# Kinetic theory and osmosis

Question: What is the kinetic theory and how does it relate to osmosis in plant cells?

The kinetic theory of matter describes the motion of molecules. It relates their motion to their mass and the temperature of the medium of which they are part.

Strong evidence for the existence of molecules is provided by the following observations:
(a) the diffusion of gases and liquids - diffusion in solids has actually also been observed: a slab of lead was clamped to a slab of gold for some years, and diffusion of each metal a few millimetres into the other was demonstrated by chemical analysis.
(b) the mixing of two liquids to give a final volume which is less than the sum of their original volumes;
(c) dissolving a solid in a liquid.

Various other phenomena may be explained in terms of the kinetic theory:
(a) evaporation - molecules in the surface of a liquid gain sufficient energy to escape from it
(b) saturated vapour pressure - there is a dynamic equilibrium between molecules entering and leaving a liquid surface in an enclosed space
(c) surface tension - the intermolecular forces at a liquid surface explain capillary rise, liquid drop shapes, the wetting of surfaces and so on
(d ) latent heat - energy is required to overcome the intermolecular attraction to change the state of a substance
(e) viscosity - the movement of molecules between adjacent layers of a moving fluid and their attraction gives the effect of viscous drag
(f) the behaviour of gases, both ideal and real, may be explained in terms of molecules.

In the kinetic theory of gases molecules are thought to have the following properties:

molecules behave as if they were hard, smooth, elastic spheres
molecules are in continuous random motion;
the average kinetic energy of the molecules is proportional to the absolute temperature of the gas;
the molecules do not exert any appreciable attraction on each other
the volume of the molecules is infinitesimal when compared with the volume of the gas
the time spent in collisions is small compared with the time between collisions.

In osmosis the motion of the molecules through a membrane can be described in terms of the kinetic theory, more massive molecules generally diffusing slower than less massive ones at a similar temperature.