1- contents of the air

8/13/2019 1- Contents of the Air

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THE AIR AROUND US


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CONTENTS OF THE AIR

Air is matter in the form of gas. It supportsall life forms on earth.

Air is mixture of nitrogen, oxygen, carbon

dioxide and inert gases. Air is invisible because air is colourless.

However moving air can be felt and is

known as wind. Earth is surrounded by layer of air called

atmosphere.


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The composition of air

The composition of

air

The average percentage of its

components by volume

Nitrogen

78%

Oxygen 21%

Carbon dioxide 0.03%

Inert gases and

others

0.97%


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Nitrogen is the largest component of air

(78%).

Oxygen is the second largest component

of air (21%).

Oxygen is needed for respiration.

Carbon dioxide constitutes 0.03% of air.

Carbon dioxide is needed by green plants

for photosynthesis.

Inert gases are unreactive; helium, argon,

xenon, neon, krypton and radon.


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Air also contains water vapour, microorganismsand dust.

Water vapour :- The percentage of water vapour in air is known

as humidity.

Air humidity increases when the content of watervapour in air increases.

Dust :-

Dust in air originates from particles of soil and

smoke emitted by factories and vehicles. Microorganisms :-

Bacteria, viruses and fungi are found in air.


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Activity 1 : To show the percentage of oxygen

in air

Procedure

1. The space above the

water level in a gasjar containing air wasdivided into 5 equalparts.

2. A light candle wasplaced inside the gas

jar


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Observation

The flame of the candle goes out after a little

while.

The water level rises to the level marked 1


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Activity 2 : To show that air contains water

vapour

Procedure

The external surface of thebeaker was wiped with a drycloth.

Some ice cubes were addedinto the beaker

The beaker was left on thetable for 5 minutes.

The droplets of colourlessliquid that form outside the

beaker are tested with a bluecobalt chloride paper.


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Observation

Droplets of the liquid are found on the

external surface of the beaker. The colourless liquid changes the colour of

cobalt chloride paper from blue to pink


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Activity 3 : To show that air contains

microorganisms

Procedure

1. Two petri dishes containingsterilised nutrient agar wereprepared.

2. One of the petri dishes was

sealed with cellophane tape.3. The other petri dish was

exposed to the air for 3 hoursbefore being covered andsealed.

4. The nutrient agar in the petridishes were observed withmagnifying glass after 3 days


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Observation

There is no colony of microorganisms on the

sterilised nutrient agar that was not exposedto the air.

There are many colonies of microorganisms

on the sterilised nutrient agar was exposedto the air for 3 hours.


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Activity 4: To show that air contains dust

Procedure

Four strips of cellophanetape with the stickysurface facing upwards

as shown. The glass slides were

placed in four differentplaces.

The cellophane tapewas observed after 2hours by usingmagnifying glass.


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Observation

Cellophane Location Observation

P In a covered petri

dish

No dust seen

Q In the laboratory A little dusty

R In the school field Dusty

S Beside the road Very dusty


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Why is air a mixture?

The composition of air is not fixed.

It changes according to :

1. The time of the day

2. The conditions of the environment3. Humans activities


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Example,

- The air in a place which is crowded with people

has a higher content of CO2than the air in a lesscrowded place.

- The air in a jungle has a higher content of oxygen

than the air in an industrial area.

All these examples show that the percentage of

the components of air is not always the same.

Therefore, we can say that air is a mixture.

1- contents of the air

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