1. Composition of Air
The Earth is surrounded by a blanket of air which we call the atmosphere. Air is an important natural resource. Not only do we use oxygen in the air to breathe, we also extract many gases from the air to
use as raw materials in industry.
What does air consist of? Air is a mixture of several gases. It contains elements and compounds that are needed by all living
things. As air is a mixture, its composition varies from time to time and from place to place. Table 1 shows the composition by volume of a typical sample of clean air.
Gas Composition by volume
Nitrogen 78 – 79
Oxygen 20
Carbon dioxide 0.03
Water vapour 0 – 5
Noble gas : argon
neon and helium 0.9
0.002
Table 1 Composition by volume of clean air
The main gases in the air are nitrogen and oxygen. The rest are the noble gases mostly argon, carbon dioxide and water vapour. The amount of water vapour is air can vary widely around the world, from
practically near 0 in a desert to about 5 in a tropical forest.
Air over a busy city also contains toxic gases such as carbon monoxide and sulphur dioxide. Air over a forested area is cleaner.
Fractional Distillation of Liquid Air
Separating air into its constituent gases is an important process, especially for obtaining nitrogen and oxygen. These two gases are widely used in industries. Air is first cooled and compressed into liquid.
Liquid air is then separated into its constituents or fractions by fractional distillation Fig 20.1. In fractional distillation, the liquid with the lowest boiling point b.p. distils over first. In this case,
nitrogen is distilled over first.
Fig 1 The fractional distillation of liquid air
Key Ideas
Test Your Self
1. Air is a mixture. The composition by volume of clean air is
78 – 79 nitrogen, 20 oxygen,
1 noble gas mainly argon 0.03 carbon dioxide
variable amounts of water vapour 2. Air can be separated into its constituent by the
fractional distillation of liquid air.
Worked Example 200 cm
3
of air in syringe A was made to pass over heated copper until the reaction was complete
The gas in syringe B was then allowed to cool to its original temperature. What is the volume of gas collected in syringe B?
A. 80 cm
3
B. 120 cm
3
C. 160 cm
3
D. 400 cm
3
Thought Process Copper reacted with oxygen in the air to form copper II oxide.
copper + oxygen →copper II oxide 2 Cus + O
2
g →2 CuOs Since oxygen makes up 20 of air, maximum volume of oxygen that
will react = 20 of 200 cm
3
= 40 cm
3
Therefore, the volume of gas collected in syringe B = 200 cm3 – 40 cm3 = 160 cm3
Answer C
Questions 1. What will you observe in the a U-tube and b limewater when
air is passed through the apparatus as shown here for a prolonged period? What conclusions about air can you draw from this
experiment?
2. In the fractional distillation of liquid air, which gas is distilled over first? Why?
2. Air Pollution Clean air is essential to life. Unfortunately, much of the air we breathe in is not clean. It contains
chemicals that may be harmful to us. Air pollution is defined as the condition in which air contains a high concentration of certain chemicals that may harm living things or damage non-living things.
Common Air Pollutants Air pollution is caused by solid particles and poisonous gases in the air. These substances are called
air pollutants. These pollutants include carbon monoxide, oxides of nitrogen and sulphur dioxide.
Carbon monoxide Carbon monoxide CO is a poisonous gas that is colorless and odorless. It is produced by the
incomplete combustion of carbon-containing fuels. Much of the carbon monoxide in the air comes from the incomplete combustion of petrol in car engines.
Oxides of nitrogen Oxides of nitrogen NO, NO
2
are produced in two ways: 1. In a car engine or chemical factory, where the temperature is very high, nitrogen combines with
oxygen in the air to form nitrogen monoxide NO. Nitrogen monoxide is also called nitric oxide. nitrogen + oxygen → nitrogen monoxide
N
2
g + O
2
g → 2 NOg Nitrogen monoxide, a colorless gas, reacts with oxygen to form a brown gas, nitrogen dioxide
NO
2
nitric oxide + oxygen → nitrogen dioxide 2 NOg + O
2
g → 2 NO
2
g 2. During thunderstorms, the heat energy released by lightning causes nitrogen and oxygen in the air
to react to form oxides such as nitrogen monoxide and nitrogen dioxide. These oxides dissolve in water to form acid rain. You will learn more about this later.
