Explain why (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
Answers
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b)It is because surface tension of mercury is more than water.
c) Surface tension only depends upon nature of liquid not area of contact.
d)The detergant Lowers the surface tension of water.
e)According to surface tension law every liquid has tendency to occupy minimum surface area.In general sphere has minimum area among all geometrical shapes so a drop of liquid under no external forces is spherical in shape.
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Answer
(a) When a liquid comes in contact with solid, three surfaces are formed namely liquid-air, solid-air and solid-liquid. The surface tensions corresponding to these interfaces are T
LA
,T
SA
and T
SL
respectively
Now angle of contact of solid with liquid is
θ=
T
LA
T
SA
−T
SL
In case of mercury-glass T
SA
<T
SL
,making cosθ negative and the angle of contact obtuse.
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.
S
SL
,S
LA
,S
SA
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,
The angle of contact θ , is obtuse if S
SA
<S
LA
(as in the case of mercury on glass). This angle is acute if S
SL
<S
LA
(as in the case of water on glass).
(b) T
LA
cosθ+T
SL
=T
SA
When θ is an obtuse angle then molecules of liquids are attracted strongly to themselves and weakly to those of solid, it costs a lot of energy to create a liquid-solid surface, and liquid then does not wet the solid. This is what happens with mercury on a glass surface. On the other hand,
if the molecules of the liquid are strongly attracted to those of the solid, this will reduce T
SL
and therefore, cosθ may increase or may decrease. In the case of water, it is an acute angle and hence water
wets the glass surface.
(c) Surface tension is defined as the force acting per unit length on either side of a imaginary line drawn tangentially on the surface of a liquid. As the force is independent of area of liquid surface taken, so is the surface tension.
(d) The cloth has narrow spaces in form of fine capillaries.The rise of liquid in a capillary tube is given by
h=
ρrg
2Tcosθ
h∝cosθ
this implies that if detergent has a small angle of contact it will have a greater value of h, implying that detergent penetrates more in the cloth to remove dirt
(e) A liquid tends to acquire the minimum surface area to minimise the energy by the virtue of surface tension. For a given volume, the sphere has the least surface area
Explanation: