Viva Voice Question
Surface Tension – Viva Voice Questions with Answer
For: Science Class 11 ( Practical Exam )

Q.1: Define surface tension?
Ans. “The tangential cohesive force acting along the unit length of the surface of a liquid”
T = F / L
Where F = total force along a line
L = length of the line
In this experiment:
F = m g = weight of the pan + weight in the pan.
L = 2 (length of the slide) + 2 (breadth of the slide).
Sometime we neglect the breadth since it is very small.

Q.2: What are units of surface tension in C.G.S. and S.I. (M.K.S.) system?
Ans. Dynes / cm (C.G.S.)
Newton / meter (M.K.S.)

Q.3: What are cohesion and adhesion force?
Ans. Cohesion force is the attractive force between like molecules, whereas, the adhesion is the attractive force between unlike molecules, e.g. attraction between glass slide and the liquid.

Q.4: What are the factors affecting the surface tension?
Ans. (a) Nature of liquid (b) Nature of the surface in contact (c) Temperature

Q.5: What is the effect of temperature on the surface tension?
Ans. Surface tension decreases with the rise of temperature.

Q.6: Define critical temperature.
Ans. The temperature at which the surface tension is zero.

Q.7: Why the free surface of water is concave but that of mercury is convex?
Ans. The free surface of water is concave because:
Cohesion force between water molecules >> adhesion force between water and gas molecules Because the free surface of mercury is convex because Cohesion force >> adhesion force

Q.8: What is the shape of free surface at critical temperature?

Ans. The surface tension will decrease.

Q.10: Define angle of contact.
Ans. Angle of contact, for a pair of solid and liquid, id define as
“the angle between tangent to the liquid surface drawn at the point of contact and the solid
surface inside the liquid.”

Q.11: Give some practical applications of surface tension.
Ans. (a) A drop of falling liquid is always in spherical shape.
(b) We use oily substances to set out hairs.
(c) We use soaps and detergent for cleaning clothes.
(d) A thin layer of water over the umbrella protects us against light rain.
(e) Capillary action e.g. rising of oil in the wick of a lamp.
(f) Flying insects can walk on water surface without getting their feel wet.