The primary assumptions are as follows:
Any gas is a collection of innumerable number of minuscule particles which are known as molecules according to Avogadro’s law.
There are no forces of attraction or repulsion among the particles or between the molecules and the surroundings.
The gas particles are always at straight, rapid, fast & random motion resulting in inevitable collisions with other particles and the surroundings that changes direction of motion.
Since the particle are spherical, solid and elastic the collisions involving them are elastic in nature as well i.e their kinetic energy is conserved even after collisions.
The total kinetic energy of the particles is proportional to the absolute temperature.
In some books two other assumptions are given as well:
1. The size or area of each particle is negligible compared to that of the container.
2. Pressure of gas is result of the continuous clash of the particles with the wall of the container.
or
The simplest kinetic model is based on the assumptions that: (1) the gas is composed of a large number of identical molecules moving in random directions, separated by distances that are large compared with their size; (2) the molecules undergo perfectly elastic collisions (no energy loss) with each other and with the walls of the container, but otherwise do not interact; and (3) the transfer of kinetic energy between molecules is heat. These simplifying assumptions bring the characteristics of gases within the range of mathematical treatment.
Such a model describes a perfect gas and is a reasonable approximation to a real gas, particularly in the limit of extreme dilution and high temperature. Such a simplified description, however, is not sufficiently precise to account for the behaviour of gases at high densities.
Based on the kinetic theory, pressure on the container walls can be quantitatively attributed to random collisions of molecules the average energy of which depends upon the gas temperature. The gas pressure can therefore be related directly to temperature and density. Many other gross properties of the gas can be derived, such as viscosity, thermal and electrical conductivity, diffusion, heat capacity, and mobility. In order to explain observed deviations from perfect gas behaviour, such as condensation, the assumptions must be appropriately modified. In doing so, considerable insight has been gained as to the nature of molecular dynamics and interactions.
