Introduction to diffussion.

Diffusion is the spreading (or flow) of molecules a substance from a region of high concentration to a region of low concentration.

Diffusion in liquids:

Experiment to show diffusion in liquids.

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Water is placed in a clean glass trough

A crystal of potassium permanganate is then introduced at the bottom using a drinking straw.


The purple crystal of potassium permanganate dissolves and spreads throughout the water forming a purple solution.


This means that potassium permanganate has diffused through the water in the glass trough.

Diffusion in gasses:

Experiment to show diffusion in gases

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Cotton wool soaked in aqueous ammonia is placed at one end and another cotton wool soaked in concentrated hydrochloric acid is placed at the other end of a glass tube of about 1m long.


White fumes of ammonium chloride forms inside the tube near the end with cotton wool soaked in HCl acid.


This means that:
(i) The gases have diffused through the air in the tube.
(ii) Ammonia diffuses faster than Hydrogen chloride.

Note: Alternatively;

An air molecule tube is inverted over brown nitrogen dioxide molecules tube.
Observation: the brown colour spreads into the upper tube at the same time the air molecules spread into the lower tube.

Experiment II: to show diffusion in gases

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Connect a water manometer to a porous pot containing air.

Pass hydrogen into the air enclosed in the porous material as shown in the diagram above.

The water level in the left arm of the manometer falls while that in the right arm rises.

The hydrogen molecules diffuse through the porous material into the air. This increases the pressure in the porous pot, which then acts on the water surface in the left arm of the manometer, thus pushing the water level down wards.

Factors that determine the rate of diffusion

(i) Size of diffusing molecules

Smaller molecules diffuse faster than larger molecules. This is because larger molecules occupy large space than small ones.

ii) Temperature
The rate of diffusion is directly proportional to temperature.

(iii) Pressure
The rate of diffusion is directly proportional to pressure.
This is because at a higher pressure, gas molecules are squeezed, move faster and collide frequently than at low pressure.

(iv) Molecular weight
Lighter molecules diffuse faster than massive molecules. The speed of diffusion in a gas depends on the speed of molecules in that; lighter molecules diffuse at greater rate e.g. the rate of diffusion of hydrogen gas is faster than that of carbon dioxide because the molecules of hydrogen move at a higher speed since they are lighter than the molecules of carbon dioxide.