The idea that molecules are in constant motion was proposed by the kinentic theory of gases. The development of this theory in the 19th century are mostly based on the theory of atoms & molecules. Since there are no real experiments during that time, many leading physicists strongly opposed the idea. However, Brownian Motion, an observation done by botanist Robert Brown eliminated any opposition to the kinetic theory of gases.
According to the Theory:
1. The gas consists of very small particles, each of which has a mass or weight in SI units.
2. The number of molecules is large such that statistical treatment can be applied.
3. Molecules are in constant and random motion.
4. The rapidly moving particles constantly collide with each other and with the walls of the container.
5. The collisions of gas particles with the walls of the container holding them are perfectly elastic.
6. The interactions among molecules are negligible. They exert no forces on one another except during collisions.
7. The total volume of the individual gas molecules added up is negligible compared to the volume of the container.
8. The molecules are perfectly spherical in shape, and elastic in nature.
9. The average kinentic energy of the gas particles depends only on the temperature of the system.
10. The time during collision of molecule with the container's wall is negligible as comparable to time between successive collisions.
11. The equation of motion of the molecules are time-reversible.