sample Sample introduction (as a fine spray) Atomisation of sample (in a flame) Absorption (by the sample) of light of characteristic wavelength by the specific element Detection – amount of light absorbed depends on amount of element in sample

Atomic Absorption Spectrometer Used to detect and measure the concentration of Heavy Metals Lead Pb2+ in water and blood Cadmium Cd2+ Mercury Hg2+

Gas Chromatography (GC) Principles Different components of a mixture have different interactions with the stationary and mobile phases Processes Injection Transport of the sample along the column Separation in the column Detection

Gas Chromatography

Gas Chromatography (GC) Mobile phase = a gas Stationary phase = non-volatile liquid coated on fine particles of an inert solid Used to separate more volatile mixtures Social and Applied Aspects Drug tests on athletes Blood alcohol tests

High Performance Liquid Chromatography (HPLC) Principle Different components of a mixture have different tendencies to adsorb onto very fine particles of a solid in the HPLC column Injection Transport of the sample along the column Separation in the column Detection Mobile phase = a solvent Stationary phase = very fine particles of silica Used to separate less volatile mixtures

Social and Applied Aspects Analysis of growth promoters in meat Analysis of vitamins in food

Infrared Absorption Spectrometry Principles Molecules of a substance absorb infrared light of different frequencies The combination of frequencies absorbed is particular to the molecules of that substance Processes Preparation of sample IR radiation passes through the sample Sample absorbs IR radiation at specific wavelengths which are detected An absorption spectrum is obtained

Social and Applied Aspects Identification of plastics Identification of drugs

Ultraviolet Absorption Spectrometry (UV) Principles Absorption of UV radiation by molecules results in the promotion of electrons from their ground state energy levels to higher energy states Absorbance is directly proportional to concentration Processes Preparation of solution of sample UV light is passed through the sample An absorption spectrum is obtained

Social and Applied Aspects Quantitative determination of drug metabolites Quantitative determination of plant pigments

X-ray Crystallography Principles The wavelengths of x-rays are comparable to the distances between atoms in a crystal Pattern of scattering of x-rays passed through a crystal is related to the structure of the crystal Processes Preparation of crystal Narrow beam of x-rays of a particular wavelength directed at crystal Reflected x-rays detected on film Pattern that is detected on the film used to work out the structure

Social and Applied Aspects Used to determine the structure of crystals e.g. vitamin B12 and penicillin