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3. Surface tension has which of the following properties?
I. It has units of force per unit length.
II. It exists whenever there is a density discontinuity.
III. It is strongly affected by pressure.
a) I only
b) II only
c) III only
d) I and II
e) I, II, and III
All 3 statements are correct
The phenomenon of surface tension arises due to the two kinds of
intermolecular forces
(i) Cohesion : The force of attraction between the
molecules of a liquid by virtue of which they are bound to each
other to remain as one assemblage of particles is known as the
force of cohesion. This property enables the liquid to resist
tensile stress.
(ii) Adhesion : The force of attraction between unlike molecules, i.e. between the molecules of different liquids or between the molecules of a liquid and those of a solid body when they are in contact with each other, is known as the force of adhesion. This force enables two different liquids to adhere to each other or a liquid to adhere to a solid body or surface.
Figure : The intermolecular cohesive force field in a
bulk of liquid with a free surface
A and B experience equal force of cohesion in all directions, C
experiences a net force interior of the liquid The net force is
maximum for D since it is at surface
Work is done on each molecule arriving at surface against the
action of an inward force. Thus mechanical work is performed in
creating a free surface or in increasing the area of the surface.
Therefore, a surface requires mechanical energy for its formation
and the existence of a free surface implies the presence of stored
mechanical energy known as free surface energy. Any system tries to
attain the condition of stable equilibrium with its potential
energy as minimum. Thus a quantity of liquid will adjust its shape
until its surface area and consequently its free surface energy is
a minimum.
The magnitude of surface tension is defined as the tensile force
acting across imaginary short and straight elemental line divided
by the length of the line.
The dimensional formula is F/L or MT-2 . It
is usually expressed in N/m in SI units.
Surface tension is a binary property of the liquid and gas or
two liquids which are in contact with each other and defines the
interface. It decreases slightly with increasing temperature. The
surface tension of water in contact with air at 20°C is about 0.073
N/m.
It is due to surface tension that a curved liquid interface in equilibrium results in a greater pressure at the concave side of the surface than that at its convex side.