ICSE Class 7 Chemistry Chapter 4 Air and Its Constituents

Air And Its Constituents

In This Chapter You Will Learn:

Constituents of air; air as a mixture; the importance of the gases present in air; air pollution and acid rain

Occurrence of oxygen

Preparation of oxygen; properties of oxygen; burning (formation of oxides) and rusting; tests and uses of oxygen; regeneration of oxygen in the atmosphere

Occurrence of carbon dioxide; preparation of carbon dioxide; properties of carbon dioxide; importance of carbon dioxide; green house effect and global warming.

Introduction

You know that, among food, water and air the most essential substance for the survival of life is air. It is used for respiration by all kinds of living beings.

Air helps in burning and producing heat energy.

Air is invisible and transparent because it is a mixture of colourless gases.

Although we cannot see air, we can feel its presence.

Occurrence

Air occurs in the atmosphere which surrounds the earth and extends to about 320 km above its surface.

Air also occurs in water in dissolved state which helps aquatic plants and animals to survive.

Air - A Mixture Of Gases

4.1 Constituents Of Air

The main constituents of air are nitrogen and oxygen. It also contains carbon dioxide and water vapour in small amounts. Inert gases and ozone are also present in trace amounts in air.

Apart from these gases, air also contains some impurities, like sulphur dioxide, hydrogen sulphide, nitrogen dioxide, dust particles, etc.

Teacher's Note

Air composition varies slightly based on location and weather patterns, which is why some areas experience different air quality on different days.

Table 4.1 Percentage Proportions Of Gases In Air By Volume

In ancient times air was considered as an element but now it has been proved that air is a mixture of gases. This was proved by Antoine Lavoisier in 1774 with the help of an experiment.

Lavoisier's Experiment To Show That Nitrogen And Oxygen Are The Main Constituents Of Air

Lavoisier took some mercury in a retort and heated it. The other end of the retort was connected to a bell jar containing air. The bell jar was kept in an inverted position over mercury contained in a trough as shown in the figure below.

The following were his observations:

1. A red layer of mercuric oxide was formed on the hot surface of mercury in the retort.

2. The level of mercury in the trough rose by 1/5 th of the total volume of the bell jar.

The following were his conclusions:

1. The 1/5 th portion of air in the bell jar was used up by mercury in the retort to form the red substance. This air was active air. This could be re-obtained when the red substance was heated. This gas supported burning better than air and also supported life. Lavoisier named this active air as oxygen.

Mercury + Oxygen (heat) - Mercury (II) oxide (Red powder)

\[2\text{Hg} + \text{O}_2 \xrightarrow{\text{heat}} 2\text{HgO}\]

Mercury (II) oxide (heat) - Mercury + Oxygen

\[2\text{HgO} \xrightarrow{\text{heat}} 2\text{Hg} + \text{O}_2\]

2. The remaining 4/5 th portion of air in the bell jar was inactive air as it did not support burning of air. It was found that this inactive air neither supports combustion (burning) nor life. The 4/5 th air in the bell jar was tested by putting a burning candle into the gas. The flame was extinguished. It did not support life. The gas was named as azote meaning unsuitable for life. Later on Lavoisier named it as nitrogen.

From above it is clear that air contains 1/5 th by volume of oxygen and 4/5 th by volume of nitrogen, i.e., nitrogen and oxygen are present in air in the ratio of 4:1 by volume. The rest of the gases are in very small amounts.

Teacher's Note

Lavoisier's experiment demonstrates how scientists use controlled reactions to discover the composition of mixtures, a method still used in modern chemistry labs today.

Activity 1

To Show That Air Contains Carbon Dioxide

Take a test tube fitted with a two-holed rubber cork. Fit a long bent tube through one hole and a short bent tube through the other hole. Pour some lime water (calcium hydroxide solution) into the test tube.

Blow air by air pump through the long tube. You will observe that air bubbles through lime water and after some time the lime water turns milky.

This shows that air contains carbon dioxide.

Note: Lime water is used as a chemical test to identify carbon-dioxide gas because the two substances react to form an insoluble white solid calcium carbonate which turns the lime water milky.

\[\text{Lime-water} + \text{Carbon dioxide} \rightarrow \text{Calcium carbonate} + \text{Water}\]

\[\text{Ca(OH)}_2 + \text{CO}_2(g) \rightarrow \text{CaCO}_3\downarrow + \text{H}_2\text{O}\]

(aq)

(white precipitate)

Precipitate: An insoluble solid formed in water (solvent) is called a precipitate.

Activity 2

To Show That Water Vapour Is Present In Air

Take a glass tumbler. Fill it half with ice cold water. You will observe that fine water droplets get deposited on the outer wall of the glass tumbler. These droplets have certainly not passed through the material of the glass tumbler from inside.

These water droplets have come from air. Due to the cold surface of the glass tumbler, the water vapours present in air get condensed into droplets.

This proves that air contains water vapours.

There are enough evidences to show that air is a mixture and not a compound.

1. The composition of air varies from place to place and from time to time.

2. The components of air retain their individual properties.

3. Liquid air has no definite boiling point.

4. No energy exchange occurs when the components of air are mixed with each other.

5. Components of air can be separated by simple physical methods.

6. Air is not represented by a chemical formula as it is a mixture.

Teacher's Note

You can observe air composition changes daily - humid days show more water vapour condensation on cold surfaces, while dry days show less, helping us understand how air quality and humidity vary naturally.

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