Surface Tension in liquids, class xi physics. Science

1. SURFACE TENSION

2. HESIVE FORCE
 THE FORCE OF ATTRACTION BETWEEN
MOLECULES OF SAME SUBSTANCE
ADHESIVE FORCE
THE FORCE OF ATTRACTION BETWEEN
MOLECULES OF DIFFERENT
SUBSTANCES E.G. FEVICOL ADHESIVE
TAPE , GUM

3. Surface tension
 Surface tension is the property of the liquid by
virtue of which the free surface of liquid at
rest tends to have minimum area and acts
like a stretched membrane.

4. DEFINITION
 SURFACE TENSION OF
A LIQUID IS MEASURED
AS THE FORCE ACTING
ON UNIT LENGTH OF A
LINE IMAGINED TO BE
DRAWN ON THE FREE
SURFACE OF THE
LIQUID AT REST

5. Surface tension on the basis of
molecular theory
 The force of attraction between molecules is due to
electrical interaction between the charges
 The maximum distance up to which the molecule can
attract some other molecule is range of molecular
forces
 Sphere of radius is equal to range of molecular forces
about given molecule as centre is called sphere of
influence
 The thin film of liquid near the surface and having
thickness equal to molecular range is called surface
film

6. MOLECULAR THEORY OF
SURFACE TENSION

7. Surface energy
 Surface energy is the work done per unit
increase in surface area
 Numerically
 Surface energy = surface tension
=> S = σ
Surface energy = work done/ΔArea
=> S = W/ΔA

8. Angle of contact
 Angle of contact @is the angle made by the
tangent to the liuid surface at the point of contact
with the solid surface inside the liquid
 Angle of contact may be
Acute
Obtuse
or 90”

9. Capillarity
 The rise and fall of
liquid in a tube of very
fine bore is called
capillarity

10. Factors affecting surtace tension
 Surface tension depends on temperature
 Surface tension decreases with increase in
temperture
 Surface tension depends on impurities present
in the liquid
highly soluble substances increase the surface
tension of water
whereas sparingly soluble substances decrease
the surface tension

11. Practical applications of surface
tension
 Cleaning action of
detergents
 Breeding of mosquitoes
is prevented by pouring
kerosene on stagnant
water
 Paints have a low value
of surface tension
 Antiseptics have low
value of surface tension

12. Explain why
 Hot soup tastes better than cold one
 Why needle coated with grease floats on the
surface of water
 Why drop of liquid under no external force is
always spherical in shape
 Small pieces of camphor dance on the water
surface