The charactetistics of gases are described fully in terms of four parameters (measurable properties) :

(i) `text(Pressure :-)` It is exerted by gas due to collision of randomly moving gas molecules with the walls of the container. Pressure can be measured in following units :

`text(Pascal)` : It is `SI` unit for pressure. Pascal is very small amount of pressure.

o (10mg weight on 1 `cm^2` area)

o 1 Pa = `1 N//m^2`

`text(Atm)` : `text(atm)` is pressure exerted by mass of the atmosphere on the earth's surface.

Various units of pressure can be interrelated as follows

`1` atm = `76` cm of Hg = `760` torr = `101325` Pa = `1.01325` bar

(ii) `text(Volume :-)` It represents free volume available for motion of gas particles.

Various units of volume are interrelated as follows

`1` L = `1000` mL = `10^(-3)` `m^3` = `1000` cc

(iii) `text(Temperature :-)` Units of temperature are interrelated as follows

`T` `K` `=` `T-C+ 273`

(iv) `text(Amount of gas :-)` Generally measured in moles of gas.

Simplicity of gases is due to the fact that the forces of interaction between their molecules are negligible. Their behavior 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. Inter-dependence of these variables leads to the formulation of gas laws.