Sulphur dioxide
heat The major source of sulphur dioxide SO2 is the combustion of fossil fuels such as coal, crude oil
petroleum and natural gas. Fossil fuels contain sulphur. When they are burnt, sulphur is converted to sulphur dioxide.
sulphur + oxygen sulphur dioxide
Ss + O
2
g SO2g
Sulphur dioxide is also produced in large quantities during volcanic eruptions.
Other Pollutants Besides carbon monoxide, oxides of nitrogen and sulphur dioxide, other air pollutants include unburnt
hydrocarbons, methane, lead and ozone. Unburnt hydrocarbons and lead are released in car exhaust. Methane is colorless and odorless gas produced when plant and animal matter decay. It is also
produced from the decay of rubbish and landfills.
Methane is a greenhouse gas. You will learn more about greenhouse gases in section 20.4.
Effect of Air Pollution Air pollution affects human health and the environment in a number of ways. Table 2 gives a
summary of air pollutants present in the environment and their effects. Acid rain in particular is quite harmful.
Air pollutants Source
Effects Carbon
monoxide CO Incomplete
combustion of fuels such as petrol
CO react with hemoglobin in blood to form carboxyhemoglobin. As a result, hemoglobin cannot
transport oxygen to the rest of the body. CO causes headaches, fatigue, breathing difficulties and
even death. Sulphur dioxide
SO
2
From volcanoes and combustion of
fossil fuels These gases irritate the eyes and cause breathing
difficulties by irritating the lungs. High levels of SO
2
and oxides on nitrogen also lead to inflammation of the lungs bronchitis.
SO
2
and NO
2
from acid rain, which destroys buildings, aquatic life and plants.
Oxides of
nitrogen NO, NO
2
Exhaust fumes from
vehicles, chemical plants and
lightning
Table 2 Effects of air pollutants of human health and the environment.
Acid Rain Acid rain is formed when acidic air pollutants such as sulphur dioxide and nitrogen dioxide dissolve
in rainwater. Sulphur dioxide dissolves in water to form sulphurous acid H
2
SO
3
sulphur dioxide + water sulphurous acid
SO
2
g + H
2
Ol H
2
SO
3
aq In the presence of oxygen in the air, this acid is slowly oxidised to sulphuric acid H2SO4
heat
Fig. 2 The pH value of acid rain
Oxides of nitrogen also contribute to acid rain. In the presence of oxygen and water, nitrogen is converted to nitric acid.
nitrogen dioxide + water + oxygen nitric acid
4 NO
2
g + 2 H
2
Ol + O
2
g 4 HNO
3
aq The pH value of unpolluted rainwater is usually slightly below 7. This is because carbon dioxide in
the air dissolves in rainwater to from carbonic acid, which is a week acid.
carbon dioxide + water carbonic acid
CO
2
g + H
2
Ol H
2
CO
3
aq However, acid rain is much more acidic than rain that only
contains carbonic acid. Acid rain has a pH value of 4 or less. Fig. 20.2 shows the pH value of acid rain compared
with the pH values of uncontaminated rainwater and other common substance. In some extreme cases, for example in
heavy industrial areas, acid rain can be more acidic than vinegar
What are the effects of acid rain on buildings, plants and aquatic life?
Acid rain reacts with metals and with carbonates in
marble and limestone. When this happens, metal bridge and stone buildings are damaged.
Acid rain can reduce the pH value of natural water bodies from between 6.5 to below 4, which kill fish and
other aquatic life. Acid rain also leaches important nutrients from the soil and
destroys plants. Without these nutrients, plant growth is stunted. In some cases, acid rain dissolves aluminium
hydroxide AlOH
3
in the soil to produce Al
3+
ions, which are toxic to plant. Forest throughout most of Central and
Eastern Europe have been destroyed in this manner by acid rain.
Key Ideas
1. Common air pollutants and their sources:
Air Pollutants Sources
Oxides lightning activity
inside combustion engines of cars Carbon monoxide
incomplete combustion of carbon-containing substances, e.g. charcoal, wood, petrol, etc.
