Samacheer Class 8 Science - Air

Physical properties of oxygen:
Oxygen is a colorless, odorless and tasteless gas.
It is a poor conductor of heat and electricity
Oxygen dissolves readily in cold water.
It is denser than air.
It can be made into liquid (liquefied) at high pressure and low temperature.
It supports combustion.

Uses of nitrogen:
Liquid nitrogen is used as a refrigerant.
It provides an inert atmosphere for conducting certain chemical reactions.
It is used to prepare ammonia (by Haber’s process) which is then converted into fertilizers and nitric acid.
It is used for inflating tyres of vehicles.

Nitrogen reacts with non-metals like hydrogen, oxygen etc., at high temperature to form their corresponding nitrogen compounds.
Non-metal + Nitrogen \(\underrightarrow { \Delta } \) Nitrogen compound
Example:

The increased green house effect is caused due to increase in the air pollutants and it results in the average increase of temperature of the atmosphere. This is called as Global warming.

Solid carbon dioxide, called as dry ice is used as a refrigerant.
The gas is so cold that moisture in the air condenses on it, creating a dense fog which is used in stage shows and movie effects.

When a limited amount of CO 2 is passed through lime water, it turns milky due to the formation of insoluble calcium carbonate.
Ca(OH) 2 + CO 2 → CaCO 3 + H 2 O
When an excess amount of CO 2 is passed through lime water, it first turns milky and the milkyness disappears due to the formation of soluble calcium hydrogen carbonate, Ca(HCO 3 ) 2 .

Carbon dioxide (CO 2 )
Sulphur dioxide (SO 2 )
Phosphorus trioxide (P 2 O 3 ) (or) Phosphorus pentoxide (P 2 O 5 )
Magnesium Oxide (MgO)
Iron Oxide (Fe 2 O 3 )
Sodium Oxide (Na 2 O)

1. Potassium combine with CO 2 to form potassium carbonate.
4K + 3CO 2 → 2K 2 CO 3 + C
2. When a limited amount of CO 2 is passed through lime water, it turns milky due to the formation of insoluble calcium carbonate.
Ca(OH) 2 + CO 2 → CaCO 3 + H 2 O
(Calcium carbonate)
Sodium hydroxide (base) is neutralized by carbon dioxide (acidic) to form sodium carbonate (salt) and water.

Acid rain affects us in many ways. Some of the consequences are given below.
It irritates eyes and skin of human beings.
It inhibits germination and growth of seedlings.
It changes the fertility of the soil, destroys plants and aquatic life.
It causes corrosion of many buildings, bridges, etc.
Preventive measures:
Acid rain and its effects can be controlled by the following ways.
Minimizing the usage of fossil fuel such as petrol, diesel etc.
Using CNG (Compressed Natural Gas).
Using non – conventional source of energy.
Proper disposal of the industrial wastes.

In soda bottle carbon dioxide gas is dissolved in water under pressure.
The gas in the bottle expands.
Hence, the pressure inside the bottle increases.
Thus the bottle may burst in hot summer.

Gills are richly supplied with blood capillaries and can readily absorb the oxygen dissolved in water.
When fishes are taken out of water, the supply of oxygen to the fishes is cut as the fishes cannot absorb and breathe using the oxygen present in the atmosphere.
Hence they die, when it is taken out of water.

Astronauts cannot breathe in space unless they carry their own oxygen with them.
They can make their own oxygen by using energy from the solar arrays to split hydrogen and oxygen from water.
Do you know?

It is because nitrogen tyres hold pressure longer as compared to compressed air. Nitrogen gas in the tyre escapes more slowly than compressed air does.