PHY2: DIFFUSION

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  DIFFUSION.

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

Factors which affect the rate of diffusion

The following factors affect the rate of diffusion

Temperature

The rate of diffusion is directly proportional to the temperature , ie the higher the temperature the faster the rate of diffusion and the lower the temperature the lower the rate of diffusion

Concentration

The rate of diffusion is directly proportional to the concentration. ie the higher the concentration the faster the rate of diffusion and vice versa

size of the diffusing molecules

smaller or lighter molecules diffuses faster than the heavier ones . This is because lager molecules occupy more space than the smaller molecules

density of the diffusing molecules

The rate of diffusion is inversely proportional to the density ie the higher the density the the lower the rate of diffusion and vice versa

NB

Diffusion is slowest in solids followed by liquids and fastest in gases

Diffusion in air

A perfume that is 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

Note:

Heavy gases like carbon dioxide diffuse more slowly than light gases

Example

Hydrogen is used in the experiment to demonstrate that light gases diffuse faster than heavy gases.

Procedure.

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.

DIFFUSION IN LIQUIDS

Water is poured in a clean beaker.

Using a thistle funnel, a crystal of copper (II) sulphate  is carefully placed at the bottom of the beaker.

Observation

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

Note:

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

SURFACE TENSION

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

Definition: It is the force per unit length acting normally along a liquid surface

A dish is filled with water and a needle on a small piece of bloating paper placed is on the water surface.

The thin piece of paper soaks and sinks leaving the needle floating on the surface of water.

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

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 lower 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.

Applications.

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.

Floating of leaves on calm water surface.

THIS VIDEO EXPLAINS MORE ABOUT DIFFUSION