Diffusion of Gases – Brownian Motion, Osmosis, Diffusion, Viscosity, Surface Tension

The molecular kinetic theory can be used to explain a number of phenomena. Some of these phenomena are:

  1. Brownian Motion
  2. Osmosis
  3. Diffusion
  4. Viscosity
  5. Surface tension

Let us consider Diffusion

Definition of Diffusion

Diffusion is the process whereby gas molecules mix up intimately with one another as a result of the kinetic energy of the molecules. According to Graham’s law of diffusion, the time rate at which gases diffuse is inversely proportional to the square root of its density provided that temperature is kept constant. This is written mathematically as:


t = rate of diffusion

d = molecular density of the gas

This means that, the heavier the gas, the slower the movement of the gas.

Pressure of a Gas

If the volume of a fixed mass of a gas is reduced at constant temperature, the molecules will be compressed and become more closely packed together as shown in the figure below. Under this condition, the time rate of collision of the molecules with themselves and the walls of the container increases thereby increasing the pressure of the gas. If on the other hand, the volume is increased by raising the piston up at constant temperature, the time rate of collision of the molecules will decrease. They will take longer time to collide with the walls of the container. This will lead to a decrease in the pressure. This agrees with Boyle’s law which states that, ‘the volume of a fixed mass of a gas at constant temperature is inversely proportional to its pressure.’

gas diffusion


Definition of Osmosis

Osmosis is the tendency of a solvent to pass from a dilute solution, through a semi permeable membrane into a concentrated solution. Semi-permeable membrane is a substance such as cellophane, parchment or vegetable material which would allow some molecules of liquid to diffuse through them but not others. Such a membrane may allow the molecules of a solvent to pass through it but not those of a solute. and Osmosis from Carla Smedberg

Demonstration of Osmosis

(i) Tie a semi-permeable membrane across the mouth of a thistle funnel and pour some concentrated sugar solution into the funnel.

(ii) Immerse the funnel in a beaker of water as shown in the figure and note the level of sugar solution in the funnel.

(iii) Allow the set up to stand for a day, and check the level of sugar solution in the funnel again. Test the water in the beaker for sugar. You will observe that the level of the sugar solution in the inverted thistle funnel has risen and that the water in the beaker is free from sugar.


The molecules of sugar (the solute) cannot pass through the semipermeable membrane, but molecules of water (the solvent) can do so. Sugar and water molecules are bombarding the membrane on one side and only water molecules on the other side. Thus, more water molecules move up into the funnel than down out of it and so the level of liquid in the thistle funnel rises.


Viscosity by definition is internal friction between layers of fluids in motion. Liquids that are dense pour more slowly than those that are less dense. E.g honey will pour more slowly than water because it is denser than kerosene. This means that honey is a more viscous liquid than water. Viscosity can be demonstrated if we consider a ball bearing falling through some liquids.

Experiment to Demonstrate Viscosity


Beaker, two different liquids say, engine oil and kerosene


Pour the engine oil into the beaker and drop the ball bearing into it. Observe the ball bearing as it moves to the bottom of the beaker. Do the same thing using the other liquid (kerosene). You will observe that the ball bearing gets to the bottom of the beaker much earlier than it does in engine oil. Therefore viscosity in engine oil is higher than that in kerosene.

Terminal Velocity

A ball that is made to fall through a liquid is under the influence of three forces namely

  1. its weight (W = mg) that acts vertically downwards,
  2. the upthrust (U) of the liquid on the ball acting upwards the viscous force (V) opposing it motion.

The resultant force acting on the ball can be written as:


Where ‘a’ is the acceleration of the ball through the liquid and m is the mass of the ball. At a certain time in the motion of the ball, its velocity becomes uniform or constant. At this stage acceleration ‘a’ is 0 so that the above equation becomes

W–V–U=0 or Mg–V–U=0V=Mg–U

The velocity-time graph of the motion of the ball is given as shown below


Applications of Surface Tension and Viscosity

  1. The knowledge of surface tension is applied in industries in making some materials such as, umbrellas, canvass, rain coats and waterproof tents
  2. It is difficult to wash dirty clothes or oily clothes with water only. That is why we use soap and detergents to wash. Soap and detergents weakens the surface tension of water and enables it to float away dirt or oil from the material
  3. Viscous liquids are used as lubricants. Examples are grease and engine oil.
  4. Viscosity is applied in the design of boats, ships and aircraft.


  1. Differentiate between viscosity and surface tension
  2. Give two applications each of viscosity and surface tension

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