ICSE Class 7 Chemistry Chapter 04 Air and Its Constituents

4 Air And Its Constituents

There is a thick blanket of air around the earth. Life on earth would not have been possible without air, which is actually a mixture of gases. In this chapter we will study about air and its constituents.

The Composition Of Air

Air contains about 78% nitrogen and 21% oxygen by volume. The remaining 1% consists of argon, carbon dioxide, water vapour and other gases. Normally, air also contains dust and smoke. The average composition of dry air is given in Table 4.1.

Table 4.1 The average composition of dry air

Air Is A Mixture, Not A Compound

That air is a mixture and not a compound is clear from the following facts.

1. As you know, the composition of a compound is fixed, but that of a mixture is not fixed. The composition of air is not fixed. It varies with location and season. For example, the proportion of carbon dioxide in air is greater in cities than in villages. The moisture content of air is greater in the rainy season than in other seasons.

2. The constituents of a compound retain their properties. So do the constituents of air. For example, oxygen present in air continues to support combustion.

3. The constituents of air can be separated by the fractional distillation of liquefied air.

4. An artificially prepared mixture of oxygen, nitrogen, carbon dioxide, etc., in the same proportion as in air, behaves in the same manner as air.

Pollution Of Air

In urban and industrial areas, the air contains substances that are harmful to us and our environment. Such air is said to be polluted, and the harmful substances present in air are known as air pollutants. The common pollutants of air are:

- Carbon monoxide (CO)

- Oxides of sulphur (SO₂ and SO₃, collectively represented as SOₓ)

- Oxides of nitrogen (NO and NO₂, collectively represented as NOₓ)

- Chemicals called chlorofluorocarbons (CFCs) used as coolants and in aerosol sprays

- Dust and soot (collectively known as suspended particulate matter-SPM). Soot is unburnt particles from fuels, which give smoke its colour.

Air pollutants come from various sources, as mentioned in Table 4.2.

Table 4.2 Sources of the chief pollutants of air

*The complete combustion of a fuel containing carbon gives CO₂, which is not poisonous.

2C + O₂ - 2CO Incomplete combustion

C + O₂ - CO₂ Complete combustion

Effects Of Air Pollution

Effect On Health

Air pollution causes many health problems.

- The oxides of sulphur and nitrogen cause breathing problems and respiratory infections. They also trigger asthma attacks.

- Carbon monoxide hampers oxygen transport in our body. In the lungs, oxygen gets attached to the haemoglobin present in the blood. From there it gets carried to the other parts of the body with blood. When carbon monoxide is inhaled, it combines with haemoglobin to form carboxyhaemoglobin. As a result, less haemoglobin is available for transporting oxygen. This causes weakness, headaches and, in extreme cases, death.

- The chlorine present in CFCs converts ozone to oxygen. CFC pollution is, therefore, reducing the amount of ozone in the ozone layer of the atmosphere. This layer shields us from the harmful effects of the ultraviolet (UV) rays of the sun. The decrease in ozone has led to an increase in skin cancer and eye problems.

- Very small particles that pollute the air get inhaled by us. These particles irritate and damage the lungs. They cause frequent attacks of asthma and bronchitis.

Acid Rain

The oxides of sulphur and nitrogen present as pollutants in the air react with the water present in the atmosphere to form acids. These acids come down with rain, making the rainwater significantly acidic. Such rain is called acid rain. The acids found in acid rain are formed as follows.

SO₂ + H₂O - H₂SO₃

sulphur dioxide water sulphurous acid

2H₂SO₃ + O₂ - 2H₂SO₄

sulphurous acid oxygen sulphuric acid

3NO₂ + H₂O - 2HNO₃ + NO

nitrogen dioxide water nitric acid nitric oxide

In some industrial areas, hydrogen chloride gas is present in the air as a pollutant. It dissolves in water to form hydrochloric acid, which comes down with the rain.

Effects Of Acid Rain

1. The water of lakes and rivers becomes acidic and, thus, unsuitable for aquatic plants and animals.

2. The soil becomes acidic and, therefore, unsuitable for cultivation.

3. Sculptures, monuments and buildings are eroded.

Teacher's Note

Acid rain is a real environmental problem in industrial regions around the world, affecting local water sources and agriculture. Students can observe how rainwater pH varies seasonally in their own area.

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