This is the simplest state of matter. Throughout our life we remain immersed in the ocean of air which is a mixture of gases.
We spend our life in the lowermost layer of the atmosphere called troposphere, which is held to the surface of the earth by gravitational force. The thin layer of atmosphere is vital to our life.
It shields us from harmful radiations and contains substances like dioxygen, dinitrogen, carbon dioxide, water vapour, etc.
Let us now focus our attention on the behaviour of substances which exist in the gaseous state under normal conditions of temperature and pressure.
A look at the periodic table shows that only eleven elements exist as gases under normal conditions.
The gaseous state is characterized by the following physical properties.
- Gases are highly compressible.
- Gases exert pressure equally in all directions.
- Gases have much lower density than the solids and liquids.
- The volume and the shape of gases are not fixed. These assume volume and shape of the container.
- Gases mix evenly and completely in all proportions without any mechanical aid.
Simplicity of gases is due to the fact that the forces of interaction between their molecules are negligible. Their behaviour is governed by same general laws, which were discovered as a result of their experimental studies.
These laws are relationships between measurable properties of gases. Some of these properties like pressure, volume, temperature and mass are very important because relationships between these variables describe state of the gas.
Interdependence of these variables leads to the formulation of gas laws. In the next section we will learn about gas laws.