Air

15. AIR

AIR

points to remember :

1. Constituents of air and their proportion : The constituents

of air are oxygen, nitrogen, carbon dioxide, water vapour and

some inert gases. Their proportion in air are nitrogen 78%,

oxygen 21%, carbon dioxide, water vapour, inert gases and other

constituents 1%.

2. Oxygen : When the mixture of potassium chlorate and

manganese dioxide in the proportion 5 : 1 (by weight) is heated

at 300 °C, oxygen gas is formed. Manganese dioxide acts as a

catalyst in the preparation of oxygen. Oxygen is a colourless,

odourless, tasteless gas. It dissolves sparingly in water. Oxygen

does not burn itself but supports burning. Oxygen is necessary

for the respiration of all living beings. Oxygen filled in cylinders,

is used as a breathing aid by deep-sea divers, mountaineers and

patients. The mixture of oxyhydrogen and oxyacetylene flames

are used for cutting and welding purposes.

3. Nitrogen : The proportion of nitrogen is maximum in the air.

The conversion of atmospheric nitrogen into compounds like

ammonia and nitrates is called nitrogen fixation. This fixation is

brought about by both biological and atmospheric factors.

Nitrogen is required for the growth of living beings. It is used for

producing fertilizers. In industry, it is used in the production of

ammonia and nitric acid. It is used in thermometers for

measuring very high temperatures.

4. Carbon dioxide : Carbon dioxide present in air is used by

plants during photosynthesis and oxygen is released into the air.

This process takes place in the presence of sunlight. The rising

proportion of carbon dioxide increases the danger of greenhouse

effect. Carbon dioxide is sparingly soluble in water. It is used in

cold drinks. Solid carbon dioxide is called dry ice. It is prepared

by applying pressure and cooling carbon dioxide to _57 °C.

5. Water vapour : Deliquescent substances absorb water vapour

from the air. This shows the presence of water vapour in the air.

Clouds and fog are formed by the condensation of water vapour.

6. Inert gases : The inert gases argon, helium and neon are

constituents of air. Argon is used in electric bulbs. Helium is

used to obtain very low temperatures. Neon is used in

advertisement signs.

7. Air pollution : Due to industrialization, the waste chemicals

from the factories in the form of gase, liquids and solids are

given out into the atmosphere, water and soil. When the

collection of such chemical substances at any place increases

beyond a certain level, the harmful effects can be seen. This is

called pollution. The contaminated water, dust, soot, chemicals,

gases released in the atmosphere by the burning of carbon and

sulphur, the harmful metals such as lead, aluminium, zinc,

acids, bases cause pollution. The atmospheric pollution causes

respiratory disorders, cancer of the intestines, disorders of

urinary bladder, high blood pressure, disorder of the eyes etc.

It leads to palpitations, breathlessness and psychological

disorders. Pollution affects both animals and plants. To reduce

pollution, efforts should be made to grow forests, treat chemical

waste before letting out into the atmosphere, avoid the

generation of harmful chemical substances, prevent the

concentration of harmful chemicals at one place and industrial

estates should be established away from cities.

8. Acid rain : The oxides of nitrogen and sulphur are emitted by

factories in the atmosphere. These oxides in the atmosphere

react with water droplets to form corresponding acids. Sulphur

dioxide forms sulphuric acid and nitrogen dioxide forms nitric

acid. These acids dissolve in rainwater and fall as acid rain.

Q. 1 Answer the following questions :

* (1) Name the constituents of air and give their proportion

in air.

Ans. The constituents of air and their proportion : Nitrogen

78%, Oxygen 21%, Carbon dioxide, water vapour and other

constituents 1%.

* About a hundred years ago, the proportion of carbon

dioxide in air was about 0.029%. Today it is 0.035%. What

is the reason?

Ans. The fuel burnt in the large number of factories

causes a continuous rise in the proportion of atmospheric

carbon dioxide. Hence, today the percentage of carbon dioxide

has increased to 0.035%.

(2) Describe the preparation of oxygen in the laboratory.

Fig. 15.1 : Preparation of oxygen

(2) The apparatus is arranged as shown in the figure. (3) The test

tube is heated to 300°C. The gas bubbles rise in the water filled gas

jar inverted over the water. (4) Oxygen displaces water in the gas

jar. As more and more gas is filled in the gas jar the level of water

decreases. When water is fully drained out, it shows that the jar is

full of gas. (5) As oxygen is slightly heavior than air, it is collected

by the downward displacement of water.

 (4) Give the physical properties of oxygen.

Ans. (1) Oxygen is a colourless, odourless, tasteless gas.

(2) It dissolves sparingly in water.

* (1) Why does the fire go out if water is thrown on the

burning wood?

Ans. When water is thrown on the burning wood it covers

the wood and separates it from oxygen of air and brings down

the temperature below the ignition temperature of wood.

