Hardness in Water

Hardness in Water

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

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-

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 (1) 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. Mixture of anion and cation exchange resins Ca2+ Cation

Exchange Resin Water is neutral Water formed H+ and OH- cancel out SO42-

Anion Exchange Resin 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) 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 Reservoir (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 Underground storage Prevents contamination Chlorination kills bacteria Fluoridation strengthens teeth

Too acid add NaOH To alkaline add CO2 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 Water in

Rubbish Fine sand Coarse sand Fine gravel Coarse gravel Cleaned by Back-washing Water out Mesh to keep in coarse gravel

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

23 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 24

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 microorganisms These digest the sewage and destroy pathogens 25

(ii) Sedimentation Liquid then discharged into rivers etc. Biologically safe i.e. you wont 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.

26 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. 27 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. 28

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.

29 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] 30 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 1950s 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 (aq) Substance : Concentration Nitrates : 50 mg / L Phosphates : 2.2 mg / L Lead : 50 mg / L Mercury :

1 g / 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 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 1. Lead Pb2+ in water and blood 2. Cadmium Cd2+ 3. Mercury Hg 2+

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

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

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