Tips & Formulas Surface Tension

SURFACE TENSION Surface tension is basically a property of liquid. The liquid surface behaves like a stretched elastic membrane which has a natural tendency to contract and tends to have a minimum possible area. This property of liquid is called surface tension. 1. Intermolecular forces (a) Cohesive force The force acting between the molecules of one type of molecules of same substance is called cohesive force. (b) Adhesive force The force acting between different types of molecules or molecules of different substance is called adhesive force. Intermolecular forces are different from the gravitational forces not obey the inverse-square law. The distance upto which these forces effective, is called molecular range. This distance is nearly 10–9 m. Within this limit this increases very rapidly as the distance decreases. Molecular range depends on the nature of the substance 2. Properties of surface tension (a) (b) (c) (d) (e) Surface tension is a scalar quantity It acts tangential to liquid surface Surface tension is always produced due to cohesive force More is the cohesive force, more is the surface tension. When surface area of liquid is increased, molecules from the interior of the liquid rise to the surface. For this, work is done against the downward cohesive force. 3. Dependency of Surface Tension (a) On Cohesive Force : Those factors which increase the cohesive force between molecules increase the surface tension and those which decrease the cohesive force between molecules decrease the surface tension. (b) On Impurities : If the impurity is completely soluble then on mixing in it the liquid, its surface tension increase e.g., on dissolving ionic salts in small quantities in a liquid, its surface tension increases. If the impurity is paratially soluble in a liquid then its surface tension decreases because adhesive force between insoluble impurity molecules and liquid molecules decreases cohesive force effectively. e.g. (a) On mixing detergent in water its surface tension decreases. (b) Surface tension of water is more than (alcohol + water) mixture. (c) On Temperature On increasing temperature surface tension decreases. At critical temperature and boiling point it becomes zero. Note : Surface tension of water is maximum at 4 oC (d) On Contamination The dust particles or lubricating materials on the liquid surface decreases its surface tension. MECHANICAL PROPERTIES OF MATTER (e) On Electrification The surface tension of a liquid decreases due to electrification because a force starts acting due to it in the outward direction normal to the free surface of liquid Definition of surface tension 4. The force acting per unit length of an imaginary line drawn on the free liquid surface at right angles to the line and in the plane of liquid surface is definedas surface tension. For floating needle 2T l sin = mg Required excess force for lift 5. • Wire Fex = 2T l • Hollow disc Fex = 2T (r1 + r2) • For ring Fex = 4rT • Circular disc Fex = 2rT • Square frame Fex = 8aT • Square plate Fex = 4aT • Soap bubble W = 8r2T Work = surface energy = TA • Liquid drop W = 4r2T Splitting of bigger drop into smaller droples • R = n1/3 r 1 1  Work done = Change in surface energy = 4R 3 T     rR 2 T n1 3  1 r R   Excess pressure Pex = Pin – Pout 6. • In liquid drop Pex = 2T R • In soap bubble Pex = 4T R ANGLE OF CONTACT (C) The angle enclosed between the tangent plane at the liquid surface and the tangent plane at the solid surface at the point of contact inside the liquid is defined as the angle of contact. The angle of contact depends the nature of the solid and liquid in contact. Angle of contact  < 90o  concave shape, Liquid rise up Angle of contact  > 90o  convex shape, Liquid falls Angle of contact  = 90o  plane shape, Liquid neither rise nor falls 7. Effect of Temperature on angle of contact 1  On increasing temperature surface tension decreases, thus cosc increases Q cos c   and cdecrease. T  So on increasing temperature, c decreases. 8. Effect of Impurities on angle of contact (a) Solute impurities increase surface tension, so cos c decreases and angle of contact c increases. (b) Partially solute impurities decrease surface tension, so angle of contact cdecreases. 9. Effect of Water Proofing Agent Angle of contact increases due to water proofing agent. It gets converted acute to obtuse angle. Capillary rise : h  10. 2T cos  rg 1 r • Zurin`s law h  • Jeager`s method T  rg  H  hd  2 The height ‘h’ is measured from the bottom of the meniscuc. However there exist some liquid above this line also. 1   rg  h  r  3   If correction of this is applied then the formula will be T  2 cos  11. When two soap bubbles are in contact then radius of curvature of the common surface r 12. r1r2  r1  r2  r1  r2 When two soap bubbles are combining to form a new bubble then radius of new bubble r  r12  r22 13. Force required to separate two plates F = 2AT d