Thus, the fire goes out.

* (2) Why does an electric light burn even when there is

no oxygen in the bulb ?

Ans. The filament (fine wire) in the bulb does not burn. It

gives out light because it becomes red hot. As electrical energy

is converted into heat energy, therefore, an electric light burns

even when there is no oxygen in the bulb.

(5) Give the uses of oxygen.

Ans. (1) Oxygen is absolutely necessary for the respiration of

all living beings. (2) Oxygen filled in cylinders is conveniently

used as a breathing aid by deep-sea divers and mountaineers.

(3) If a person cannot breathe, then oxygen is supplied artificially.

(4) The mixture of oxyhydrogen (2800°C) and oxyacetylene (3300°C)

flames are used for cutting and welding purposes.

Answer to the Question given on page 150 of the textbook

* What is the function of the blacksmith’s bellows ?

Ans. Bellows used by the blacksmith is a kind of pump.

By operating it, sufficient air is supplied to increase the

intensity of fire.

* (6) What is meant by nitrogen fixation ? How does it

happen?

Ans. The atmospheric nitrogen cannot be used directly by

living beings. The conversion of atmospheric nitrogen into

nitrogenous compounds like ammonia and nitrates which can be

used by living things is called nitrogen fixation. It is brought about

by both atmospheric and biological factors.

 (7) How does biological fixation take place ?

Ans. (1) There are two kinds of microorganisms that bring

about nitrogen fixation. One of these types of microorganisms are

found in the nodules on the roots of certain plants while the others

are found in the soil. Nitrogen from the air is absorbed by the

microorganisms present in the nodules and it is converted into

nitrogen compounds.

Nitrogen from the air is absorbed by the microorganisms

present in the soil and it is converted into ammonia, nitrous acid,

nitric acid which in turn forms nitrates. Thus, nitrogen in the air

becomes available to living things after biological fixation.

(8) How does atmospheric fixation take place ?

Ans. During lightning, atmospheric nitrogen combines with

the oxygen to form nitric oxide gas. This nitric oxide oxidises to

form nitrogen dioxide. Nitrogen dioxide dissolves in rainwater to

form nitric acid.

When it rains, the nitric acid is washed down to earth, where it

reacts with the salts in the soil to form soil nitrates. Thus, nitrogen

in the air becomes available to living things after atmospheric

fixation.

(9) How does the proportion of nitrogen in the atmosphere

remain constant ?

Ans. (1) Nitrogen in air is used by living beings through

nitrogen fixation. (2) Due to excretion, decay of living things and

burning, nitrogen compounds are decomposed. Thus, nitrogen gets

released into the atmosphere. This is how the proportion of

nitrogen in the atmosphere remains constant.

(10) Give the important uses of nitrogen.

Ans. (1) Nitrogen is used for producing fertilizers. (2) Plants

get nitrogen from the soil in the form of nitrates and fertilizers. This

helps plants to grow. Plants use this nitrogen to produce

proteinaceous substances. Animals or other living things use plants

as food for their growth. (3) In industry, nitrogen is used in the

production of ammonia and nitric acid. (4) Nitrogen is used in

thermometers for measuring very high temperatures.

* (11) Of what use is carbon dioxide to plants ?

Ans. Carbon dioxide from air is used by plants to prepare their

own food during photosynthesis and release oxygen into air. This

process takes place in the presence of sunlight.

* (1) How does carbon dioxide cause global warming?

Ans. The main cause of global warming is carbon dioxide.

Because of carbon dioxide, heat, instead of being given out, is

trapped in the atmosphere and the atmospheric temperature

rises.

* (2) Why does common salt becomes wet in the rainy

season ?

Ans. Common salt being a deliquescent substance

absorbs water vapour from the air. During rainy season, the

content of water vapour in the atmosphere is very high.

Therefore, common salt becomes wet in the rainy season.

* (3) A pouch of chemical powder is placed in some

bottles of medicines. What is this chemical? Why is it

placed there ?

Ans. Silica gel is placed in some bottles of medicines. The

constituents of some medicines are deliquescent. When a

medicine bottle is opened, due to water vapour from the air,

the medicine may get spoiled. To avoid this, silica gel pouch is

kept in a medicine bottle.

(12) What is meant by pollution ?

Ans. Due to industrialization, the waste chemicals from the

factories in the form of gaseous, liquid and solid state are given out

into atmosphere, water and soil. When the collection of such

chemical substances at any place increases beyond a certain level,

the harmful effects can be seen. This is called pollution.

(13) Name the substances which cause pollution.

Ans. Substances which cause pollution : The contaminated

water, dust, soot, chemicals, gases released in the atmosphere by

the burning of carbon and sulphur; the harmful metals such as lead,

aluminium, zinc, acids, bases, etc.

* (14) What ill-effects of pollution are being seen ?

