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Viscosity ( ). It is the internal resistance of a liquid to its flow. It is the property of a liquid by virtue of which it opposes the relative motion of its different layers. The force of friction F resisting the relative motion of the two layers is given as:
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Viscosity () It is the internal resistance of a liquid to its flow. It is the property of a liquid by virtue of which it opposes the relative motion of its different layers. The force of friction Fresisting the relative motion of the two layersis given as: A = areadx = distance between the layersdv = velocity difference Here is the coefficient of viscosity defined as “the force per unit area required to maintain the unit difference of velocity between two parallel layers of a liquid, unit distance apart at a given temperature”.
Units of Viscosity () 1 poise = 0.1 kg m-1 s-1 1 poise = 100 centipoise Fluidity: Reciprocal of coefficient of viscosity is called as fluidity Fluidity = 1/ Factors Affecting Viscosity () (i) Intermolecular forces (ii) Temperature (iii) shape of molecules
Measurement of Viscosity () Viscosity can be determined with the help of Poiseulle’s equation Where “v” is the volume of the liquid having viscosity flowing through a capillary tube of radius “r” and length “L” in time “t” seconds under applied pressure”P” Instead of absolute coefficient of viscosity, we find relative viscosity w.r.t. water Measured by Ostwald’s Viscometer
In homologous series, the increase in viscosity per CH2 group is approximately constant Rheochor It is the molar volume of the liquid at the temperature at which its viscosity is unity Rheochor is additive and constitutive property Molecular mass of Polymers [] is intrinsic viscosity which is the limiting viscosity number when conc. of the polymer in a given solvent approaches zero k & a are empirical parameters which depend upon the nature of solvent and solute a is the shape factor a = 0.50 for randomly coiled molecules = 2 for rod like molecules = 0 for spherical molecules
Refractive Index It is the ratio of the velocity of light in vacuum or air to the velocity of light in the given medium i • R. I. is a characteristic property of a liquid. • It varies with temperature and wavelength of the light used.Normally it is measured at 20oC using D-line of sodium and represented as: r Critical Angle: “r” is maximum when “I” = 90o. This value of the angle of refraction is called as critical angle.If the angle of incidence is greater than 90, then the ray is totally reflected.
Specific Refraction (or Refractivity) Lorentz and Lorenz (1880) derived a relationship between refractive index and density of a liquid • R is the specific refraction or refractivity which is independent of temp. • “n” is the refractive index and “d” is the density of the liquid. For comparison Molar Refraction or Molar Refractivity (Rm) is employed which is the product of specific refraction and molecular mass i.e. • Rm is also temp. independent but it depends upon the wavelength of light. • Its units are the units of M/d (i.e. m3 mol-1) • It is an additive an constitutive property.
Measurement of Refractive Index • Abb’s Refractometer: Its optical system consists of(i) a mirror (ii) two prisms housed in a box (iii) a fixed telescope and an eyepiece • Pulfrich Refractometer:. Factors • Molecular Structure • Intermolecular Structure • Temperature
