Components of atmosphere :

Water Vapour : 
Moisture is present as water vapour in the atmosphere. Water vapour rise high up into the atmosphere and get cooled to form clouds. These bring  rain both on land and oceans. Snow is produced in cooler regions. The water vapour control the climatic conditions.
The presence of  moisture can be felt in our daily life.
When air comes in contact with a cool surface, it condenses and drops of water appear on the cooled surface. This shows that water is present in air.

Oxygen : Oxygen is extremely important for the survival of organisms as organisms respire in it. Requirement of oxygen is an essential condition for burning of a substance hence it plays an important role in combustion.
The presence of  oxygen in air can be felt by a simple experiment.

Activity –2
AIM  : To show the presence of oxygen.
Material required : (i) two candles (ii) two shallow containers (iii) two glasses of different size 
(iv) water
Mathod : Fix two small candles of the same size in the middle of two shallow containers. Now, fill the containers with some water. Light the candles and then cover each one of them with an inverted glass (one much taller than the other) as shown in the figure.

Burning can occur only in the presence of oxygen. We see that, one component of air is oxygen. Now, the amount inside each glass in our experiment, is limited. When most of this oxygen is used up by the burning candle, it can no longer burn and blows out. Also, water rises up in the glass once the candle blows out.

Nitrogen : In  the above activity we observe that a major part of air is still present in the glass bottle even after the candle blew out. This indicates the presence of some component in the air, which does not support burning. 
It is an important constituent of living organisms. It cannot be utilized by living organisms in elemental form. The elemental nitrogen is converted into its usable compounds like nitrate by nitrogen fixation processes. 
Carbon dioxide :  In a closed room, if there is some material that is burning, we feel suffocation. This is due to excess of carbon dioxide that may be accumulating in the room, as the burning continues. Carbon dioxide makes up a small component of the air around us. Plant and animals consume oxygen for respiration and produce carbon dioxide. Plant and animal matter on burning, also consume oxygen and produce  mainly carbon dioxide and a few other gases.

It occurs in carbohydrates, fats, proteins, nucleic acids, enzymes and hormones. Carbon dioxide molecules reflect back the heat radiations and help the earth not to radiate heat very rapidly at night. Carbon dioxide partially dissolves in water and helps in the formation of carbonate salts. These give taste to natural water. Carbon dioxide is essential for  photosynthesis.

Dust and smoke : The burning of fuel also produces smoke. Smoke contains a few gases and fine dust particles and is often harmful. 
Dust particles are always present in air.