Ans. (1) The atmospheric pollution causes respiratory

disorders, cancer of the intestines, disorders of the urinary bladder,

high blood pressure, disorders of eyes, etc. It leads to palpitations,

breathlessness and psychological disorders. (2) The pollution

affects plants. The fall in the photosynthesis production, falling of

leaves, absence of fruiting, these effects can be seen on plants.

(3) The pollution affects animals. The lifespan of animals, their

capacity for producing milk is seen to be decreasing.

* (15) Which measures are necessary for arresting

pollution ?

Ans. (1) Efforts should be made to grow forests to help in

reduction of pollution of air. (2) A law has been enacted making it

mandatory to treat chemical wastes before letting out into the

atmosphere, water and soil. (3) To avoid the generation of harmful

chemical substances, efforts should be made to make necessary

changes in the chemical processes themselves. (4) Precautions

should be taken to prevent the concentration of harmful chemicals

at one place. (5) To prevent the air pollution, industrial estates

must be established away from cities. The chimneys should be of

proper height. (6) To prevent air pollution, engines of motor

vehicles should be serviced regularly and maintained properly.

* (1) What dangers will we have to face if the

proportions of oxygen and nitrogen in the air undergo a

change?

Ans. If the proportion of oxygen increases there is the

danger of fires. If nitrogen increases there is the danger of

suffocation.

* (2) What is the use of atmospheric ozone?

Ans. Atmospheric ozone stops the harmful rays from the

sun and prevents them from reaching the earth.

(16) What is meant by acid rain ?

Ans. The oxides of nitrogen and sulphur are emitted by

factories in the atmosphere. These oxides in the atmosphere react

with water droplets to form the corresponding acids. Sulphur

dioxide forms sulphuric acid and nitrogen dioxide forms nitric

acid. These acids dissolve in rain water and fall down on the ground

as acid rain.

* (17) What function does an observatory perform?

Ans. An observatory records observations of atmospheric

factors such as temperature, pressure, velocity of winds and

humidity. It keeps a record of changes taking place in them and

forecasts the weather based on the changes observed.

Q. 2 Write short notes on : (1) Nitrogen fixation (2) Air

pollution.

(1) Nitrogen fixation :

Ans. See the answers to Q. 1 (6), (7), (8).

(2) Air pollution :

Ans. See the answers to Q. 1 (12), (13), (14), (15).

* Q. 3 Give scientific reasons :

(1) The danger from the greenhouse effect is rising.

Ans. (1) The amount of carbon dioxide in the atmosphere is

increasing due to increased population and industrialization.

(2) Carbon dioxide traps sun’s heat rays causing a rise in the

earth’s temperature. This is called greenhouse effect, Due to

increase in the levels of carbon dioxide, the danger from

greenhouse effect is also rising.

(2) The proportion of nitrogen in air remains constant.

Ans. (1) Nitrogen in air is used by living beings through

nitrogen fixation. (2) Due to excretion, decay of living things and

burning, nitrogen compounds are decomposed. Thus, nitrogen gets

released back into the atmosphere. This is how the proportion of

nitrogen in the atmosphere remains constant.

(3) When a bottle of an aerated drink is opened, the drink

inside gushes out in a stream of bubbles.

Ans. (1) Carbon dioxide dissolves sparingly in water.

Therefore, carbon dioxide has to be dissolved in water under

pressure. (2) Since cold drinks are prepared using carbon dioxide

and water, when a bottle of aerated drink is opened, the pressure is

released and carbon dioxide inside gushes out in a stream of

bubbles.

Q. 4 Answer the following questions in one sentence

each :

 Name the chemicals used in the preparation of oxygen.

Ans. Potassium chlorate and manganese dioxide are used in

the preparation of oxygen.

  State the proportion of potassium chlorate and

manganese dioxide in the preparation of oxygen.

Ans. The proportion of potassium chlorate and manganese

dioxide : 5 : 1 (by weight).

 How is oxygen collected ?

Ans. Oxygen is collected by the downward displacement of

water.

 Name the catalyst used in the preparation of oxygen.

Ans. The catalyst manganese dioxide is used in the

preparation of oxygen.

  Name the constituent which has maximum proportion

in air.

Ans. In air, of all the constituents, the proportion of nitrogen

is maximum.

 Name the gases used as mixtures in the form of flame

in welding.

Ans. In welding, the oxy-acetylene flame (mixture of

oxygen_acetylene) and oxy-hydrogen flame (mixture of

oxygen_hydrogen) are used.

Which gas is used in the production of fertilizers ?

Ans. Nitrogen gas is used in the production of fertilizers.

What is dry ice ?

Ans. Solid carbon dioxide is called dry ice.

How is dry ice prepared ?

Ans. Dry ice is prepared by applying pressure and cooling

carbon dioxide to _57°C.

