Surface Tension
Surface Tension
It is well known fact that liquids assume the shape of the
container. Why is it then small drops of mercury form spherical bead instead of
spreading on the surface. Why do articles of soil at the bottom of river remain
separated but they stick together when taken out? Why does a liquid? All these
phenomena are caused due to the characteristic property of liquids,
called Surface tension. A molecule in the bulk of liquid
experiences equal intermolecular forces from all sides. The molecule, therefore
does not experience any net force. But for the molecule on the surface of
liquid, net attractive force is towards the interior of the liquid, due to the
molecules below it. Since there are no molecules above it.
Liquids tend to minimize their surface area. The molecules
on the surface experience a net downward force and have more energy than the
molecules in the bulk, which do not experience any net force. Therefore,
liquids tend to have minimum number of molecules at their surface. If surface
of the liquid is increases by pulling a molecule from the bulk, attractive
forces will have to be overcome. This will require expenditure of energy. The
energy required to increase the surface area of the liquid by one unit is
defined as surface energy.
Liquid tends to rise (or fall) in the capillary because of
surface tension. Liquids wet the things because they spread across their
surfaces as thin film. Moist soil grains are pulled together because surface
area of thin film of water is reduced. It is surface tension which gives
stretching property to the surface of a liquid. On flat surface, droplets are
slightly flattened by the effect of gravity; but in the gravity free
environments drops are perfectly spherical.
The magnitude of surface tension of a liquid depends on the
attractive forces between the molecules. When the attractive forces are large,
the surface tension is large. Increase in temperature increases the kinetic
energy of the molecules and effectiveness of intermolecular attraction
decreases, so surface tension decreases as the temperature is raised.
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