SURFACE TENSION Presentation by Mrs. V. T. Patil

SURFACE TENSION INTRODUCTION DIFFERENT FORCES MOLECULAR THEORY DEFINATION & UNIT ANGLE OF CONTACT SIGNIFICANCE CAPILLARY ACTION AND EXAMPLES EFFECT OF TEMP & IMPURITY EXPRESSION OF CAPILLARY RISE APPLICATIONS OF SURFACE TENSION

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

Examples of Surface Tension

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.

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.

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.

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)

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.

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.

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.

EFFECT OF TEMP. & IMPURITY 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

Capillary rise Method

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

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.