State the use of dry ice.

Ans. Dry ice is used for preserving foodstuffs by keeping the

temperature very low. [Below 0°C]

Name the gas which gushes out of a bottle of an

aerated drink when it is opened?

Ans. Carbon dioxide gushes out of a bottle of an aerated drink

when it is opened.

  Name three inert gases present in air.

Ans. Inert gases in air : Argon, Helium and Neon.

State the use of argon.

Ans. Argon is used in electric bulbs.

State the use of helium.

Ans. Helium is used to obtain very low temperatures.

State the use of neon.

Ans. Neon is used in advertisement signs.

Name two nonmetals responsible for pollution.

Ans. Nonmetals responsible for pollution : (1) Carbon

(2) Sulphur.

Name two metals responsible for pollution.

Ans. Metals responsible for pollution : (1) Lead

(2) Aluminium.

Name two gases responsible for pollution.

Ans. Two gases responsible for pollution : (1) Carbon dioxide

(2) Sulphur dioxide.

Name two diseases caused by pollution.

Ans. Two diseases caused by pollution : (1) Cancer of

intestines. (2) Respiratory disorders.

Q. 5 State whether the following statements are True or

False. If a statement is false, correct it and rewrite :

(1) The main constituents of air are nitrogen, oxygen and

carbon dioxide.

(2) Oxygen is collected by the downward displacement of air.

(3) Living things need nitrogen for respiration.

(4) Oxygen is used for producing fertilizers.

(5) Nitrogen is an important constituent of proteins.

(6) Nitrogen and oxygen from air combine to form nitrogen

dioxide.

(7) Helium is used in electric bulbs.

(8) In industries, pollution is caused by contaminated water,

dust, soot and chemicals.

(9) Forests should be grown to help increase pollution of air.

(10) Acid rain causes trees to die and fall.

Ans. (1) True. (2) False. Oxygen is collected by the downward

displacement of water. (3) False. Living things need oxygen for

respiration. (4) False. Nitrogen is used for producing fertilizers.

(5) True. (6) False. Nitrogen and oxygen from air to combine to

form nitric oxide. (7) False. Argon is used in electric bulbs.

(8) True. (9) False. Forests should be grown to help reduce

pollution of air. (10) True.

Q. 6 Fill in the blanks :

*1. (1) Oxygen ……… burn, but ……… burning.

(2) Some ……… substances absorb ……… from the air.

(3) It is mandatory to treat …… before letting them out.

(4) Solid carbon dioxide is also called ……… .

Ans. (1) does not, supports (2) deliquescent, water vapour

(3) chemical waste (4) dry ice.

-- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --

2. (1) ……… is important for the growth of living things.

(2) ……… is used in electric bulb.

(3) Nitric oxide is oxidised to form ……… .

(4) Oxygen is found in air in the ……… state.

(5) Manganese dioxide is used as ……… in the preparation of

oxygen.

Ans. (1) Nitrogen (2) Argon (3) nitrogen dioxide (4) free

(5) catalyst.

Q. 7 Choose the correct alternatives and complete the

following statements :

(1) The survival of living things depends upon ………

(a) nitrogen (b) carbon dioxide

(c) water (d) nitrogen dioxide

(2) In the preparation of oxygen, the mixture of potassium

chlorate and manganese dioxide is used in the proportion

……… by weight.

(a) 5 : 1 (b) 4 : 1 (c) 1 : 4 (d) 1 : 5

(3) ……… is required for the growth of living beings.

(a) Carbon dioxide (b) Oxygen

(c) Nitrogen (d) Nitric oxide

(4) In advertisement signs, ……… gas is used.

(a) neon (b) helium (c) argon (d) oxygen

 (5) ……… is used to obtain very low temperature.

(a) Helium (b) Argon (c) Neon (d) Xenon

Ans. (1) water (2) 5 : 1 (3) Nitrogen (4) neon (5) Helium.

* Q. 8 Match the following :

‘A’ ‘B’

(1) Potassium chlorate

(2) Microorganisms

(3) Greenhouse effect

(4) Fertilizers

(5) Manganese dioxide

(a) Catalyst

(b) Nitrogen

(c) Carbon dioxide

(d) Oxygen

(e) Nitrogen fixation

Ans. (1) Potassium chlorate – Oxygen (2) Microorganisms–

Nitrogen fixation (3) Greenhouse effect – Carbon dioxide

(4) Fertilizers – Nitrogen (5) Manganese dioxide – Catalyst.

* Q. 9 Who am I ?

(1) I am found in clouds and mist.

(2) I am used in aerated drinks.

(3) I am used in electric bulbs.

(4) I form 78% of air.

(5) I am used in the production of nitric acid.

Ans. (1) Water vapour (2) Carbon dioxide (3) Argon

(4) Nitrogen (5) Nitrogen dioxide.