Surface tension
Intermolecular forces
The force between two molecules of a substance
is called intermolecular force. This intermolecular force is basically electric
in nature. When the distance between two molecules is greater, the distribution
of charges is such that the mean distance between opposite charges in the
molecule is slightly less than the distance between their like charges. So a
force of attraction exists. When the intermolecular distance is less, there is
overlapping of the electron clouds of the molecules resulting in a strong
repulsive force.
The intermolecular forces are of two types.
They are (i) cohesive force and (ii) adhesive force.
Cohesive force
Cohesive
force is the force of attraction between the molecules of the same substance. This cohesive force is very strong in solids, weak in liquids and extremely weak in gases.
Adhesive force
Adhesive
force is the force of attraction between the moelcules of two different
substances. For example due to the adhesive force, ink sticks to paper while writing. Fevicol,
gum etc exhibit strong adhesive property.
Water wets glass because the cohesive force
between water molecules is less than the adhesive force between water and glass
molecules. Whereas, mercury does not wet glass because the cohesive force
between mercury molecules is greater than the adhesive force between mercury
and glass molecules.
Molecular range and sphere of influence
Molecular
range is the maximum distance upto which a molecule can exert force of
attraction on another molecule. It is of
the order of 10-9 m for
solids and liquids.
Sphere
of influence is a sphere drawn around a particular molecule as centre and
molecular range as radius. The central molecule exerts a force of attraction on all the
molecules lying within the sphere of influence.
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