(a) The angle of contact of mercury with glass is obtuse,
while that of water with glass is acute.
(b) Water on a clean glass surface tends to spread out
while mercury on the same surface tends to form drops. (Put
differently, water wets glass while mercury does not.)
(c) Surface tension of a liquid is independent of the area
of the surface
(d) Water with detergent dissolved in it should have small
angles of contact.
(e) A drop of liquid under no external forces is always
spherical in shape
(a) The angle between the tangent to the liquid surface at
the point of contact and the surface inside the liquid is called
the angle of contact (θ), as shown
in the given figure.
Sla, Ssa, and
Ssl are the respective interfacial tensions
between the liquid-air, solid-air, and solid-liquid interfaces.
At the line of contact, the surface forces between the three
media must be in equilibrium, i.e.,
The angle of contact θ , is obtuse
if Ssa < Sla (as in the
case of mercury on glass). This angle is acute if
Ssl< Sla (as in the case
of water on glass).
(b) Mercury molecules (which make an obtuse angle with
glass) have a strong force of attraction between themselves and a
weak force of attraction toward solids. Hence, they tend to form
On the other hand, water molecules make acute angles with glass.
They have a weak force of attraction between themselves and a
strong force of attraction toward solids. Hence, they tend to
(c) Surface tension is the force acting per unit length at
the interface between the plane of a liquid and any other
surface. This force is independent of the area of the liquid
surface. Hence, surface tension is also independent of the area
of the liquid surface.
(d) Water with detergent dissolved in it has small angles
of contact (θ). This is because for
a small θ, there is a fast
capillary rise of the detergent in the cloth. The capillary rise
of a liquid is directly proportional to the cosine of the angle
of contact (θ). If
θ is small, then
cosθ will be large and the rise of
the detergent water in the cloth will be fast.
(e) A liquid tends to acquire the minimum surface area
because of the presence of surface tension. The surface area of a
sphere is the minimum for a given volume. Hence, under no
external forces, liquid drops always take spherical shape.
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