Viscosity is characteristic property of liquid, viscosity describes the flow of a liquid.
   Viscosity is defined as the resistance in the flow of a liquid Or
Internal friction present between two layers of a liquid
which resists the flow of liquid is commonly known as Viscosity.

   ∑ A liquid with high viscosity is thick and flows slowly.

   ∑ A liquid with low viscosity is thin and flows quickly.

   ∑ Different liquids have different viscosities.

   Size of molecules
   Viscosity of a liquid having large molecules is high whereas the viscosity of those liquids that have small    molecules is low.    
Shape of molecules
   Spherical molecules provide resistance but oval shaped or disc like molecules provide greater resistance in    the flow of liquid. Thatís why viscosity of liquids having spherical molecules is low.
   Inter molecular forces
   Liquids having large inter molecular forces have greater viscosity.

   Viscosity of liquid decreases with increase in temperature. Because an increase in temperature, reduces the forces of attraction between molecules.
   ∑ Most common unit is "POISE"

   ∑ Small unit is "CENTI POISE"

   ∑ In S.I system unit is N.S/m2

   Conversion factors
   1Centipoise = 10-3 NS/m2 or 0.001 NS/m2


   Surface tension is a characteristic property of a liquid.


   "Perpendicular force acting on the unit length of the surace of a liquid is called SURFACE TENSION".

   Surface tension = force/length

   g = F/L

   2nd definition:

   "Energy per unit area on the surface of a liquid is called SURFACE TENSION"

   s = energy /area

   Unit of surface tension

∑ N/m (in S.I system)

∑ Dyne/cm (in C.G.S system)

∑ Joule/m2 (in S.I system)

∑ Erg/cm2 (in C.G.S system)


   Inter molecular forces

   If force of attraction between molecules is high then surface tension will also be high.

   Hydrogen bonding

   Liquids that have H-bond such as water, have high values of surface tension.


   Surface tension of a liquid decreases with the increase in temperature because an increase in temperature, reduces force of attraction between molecules.