Kinetic molecular theory is the scientific idea that the basic forms of matter — gas , liquid , and solid —all consist of tiny particles and that those particles are in constant motion . The particles may be atoms or molecules (combinations of atoms). The theory also describes the relationship between the temperature of matter and the energy of motion in its particles. Scientists have used the theory as a basis for describing many properties of matter, including the behavior of a gas under various conditions of pressure , temperature, and volume .
The theory holds that in a gas, the particles move separately in random directions at various speeds. In a liquid, the particles are in contact with—but not bound to—one another and move at various speeds. In a solid, the particles are bound together and vibrate in place.
The most important aspect of the kinetic molecular theory relates the motion of particles to the temperature of matter. Temperature is related directly to the average kinetic energy (energy of motion) of the particles. The higher the temperature, the higher the average kinetic energy in the particles. As the temperature approaches absolute zero , the speed of the particles—and hence their kinetic energies—approach zero. Absolute zero is 0 degrees on the Kelvin scale.
The primary application of kinetic molecular theory is to explain the ideal gas law. The ideal gas law relates the pressure, volume, and temperature of a gas. However, the law was discovered before people widely accepted that matter is made of atoms and molecules. Applying kinetic molecular theory reveals the underlying atomic reality behind the ideal gas law. For a container of gas with a given volume, the theory holds that the gas’s particles move randomly with various speeds. The particles are also assumed to bounce off one another and the sides of the container with no energy loss. The laws of motion, described by the English scientist Isaac Newton, can be applied to the forces the particles exert on the container walls. The result elegantly confirms the ideal gas law—with an important addition. The gas’s temperature is revealed to be a measure of the average kinetic energy of its particles.
