Revision: Surface Tension Physics HSC Science (General) 12th Standard Board Exam Maharashtra State Board

The sphere of influence of a molecule is defined as an imaginary sphere with the molecule at its centre and a radius equal to the range of molecular attraction.

The angle between tangents drawn at the point of contact to the liquid surface and the solid surface inside the liquid is called the angle of contact for a pair of solid and liquid. It is denoted by θ.

Surface tension is defined as the force acting on a unit length of an imaginary line drawn on the free surface of the liquid, the direction of the force being perpendicular to the line so drawn and acting parallel to the surface.

When a liquid is in contact with a solid, the angle between the tangent drawn to the free surface of the liquid and the surface of solid at the point of contact measured inside the liquid is called the angle of contact.

Surface tension is defined as the force per unit length acting at right angles to an imaginary line drawn on the free surface of the liquid. 

A tube with a hole of very small diameter is called a capillary tube or capillary.

The rise or fall of level of liquid in a capillary tube is called capillary action or capillarity.

The phenomenon where a liquid in a capillary tube either ascends or descends relative to the surrounding liquid when a tube of very narrow bore is dipped in it is called capillarity.

The phenomenon in which a liquid rises in a capillary tube when the angle of contact is acute, or falls when the angle of contact is obtuse, due to the interplay of pressure caused by the liquid column and pressure difference due to surface tension, is called capillary ascent (or descent).

The vertical height attained by a liquid in a capillary tube at equilibrium, which is independent of the shape of the capillary provided the radius of meniscus remains the same, is called the capillary rise height (h).

For two different liquids in the same tube:

\[\frac{h_1}{h_2}=\frac{\rho_2T_1}{\rho_1T_2}\]

For the same liquid in tubes of different radii:

h₁​r₁​ = h₂​r₂​

Principle: Pressure due to liquid column = Pressure difference due to surface tension

h = \[\frac {2T}{Rdg}\] = \[\frac {2T cos θ}{rdg}\]

where r = radius of capillary tube and θ = angle of contact.

• A highly soluble impurity increases surface tension, while a partially soluble impurity (e.g., detergent) decreases it; a waterproofing agent increases it.
• Surface tension decreases with increase in temperature, given by T = T₀(1 − αθ), where T₀​ is surface tension at 0°C and α is the temperature coefficient.
• When a soap bubble is charged (positively or negatively), force acts outward on the surface, increasing its radius — thus electrification always decreases surface tension.