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1. How can insect walk on water surface?

2. Why shape of rain drop is spherical?

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3. Why rain drop can not pass throw holes of the

umbrella?

4. If we place a needle on water surface gently it will

float. How?

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5. How mercury drop forms small drops after falling

on around surface.

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Ans : Surface tension.

Intermolecular force of attraction

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There are two types of intermolecular forces:

1. Cohesive force

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The force of attraction between two molecules of

the same substance is called cohesive force.

2. Adhesive force

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The force of attraction between two molecules of

different substances is called adhesive force.

Important Definitions

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i. Range of molecular attraction

(Molecular Range) (R)

The maximum distance between two molecules

upto which, the molecular force of attraction is

effective is called range of molecular attraction or

molecular range.

It is of the order of 10-9 m.

ii. Sphere of influence

An imaginary sphere having drawn around the

molecule with the molecule at its centre and

radius equal to the range of molecular force of

attraction is called sphere of influence.

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Surface tension on the basis of molecular theory

Surface tension

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Surface tension is defined as the force acting per unit

length of an imaginary line drawn on free liquid

surface at right angles on either sides of the line.

Explanation:

Units: S.I. : N/ m.

C. G. S. : dyne / cm.

Multiple Choice Questions

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1. The force of attraction between the molecules of

same substance is ……..

(a) adhesive force

(b) cohesive force

(c) molecular force

(d) intermolecular force

2. The maximum distance upto which the force of

cohesion between the two molecules can act is

called ……..

(a) radius of molecule

(b) sphere of influence

(c) molecular range

(d) intermolecular range

3. Dimensions of surface tension are ………

(a) [ M1 L0 T -2 ] (b) [ M-1 L0 T -2 ]

(c) [ M1 L1 T -2 ] (d) [ M0 L0 T -2 ]

Ans1:

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(b) cohesive force

Ans2:

(c) molecular range

Ans3:

(a) [ M1 L0 T -2 ]

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Relation between Surface Energy and Surface

Tension

∴ F = 2Tl [Factor 2 appears for 2 films, one above

and the other below the frame.]

Work = Force x displacement = 2Tlx

But, 2lx = area (A) created.

∴ W = TA

This work is stored in the film in form of potential

energy (E).

∴ E = TA

Potential energy stored in the liquid surface, per

unit surface area is called surface energy.

ESurface Energy T

A

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∴ Surface Energy = Surface Tension

Form the formula, W = TA,

T = W/A.

Thus, we get two more units for surface tension as

J / m2 and erg / cm2. However, the dimensions of

surface tension remain the same.

Applications of surface tension

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i. Tooth paste on the brush spreads freely within

the mouth and cleans the teeth more easily as it

contains soap which reduces the surface tension.

ii. If detergent is added to water, it is soluble to

some extent in water and moves into the water.

The surface tension of soap solution reduces

which increases the area of contact and the

cleaning ability of soap surface tension.

iii. Surface tension phenomenon is used in dying

process.

iv. It is used to detect jaundice in medical

laboratories.

v. It is used in blending of different types of oils.

vi. Human skin cuts are wound by antiseptic which

spreads more easily, thus lowering the surface

tension.

vii. Due to surface tension of water, the mosquito

eggs float on the water surface. But if kerosene is

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sprayed on it, the surface tension is lowered and

the eggs go down and mosquitoes breed down

into water.

Home Work Questions

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1. A needle of length 5 cm can just rest on the

surface of water of surface tension 0.073 N / m.

Find the vertical force required to detach this

floating needle from the surface of water.

2. A horizontal circular loop of wire of diameter 0.08

m is lowered in to a oil. The force due to surface

tension required to pull the loop out of the liquid is

0.0226 N. Calculate the surface tension of the oil.