This unit is about Diffusion in liquids and gas among other things.


Diffusion is the movement of molecules from regions of high concentration to the areas of low concentration.

Diffusion in air

A perfume that in sprayed in one corner of a room is detected in another corner of the room

An experiment to show that light gases diffuse faster than heavy gases




Heavy gases like carbon dioxide diffuse more slowly than light gases


Hydrogen in the experiment to demonstrate that light gases diffuse faster than gases a porous pot is fitted with a rubber bung and a length of glass tubing and is set up vertical into a beaker of water.

When the porous pot is surrounded by gas from the lab supply shown above, the gas diffuses into the pot more rapidly than air molecules diffuse outwards.

The pressure in the pot therefore increases and as a result bubbles appear at the end of the tube as illustrated in A.

If the inverted beaker containing the gas is taken away the outward diffusion of gas already inside the pot is more rapid than the inward diffusion of air.

Consequently the reduced  pressure inside the water is forced up the tube as illustrated in B.


Put water in a beaker, using a thistle funnel, put a solution of copper sulphate at the bottom of the beaker.

diffusion 2


After 24hours, the blue colour of copper sulphate is seen upwards in the water. Copper sulphate molecules diffuse upwards into the water.


The rate of diffusion depends on the mass of the molecules and the temperature.


This is tendency of a liquid surface to behave like a stretched elastic skin trying to contract.

diffusion 3

A dish is filled with water and a needle on a small piece of paper pierced on the water surface. The thin piece of paper soaks and sinks/ easing the needle floating on top of the surface.

The needle rests on a sight depression. The water surface behaves like a stretched elastic skin trying to contract due to the force of attraction in between the liquid molecules.

diffusion 4

Molecules in the interior of liquid A experience equal forces from neighbor molecules from all directions but those on surface B only experience forces from molecules low them and on the sides. They experience a net downward force hence the surface tends to contract to acquire a state of tension.

Surface tension can be reduced by;-

– Adding a detergent

– Increase in temperature

In each case the increase of forces of attraction in the molecules is reduced.

Water form spheres as it drips e.g. falling from a top.

Some insects are able to walk on the surface of water without sinking or getting wet.



ASSIGNMENT : DIFFUSION assignment MARKS : 10  DURATION : 1 week, 3 days


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