Sulphur dioxide combustion of fossil fuels in motor vehicles, power stations and
factories volcanic eruptions
2. Other air pollutants are methane, lead, ozone, and unburnt hydrocarbons.
3. Carbon monoxide can cause breathing difficulties and even death. 4. Nitrogen dioxide and sulphur dioxide in the air react with rainwater to form acid rain.
5. Air pollution can have harmful effects on buildings, human health, plant life and aquatic life.
Test Yourself
Worked Example
A sample of air in city was found to contain the following gases: oxides of nitrogen, sulphur dioxide and carbon monoxide.
Which of these gases
a do not corrode metal structure? b are produced during lightning activity?
Thought Process
a Acidic gases will corrode metal structures. Non-acidic gases will no corrode metal structures. Carbon monoxide is a neutral oxide and is therefore a non-acidic gas.
b The atmosphere contains nitrogen and oxygen. These two gases will only react at high temperatures to form nitrogen oxides.
Answer
a Carbon monoxide b Oxides of nitrogen
Questions
1. Explain the following sentences observations: a When clean air is bubbled through pure water, the pH of the water changes gradually.
b Sitting in a parked car with all the windows closed and the engine running can cause death. 2. a Identify X in the following equations:
SO
2
g + water Xaq
Xaq + O
2
g in air H
2
SO
4
aq b Balance the following equation:
SO
2
g + H
2
Ol + O
2
g H
2
SO
4
aq c Explain why acid rain damages buildings and causes farmland to become less fertile.
Depletion of The Ozone Layer
Ozone was discovered in 1840. It was first detected in the upper layer of the atmosphere the stratosphere in 1889. It is produced by the photochemical reaction between oxygen molecules and
oxygen atoms in the atmosphere.
oxygen atom + oxygen ozone
Og + O
2
g O
3
g When ozone is formed at low altitudes, it can cause severe pollution problems. However, in the
stratosphere, ozone becomes very important to us. How is this so?
There is actually a thin layer of ozone in the stratosphere, about 20 – 50 km above the Earth. Here, the ozone acts as a kind shield, or giant sunscreen, filtering out some the harmful ultraviolet radiation
from the sun. It this radiation reaches the surface of the Earth, there could be a drastic increase in the number of cases of skin cancer, genetic mutations and eye damage e.g. cataracts being formed.
Ultraviolet radiation may also be harmful to marine life.
Since 1976, there has been an alarming decrease in the amount of ozone in the stratosphere over the South Pole. In recent years, a similar phenomenon has been occurring over the North Pole.
How is ozone different from oxygen? Ozone is an allotrope of oxygen. Unlike oxygen, which has two atoms in each molecule, ozone has
three atoms of oxygen per molecule. Therefore, its molecular formula is O
3
. Ozone is a pale blue, almost colorless gas with a characteristic odor. In small concentrations, it is non-toxic, but in
concentrations above 100 ppm parts per million, it is toxic. Breathing in air that contains high concentrations of ozone can be dangerous, especially for people with asthma.
What is causing the depletion of ozone in the stratosphere? Scientists have discovered that the depletion of the ozone layer is caused by chlorofluorocarbons.
Chlorofluorocarbons, commonly called CFCs, are compounds containing the elements carbons, fluorine, and chlorine. CFCs were widely used as propellants for aerosols and as coolants in
refrigerators and air conditioners. They were also used in the manufacture of packing foam.
Within the last decades, large amounts of CFCs have been released into the atmosphere. CFCs are very stable and can remain in the atmosphere for a very long time. Over the years, they slowly diffuse
through the air and react with ozone, destroying the ozone layer.
Is there a solution to the problem of ozone depletion? Many countries have now agreed to ban the use of CFCs. In 1992, an international agreement was
reached for a complete ban on the release of CFCs by 1996. Until now, most of the countries in the world have completely banned the use of CFCs. However, even if the use of CFCs is totally stopped
at once, the depletion of the ozone layer will continue for many years due to the CFCs already present in the atmosphere.
Key Ideas
1. The ozone layer filters out some of the harmful ultraviolet radiation from the sun. 2. Chlorofluorocarbons CFCs react with ozone in the stratosphere and cause the depletion of the
ozone layer. 3. The increase in ultraviolet radiation reaching the Earth could lead to increased case of skin cancer,
genetic mutations and eye damage.
3. Reducing Air Pollution
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