The comparison between Adhesion and cohesion
The force of attraction between molecules of the same substance is called Cohesion, as distinct from Adhesion or the force between molecules of different substances.
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| Osomosis animation |
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| Comparison between water and mercury |
The difference between the adhesive and cohesive properties of water and mercury explains why meniscus of water curves upwards and that of mercury curves downwards when these liquids are poured into clean glass vessels ( as shown in the picture below).
To understand Cappillary attraction
If a piece of ordinary glass tubing is heated in a gas flame until it soften it may be drawn out into a tube of very fine bore. This is called a capillary tube ( from the Latin capilla = hair ).
On dipping the tube into water, alcohol or any other liquid which wets glass it is noticed that the liquid rises in the tube to a height of several centimetres. Mercury, however, gives a capillary depression.
In the case of liquids which wet the glass the adhesion of the molecules for glass is greater than their cohesion. Consequently, the meniscus curves upwards and the liquid rises in the tube.
Conversely, the cohesion of mercury molecules is greater than their adhesion to glass. The meniscus therefore curves downwards, and this is accompanied by a capillary depression.
Blotting-paper owes its absorbent properties to the capillary rise which occurs in the narrow interstices between the soft fibers from which is made. Oil rises up a lamp wick for the same reason.
Application in the house
All well-constructed houses possess a lamp-course. This is a layer of salte, bitumenize and lead foil or other suitable material inserted between the wall bricks just above ground, but below floor level. Being impervious to water, the damp-course prevents the capillary attraction of moisture from the ground, which would otherwise make the walls damp and unhealthy.Osmosis
Before cooking dried fruits such as prunes and apricots it is customary to leave them soaking in cold water for some houres so that they swell up and become restored, as far as possible, to their original condition. Durig this time water passes through the cell walls of the fruit by a process called osmosis.The osmotic pressure ( sugar and water)
Osmosis can be demonstrated by the following experiment ( see the animation above) . A thistle funnel, with its mouth covred with parchment, is partly filled with strong sugar solution and then placed, mouth downwards, in a beaker of water. Very soon, the liquid level in the stem of the funnel begins to rise, showing that water is passing through the parchment into the sugar solution. The osmotic pressure of the slution is measured by the hydrostatic pressure, h, of the liquid column in the tube.Why a prune swells in water
Parchment is described as a semi-permeable membrane, i.e., it permits water molecules to pass through its pores but obstructs the passage of the large sugar molecules. Water molecules therefore pass through in both direction, but owing to the space occupied by the sugar molecules the concentration of water molecules is less on the sugar side of the membrane than on the water side.Consequently, the rate of passage of water molecules is greater from the water side to the sugar side. The reason why a prune swells in water now becomes apparent. The cells of the fruit contain a high concentration of sugar and are surrounded by a semi-permeable membrane.