Environmental Chemistry Flashcards
hard water
soft water
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
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(g) + H2O(l) = H2CO3(aq)
its a weak acid that reacts with limestone a calcium carbonate
to form calcium hydrogen carbonate which is soluble and dissolves in the water.
CaCO3(s) +H2CO3(aq) =Ca(HCO3)2
Types of Hardness
temporary
Hardness that can be removed by boiling
Caused by dissolved calcium [or magnesium] hydrogen carbonate
water is heated and it breaks down to form calcium carbonate called limescale or 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
Limescale (CaCO3) = ‘furring’
In kettles, washing machines and boilers.
Limescale is a bad conductor of heat so it also makes the boilers inefficient
Blocking pipes in boilers and washing machines
Permanent Hardness
NOT removed by boiling Caused by Calcium sulphate (CaSO4) Magnesium sulphate (MgSO4) Calcium chloride (CaCl2) Magnesium chloride (MgCl2)
Removal of Hardness
Boiling only removes temporary hardness
Ion exchange
Removes both Temporary and Permanent Hardness
Deionized water
VS
Distilled water
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
Steps involved in purifying our water
- screening
Water is collected in reservoirs, from rivers and from wells - flocculation
AL2SO4 (flocculating agent) Al2SO4 makes them clump together and sink to the bottom - sedimentation
mixture passes through a series of tanks
It moves very slowly
This allows flocculated particles time to sink - filtration
Water is now passed through a filter bed of sand on top of gravel
Removes any very small particles that did not sediment out - additives
chlorination- add chlorine to kill bacteria
fluorination- add fluoride for strong teeth
pH -
too acid - NaOH added
(i) Acid corrodes pipes
(ii) Brings poisonous lead ions (Pb2+) into solution
too alkaline CO2 added
Dangers of adding Too Much additives
- Chlorine
poisonous
bad smell and tase - Fluoride
poisonous
bad smell and tase - CO2
pH too low
Corrosion (Pb dissolving)
Tooth decay - NaOH
pH too high
Causes hardness
storage
underground to prevent recontamination by
dead bodies
rubbish
rat urine
sewage treatment
primary treatment
Screening- removing lumpy bits
Sedimentation
secondary treatment
biological oxidation - aerobic microorganisms digest the sewage and destroy pathogens
tertiary treatment
precipitation
ion exchange
eutrophication
Excess plant growth caused by excess nutrients in water
sources of nutrients
- Fertilizers washed off land
- Silage effluent
- Pig slurry
- Sewage
BOD
Amount of oxygen consumed in p.p.m. (mg l-1) when sample kept in the dark for five days at 20oC
difference between the two oxygen levels is the B.O.D.
