Friday, January 27, 2012

Graham's law of diffusion - Further illustration of surface tension

Graham's law of diffusion

The speed with which molecules move about inside a gas depends on their mass and temperature. Thus, at a given temperature the heavy molecules of carbon dioxide " CO2" move more slowly than the light molecules of hydrogen.

This has an important bearing on the rate at which one gas will diffuse into, or mix with, another.

    Tomas Graham found that,

at constant temperature gases diffuse at rates which are inversely proportional to the square roots of their densities. This is known as Graham's law of diffusion. This video demonstrates this law

Diffusion experiment 

    The difference in the rates of diffusion of two gases through a porous partition may be demonstrated by a simple experiment.- I'm sorry but you have to use your imagination :) -.

 An unglazed earthenware pot is fitted with a rubber bung and a length of glass tubing and set up vertically with the tube dipping into a beaker of water. When the porous earthenware pot is surrounded by gas from the laboratory supply in the manner shown the gas diffuses through into the pot more rapidly than the air diffuses outwards. The pressure in the pot therefore increases, and bubbles make their appearance at the end of the glass tube.

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, there is a reduction of pressure inside, and water is now forced up the tube. A device based on this principle has been used for detecting the presence of the dangerous gas methane in coal-mines.

Diffusion in a liquid can be shown by placing some blue copper sulphate crystals at the bottom of a tall beaker containing water. If the beaker is placed where it will be undisturbed the crystals will dissolve and form a dense blue solution at the bottom. During succeeding weeks the blue coloration gradually extends upwards. In spite of their comparatively large mass, the copper sulphate molecules have sufficient energy to enable them to overcome the force of gravity and diffuse upwards through the water.

Surface tension

    Most people are aware that an ordinary sewing needle can be made to float on water. The experiment works best if the needle is slightly greasy. Usually, the needle is placed on a small piece of filter paper, which is then gently placed on the water surface. Within a few seconds the paper sinks to the bottom and the needle is left floating.

Close examination reveals that the needle rests in a slight depression. The surface of the water behaves as though covered with an elastic skin. This property of a liquid is called surface tension.
A piece of gauze cut and bent into the form of a rectangular box will also float, but if a new drops of alcohol, soap solution or detergent are added to the water the surface tension is decreased and the gauze and needle will sink.
A tent keeps out water owing to the tension which exists in the lower surface of the rain-water in contact with the canvas. Campers soon learn that it is important not to touch the inside of the canvas, or the surface film may be broken. When this occurs, the water spreads round the strands of the material and, having nothing to support it, begins to drip through.

Further illustration of surface tension

We have already seen that soap solution, detergents and alcohol will lower the surface tension of water. Camphor has a similar effect. When a piece of camphor is thrown on to the surface of water it slowly dissolves and causes a reduction of the surface tension in its immediate neighborhood. The stronger pull exerted by the surrounding uncontaminated water sets up a movement of the surface and the camphor is carried along with it. After a time the concentration of the surface solution becomes uniform and all movement ceases.

The above experiment illustrates the principle of a toy duck, made of light plastic material, which swims about on the surface of water when a small piece of camphor is attached to it. A similar toy in the shape of a small metal boat has a reservoir of methylated spirit. A short wick dips in the spirit and trails over the stern. Reduction of surface tension occurs where the spirit contaminates the water and the boat moves forward.

Quantitatively, the surface tension is defined as tangential force in the surface acting normally per unit length across any line in the surface. ( The word "normally" here means " perpendicularly".)

The tension in the surface of a liquid is well illustrated by a soap film. A wire frame with a piece of cotton tied across it is dipped into soap or detergent solution so that a film is formed. When the film on one side of the cotton is removed by touching it with filter-paper the tension in the film on the opposite side pulls the cotton into an arc of a circle. A similar frame has a length of cotton with a loop in it. The loop is pulled into the form of a circle when the film is removed from its center.  I hope that was informative.