2. SURFACE TENSION
INTRODUCTION
DIFFERENT FORCES
MOLECULAR THEORY
DEFINATION & UNIT
ANGLE OF CONTACT
SIGNIFICANCE
CAPILLARYACTION AND EXAMPLES
EFFECT OF TEMP & IMPURITY
EXPRESSION OF CAPILLARY RISE
APPLICATIONS OF SURFACE TENSION
3. INTRODUCTION
Surface tension is a property of liquid and has a tendency to contract hence
liquid occupy the minimum surface area and surface of liquid is under
tension due to force of attraction.
Due to force of attraction of liquid molecules the free surface of liquid
behaves like elastic membrane or rubber sheet. And a particular kind of
tension produces on the surface of liquid.
Following are the examples by which we can understand the property of
surface tension
1 The small insects are easily walking on the surface of liquid
2 The small drop of water and mercury are in spherical shape.
3 A painting brush dipped into water and taken out their fibers comes very
close together due to surface tension.
4 A small niddle easily floats on the surface of water due to surface tension
5. DEFINATIONS
Adhesive force:- The intermolecular force of attraction between two
different substance is known as adhesive force.
Ex.:- Force of attraction between water molecule and glass molecule.
Cohesive force:- The intermolecular force of attraction between two same
substance is known as adhesive force.
Ex.:- Force of attraction between mercury molecule and glass molecule.
Sphere of influence :- It is an imaginary sphere surrounding a molecule is
known as sphere of influence
Molecular range:- It is the maximum distance up to which there is a
existence of cohesive force.
6. MOLECULAR THEORY
Molecule A:- In molecule A, the sphere of influence is totally inside the
liquid then the molecule A is attracted by the side way molecules with
equal force of attraction hence the resultant force acting on molecule A
is zero.
Molecule B:- In molecule B, the small portion of sphere of influence lies
above the surface of liquid and major portion inside the liquid which
contains large no. of liquid molecules so molecule B is attracted from
downward directions and hence resultant force acts on molecule B is in
downward direction.
Molecule C:- In molecule C, the half portion of sphere of influence lies above the
surface having no liquid molecules and half portion inside the liquid which
contains no. of liquid molecules so molecule C is attracted from downward
directions and hence resultant force acts on molecule C is in downward
direction.
7. DEFINATION AND UNIT
Surface Tension:- The force acting per unit length at right angles to an
imaginary line which drawn on the surface of liquid is called as surface
tension.
Surface Tension = Force
Length
T = F / L
Unit:- N /m- S.I.
Dyne / cm- C.G.S.
8. ANGLE OF CONTACT
Defination:- The angle between the tangent to the curve part of liquid surface at the
point of contact and solid part of glass inside the liquid is known as angle of contact.
It is represented by θ and measured in radian.
It depends on the nature of liquid and solid used.
Following are the values of θ for different liquids.
1 Pure water = θ = 0
2 Chloroform = θ = 0
3 Impure water = θ= 27 (acute)
4 Mercury = θ = 140 (obtuse)
9. SIGNIFICANCE OF θ
If the angle of contact is acute, i.e. less than 90 , the liquid stick to the wall of
glass container due to strong adhesive force. Hence liquid surface is concave.
Ex: In water and glass θ is acute
If the angle of contact is obtuse i.e. greater than 90, the liquid does not stick to
the wall of glass container due to strong cohesive force. Hence liquid surface is
convex.
Ex: In mercury and glass θ is obtuse.
If liquid is completely stick to the glass container then θ is zero
Ex: For pure water and chloroform, θ = zero.
10. CAPILLARY ACTION
A Small glass tube with narrow bore is called as capillary tube.
When a capillary tube is dipped in water then water rises up in the
capillary tube and it forms a concave shape.
And the same capillary tube is dipped in mercury, the mercury is depressed
down and it forms a convex shape which is shown below.
11. Defination:- The rise or fall of liquid inside the
capillary tube is called as capillary action or capillary
tube.
Ex:- 1 Oil rises in the oil lamp up to wick end
2 Water rises in the plant up to the leaves
3 Blotting paper absorbs ink due to capillary action
4 Absorption of water by a peace of sponge.
5 Ink pen absorbs ink due to capillary action.
12. EFFECT OF TEMP :- As temp increases, the surface tension of liquid
decreases and vice versa.
Surface tension α 1/temp
EFFECT OF IMPURITY:- If the impurity is added in the liquid and it is
completely soluble then surface tension of liquid increases.
But impurities not completely soluble then surface tension of liquid decreases.
Ex:- Use of detergent powder for washing the cloths
14. Surface Tension Formula
T = __rhdg___
2cosθ
Where,
T = Surface Tension of Liquid
r = Radius of Capillary tube
h = Height of liquid in the Capillary
d = Density of liquid
θ = Angle of contact
g = Acceleration due to gravity
15. APPLICATIONS OF SURFACE TENSION
The property of surface tension is used
1 To prepare the ball bearings or bullets .
2 Use of detergent powder for washing the clothes.
3 The purity of water can be checked using the value of surface
tension.
4 Oil rises in the oil lamp up to tip of wick end.
5 Low surface tension oil used for lubrication of machine parts to
reduce the friction.
6 To detect the small cracks on the surface of metal parts in NDT
method the low surface tension is used.
7 Use of towel to dry our body after bath the towel absorbs water due
to capillary action.
