10.5
Acid rain 
the invisible threat
p.29
What is the pH value of rainwater?
Why is rainwater acidic?
pH value of rainwater is about 5.6
Carbon dioxide +
(acidic gas)
rainwater
rain
(with carbonic acid)
weak acid
Acid rain: pH value is
lower than 5.6
Cause of acid rain
sulphur dioxide + oxygen +
p.29
rainwater
(acidic gas)
Air pollutants
nitrogen oxides
(acidic gas)
sulphuric
acid
(strong acid)
+
rainwater
nitric acid
(strong acid)
Sulphuric acid and nitric acid make pH
value of rainwater lower than 5.6
Vehicles、factories and
power stations burn coal
and crude oil and so acidic
gases are formed.
- Coal and crude oil contain
sulphur.
sulphur
+ oxygen
sulphur
dioxide
When power stations and vehicles
burn these fuels, sulphur dioxide will
be released.
- Under high temperature of burning,
nitrogen and oxygen in air will have
chemical reaction and nitrogen oxides will
be formed.
nitrogen + oxygen
nitrogen oxides
Parthenon (巴特農神殿) is made of
marble (大理石).
After 24 centuries, it was only a
little corroded.
The speed of corrosion increased rapidly in
the recent 30 years. Some parts of the
building collapsed because of acid rain.
Some natural phenomena may also
form acidic gases.
Volcanic explosion
releases sulphur dioxide
sulphur
+ oxygen
sulphur
dioxide
Lightning forms
nitrogen oxides
nitrogen + oxygen
nitrogen oxides
p.29
What are the effects of acid rain on
humans and the environment?
Effects of acid rain
p.30
Corroding
buildings
Corroding
metals
Damaging
plants
Killing aquatic
life
Corroding buildings
- From the results of Experiment Centre 10.9
and 10.10, we know that acids are corrosive
to metals and calcium carbonate.
=> Acid rain corrodes buildings
or statues made of metals or marble.
lime
- If the rain is more water
metal
acidic, the speed of
corrosion will be higher.
dilute hydrochloric acid
Damaging our forests
- Acid rain:
=> dissolves some essential nutrients in soil.
=> affects absorption of required nutrients
of the plants.
=> affects healthy growth of plants and kills
them.
In the New Territories, some farmers
found that leaves of vegetables turn
yellow and ‘get burns’ after raining.
Killing aquatic life
- Acid rain falls into rivers and lakes
and makes the water acidic.
- When the pH value of water is
below 5, most fishes will die.
- Besides, acid rain:
=> dissolves some toxic minerals in
soil.
=> leads the toxic minerals flowing
into rivers and lakes.
=> kills aquatic life and destroys
eggs of fishes.
Effects of acid rain on humans
- Acid rain may cause respiratory
diseases and skin allergy.
Rainwater with the
highest acidity
In 1983, pH value of
acid rain in Scotland is
as low as at 1.87.
More acidic than lemon juice!
Is the acid rain problem serious?
p.33
- Acid rain is not new.
- Cities
continuously.
It wasand
firstindustries
observeddevelop
100 years
ago
=>
Damaging
effect
acid rain is more
when
factories
wereofconcentrated
in
obvious.
European cities.
=> Concentration of air pollutants increases
- The following map shows
‘Global acid rain pollution’ problem:
pH lower than 4.2
pH 4.2 - 4.5
pH 4.6 – 5.0
region with mild acid rain
Is there any acid rain problem in
Hong Kong?
The following questions may help you to
find the answer.
> What is the main source of air pollutants in
Hong Kong?
> What is the situation of air pollution in China?
> Will air pollutants in China affect Hong Kong?
How can we control acid rain?
p.33
- To control acid rain, we should take
appropriate measures to reduce the formation
of acidic gases.
What are the methods to reduce the
formation of acidic gases?
Methods of reducing acidic gas exhaust:
1 Save resources and reduce burning
of fossil fuels.
2 Use fuel with lower sulphur content.
3 Use the energy sources without
producing acidic gases.
e.g. solar power, wind power...
4 Remove the acidic gases after burning.
e.g.catalytic converter in cars
catalytic converter
5 Educate the public on effects of acid rain
and the methods of reducing acid rain.
- Acid rain problem in Hong Kong is not a
local problem, but a regional problem.
- The HKSAR government and the Guangdong
Provincial government join together to
control acid rain.
e.g. Set air pollutant emission
reduction targets
=> Reduce the regional emission of
sulphur dioxide by 40% and nitrogen
oxides by 20% before 2010.