How would you explain the kinetic molecular theory for gases, liquids, and solids?

1 Answer
Feb 29, 2016

The kinetic molecular theory (particle model) contains the following premises :

  1. All matter consists of particles.
  2. The particles are in constant random motion.
  3. There are spaces between the particles.
  4. There are forces of attraction and repulsion between the particles.

We may now characterize the 3 different phases of matter, as well as explain phase changes and even other physical and chemical properties, in terms of these particle model premises.

  • In solids , the particles remain in fixed positions in their crystal lattice structure and only vibrate randomly. The spaces between the particles are very small.
    Consequently, the forces of attraction and repulsion between the particles are very large, since these forces decrease in the inverse square relationship with distance between particles. (By Newton's Law of Universal Gravitation attraction as well as Coulomb's Law of electrostatics).

  • In liquids , there are larger spaces between particles and the particles move freely. Forces between particles are weaker. Liquids may be compressed.

  • In gases , the particles are very far apart and move very fast and freely. The forces between the particles are very weak. Gases always fill the containers into which they are placed.

In addition to this, we also can define the following 2 concepts in terms of the particle model :

  1. Temperature is a measure of the average kinetic energy of the particles, where #E_k=1/2mv^2#.
  2. Pressure is a measure of the number of collisions per unit time between the particles.

We can now explain various physical and chemical phenomena in terms of the above. I will give 2 examples :

Evaporation in terms of particle model
As a liquid is heated, energy is transferred from the flame to the liquid particles which cause them to increase their kinetic energy and hence move faster and further apart.
Eventually they move so far apart that they are able to overcome the forces of attraction holding them together in the liquid phase and they exist in the gaseous phase.

Boyle's Law in terms of particle model
When the volume of an enclosed gas is decreased, the gas particles have less space in which to move and hence they collide more often with each other, hence causing the pressure of the enclosed gas to increase.

I will also include a little rough sketch to summarize the particle model for the different phases of matter below :

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