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Video Example : Kinetic molecular theory

Kinetic molecular theory


  • identical particles
  • point masses
  • ,
  • collision
  • kinetic energies
  • distribution of speeds
  • variable kinetic energy
  • elastic

Definition of Kinetic molecular theory

The kinetic molecular theory is an alternative approach to understand the behaviour of gases

More About Kinetic molecular theory

  • Conducting careful experiments is an important aspect of scientific method and it tells us how the particular system is behaving under different conditions
  • A theory is a model that enables us to better understand our observations
  • The theory that attempts to elucidate the behaviour of gases is known as kinetic molecular theory
  • Assumptions or postulates of the kinetic molecular theory of gases are
  • Gases consist of large number of identical particles (atoms or molecules) that are so small and so far apart on the average that the actual volume of the molecules is negligible in comparison to the empty space between them
  • They are considered as point masses. This assumption explains the great compressibility of gases
  • There is no force of attraction between the particles of a gas at ordinary temperature and pressure
  • The support for this assumption comes from the fact that gases expand and occupy all the available space
  • Particles of a gas are always in constant and random motion
  • If the particles were at rest and occupied fixed positions, then a gas would have had a fixed shape which is not observed
  • Particles of a gas move in all possible directions in straight lines
  • During their random motion, they collide with each other and with the walls of the container
  • Pressure is exerted by the gas as a result of collision of the particles with the walls of the container
  • Collisions of gas molecules are perfectly elastic
  • The total energy of gas molecules before and after collision is the same
  • There may be exchange of energy between colliding molecules, their individual energies may change, but the sum of their energies remains constant
  • If there was loss of kinetic energy, then the motion of the molecules will stop and gases will settle down
  • At any particular time, different particles of gas have different speeds and hence different kinetic energies
  • This assumption is reasonable because particles have different speeds which change when they collide
  • It is possible to show that though the individual speeds are changing, the distribution of speeds remains constant at a particular temperature
  • If a molecule has variable speed, then it must have a variable kinetic energy
  • In kinetic theory it is assumed that average kinetic energy of the gas molecules is directly proportional to the absolute temperature
  • On heating a gas, the kinetic energy of gas molecules increases and the molecules strike the walls of the container with higher speed


  • The most important advantage of liquefying gases is that they can then be stored and transported in much more compact form than in the gaseous state.
  • liquid oxygen and liquid hydrogen are used in rocket engines. Liquid oxygen and liquid acetylene can be used in welding operations