Composition of Air
From Priestley's experiment which was followed by Ingenhousz and Rutherford, we came to know that air was not just one substance. We will now describe what air is made up of. This is called composition of air.
The major component of air is nitrogen. Almost four – fifth of air is nitrogen. The second major component of air is oxygen. About one – fifth of air is oxygen. In addition to nitrogen and oxygen gases, air also contains small amount of carbon–di– oxide, water vapour and some other gases like argon, helium etc. The air may also contain some dust particles.
The composition of air in terms of percentage of its various components can be written as follows:
The composition of air changes slightly from place to place and also from season to season. For example,
* Air over industrial cities usually has a higher amount of carbon-di-oxide in it than the air over open spaces.
* Air in coastal areas may have more water vapour than inland areas.
* Air also contains more water vapour in rainy season.
* The amount of dust in the air is more in windy places than other areas.
Test for the presence of dust particles in air
You might have seen the sunlight entering into a dark room through a narrow slit and making shiny dust particles dancing merrily on the path of sunlight. Actually, the air in a room always contains some dust particles, but they are so small that normally they are not visible to us. When a beam of sunlight falls on them, the tiny dust particles become visible.
Shall we do an activity to calculate the amount of dust particles in air from our area?
Take a graph sheet. Using marker pens draw a 5x5 cm square on the graph. Apply a thin film of grease on the graph sheet. This sheet will serve as dust collector. Make four or five graph sheets.
Discuss in the whole class, as where to place the dust collectors, how long to collect dust particles and place the dust collectors in agreed positions.
Ensure that the dust collectors do not get blown away. After the time scheduled for performing this activity is reached, remove the paper and count the number of collected dust particles in the marked area in all the sheets, using a magnifying glass at the dust collector. We can see something similar to the diagram below:-
Then, calculate the mean number of dust particles in the marked area.
The range of the dust can also be calculated as given below:-
Range = Maximum value – minimum value
Collect details from all the areas where we have kept the dust collector sheets. Tabulate the recordings in the table given below:-
* Which area do you think will have the most dust?
* Which area do you think will have the least dust?
Test for water vapour in air
Take a few ice cubes in a glass. Keep it on the table for a few minutes. Observe what happens. You could see tiny droplets of water all over the outer surface of the glass. From where do these droplets come?
The water vapour present in the air condenses on the cold surface of the glass. This shows that air contains water vapour.