1. Hardness in Water

2. 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.

3. 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]

4. How do calcium ions get into the water?

5. 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

6. 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

7. 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

8. 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.

9. Removal of Hardness

10. (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

11. 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

12. 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

13. (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)

14. Making water safe to drink
Water Treatment
Making water safe to drink

15. 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.

16. Steps involved in purifying our water

17. 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

18. 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.

19. 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]

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

21. 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

22. 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

23. 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

24. 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

25. 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

26. (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.

27. 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.

28. 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.

29. 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.

30. 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]

31. Heavy Metal Pollution

32. 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

33. 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”

34. 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

35. 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

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

37. Water Analysis
Instrumental Methods

38. 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

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

40. 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

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

42. 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

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

44. 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

45. Gas Chromatography

46. 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

47. 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

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

49. 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

50. Social and Applied Aspects
Identification of plastics
Identification of drugs

51. 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

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

53. 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

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