Chemistry Reference
In-Depth Information
coagulate into larger particles that separate more
rapidly. The sand and grit often contain large
amounts of useful chemicals which, by the action of
selected microbes, can be used as fertilisers.
3
The impure water is then removed and sent to a
trickling fi lter, where it is allowed to drain through
gravel on which microbes have been deposited.
These kill off any remaining bacteria in the water
by aerobic processes. This stage is known as
biological fi ltration.
4
The treated water is then chlorinated and returned
to a river, after checking.
5
Anaerobic bacteria digest what remains from the
other stages. Methane gas is produced, which can
be used as a fuel.
Water is a renewable resource but care must be
taken to avoid excessive water pollution, as this
could cause serious health issues such as typhoid
or cholera.
Questions
1
Make a list of four major water pollutants and explain
where they come from. What damage can these pollutants
do?
2
In the treatment of water for public use, state the purpose
of the addition of:
a
aluminium sulfate
b
chlorine
c
sodium hydroxide
d
sulfur dioxide.
3
Many industries use water as a coolant. Suggest the sorts
of problems that may be created by this 'thermal pollution'.
A summary of the main sources of atmospheric and
water pollution is shown in Table 11.9.
Table 11.9
Main sources of atmospheric and water pollution.
Pollutant
Source
Effects
Carbon dioxide, CO
2
Complete combustion of fossil fuels (p. 92)
CH
4
(
g
)
+
2O
2
(
g
)
→
CO
2
(
g
)
+
2H
2
O(
fi
)
Contributes to global warming (p. 212)
Carbon monoxide, CO
Incomplete combustion of fossil fuels (p. 95)
Poisonous, colourless, odourless gas
Oxides of nitrogen, NO
x
Car exhaust fumes (p. 182)
N
2
(
g
)
+
O
2
(
g
)
→
2NO(
g
) 2NO(
g
)
+
O
2
(
g
)
→
2NO
2
(
g
)
Oxides of nitrogen cause photochemical smog
Particulates, carbon C
Incomplete combustion of fossil fuels (p. 95)
Covers surfaces with carbon particles
Lead and lead oxides
Car batteries
Lead and its compounds are toxic and particularly
affect the nervous system
Lead compounds
Lead additives are added to petrol (use is decreasing),
resulting in exhaust fumes
Lead and its compounds are toxic
Chlorofl uorocarbons (CFCs)
Aerosol sprays and refrigerators (p. 221)
Destroy the layer of ozone around the Earth
Nitrates NO
3
−
and phosphates PO
4
3−
Fertilisers (p. 180)
Eutrophication*
Sodium phosphate
Detergents and water softeners
Eutrophication*
DDT*
Insecticides
Accumulates in food chain till eventually kills top
predators
Paraquat* and glyphosate*
Herbicides
Kill harmless plants
*DDT is an organochlorine herbicide. It has high toxicity to insects but has low toxicity to mammals. However, it is very chemically stable
so it accumulates in the environment and can become more concentrated in food chains. For this reason it is banned in many countries.
However, as it is cheap many developing countries continue to use it.
*Paraquat is a total herbicide. It destroys all green plant material. Paraquat is inactivated as soon as it touches the soil but it does kill any
plant whose leaves it touches.
*Glyphosate is a herbicide sprayed onto the outside of leaves and it then spreads through the plant to the roots. It is non-selective and
kills useful plants as well as weeds.
*Eutrophication occurs when fertiliser drains into lakes and rivers, causing algae to multiply rapidly and the water to turn green. It results
in fi sh and other organisms suffocating and dying through lack of oxygen in water.
