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viscosity definition chemistry

What is Viscosity? viscosity meaning

viscosity Every liquid has a tendency to flow. The rate of flowing of all liquid is not equal. Due to viscosity some liquid like glycerin, castor oil, coal tar etc. flow slowly. While some other liquids like water, ether, alcohol, milk etc. flow rapidly. However, the forces of friction between the layers offer to offer resistance to the flow. This indicates that every liquid has some internal resistance to flow possessed by a liquid is called a viscosity.

The liquids, which flow rapidly, have low internal resistance, which is due to weak intermolecular forces, and are said to have low viscosity.

Watch video on Viscosity example

Similarly, The liquids that flow slowly have high internal resistance, which is due to strong intermolecular forces, and are said to have high viscosity.

Let us examine a liquid flowing on a glass surface. The molecular layer on contact with the stationary surface has zero velocity. Each succeeding layer of the liquid move with gradually increasing velocity. The Liquid layer situated at the center will obviously have maximum velocity.

Let us consider two adjacent moving layers of a liquid. Let these be separated by a distance “d” cm and have a velocity difference “v” cms-1. The force of friction (f) resisting the relative motion of the two layers is

• Directly proportional to the cross-sectional area, A, of Glass. fαA
• Directly proportional to the difference of velocity, v fαv
• Inversely proportional to the distance between two layers, d

Formula of viscosity

fα1/d

fαA.V/D

or, F=η.A.V/d

Where η (Greek letter eta) is the proportionality constant. It is known as the coefficient of viscosity if d=1 cm, A=1 sq. cm and v= 1 cm/sec, then

f=η. Hence, the coefficient of viscosity may be defined as the force of resistance required to maintain a velocity difference of 1 cm apart and each having an area of 1 sq.cm. The coefficient of viscosity is expressed in dynes cm–2 and netwon m-2.

Application of viscosity

• In asphyxia, failure of breathing leads to an accumulation of CO2 in blood. This swells the blood corpuscles. A rise in viscosity of blood occurs which may result in heart failure. A medicine is given to the patient, Which lowers the viscosity of blood.
• Blood pressure is caused due to the strain in pumping blood through constricted and hardened arterial capillaries. A patient suffering from the blood pressure is given a medicine which lowers the blood viscosity.

Effect of temperature on  the viscosity of liquid:

The viscosity of a liquid decreases with increase om temperature. When temperature increases, There is an increase in kinetic energy of liquid molecules, thereby decreasing intermolecular forces, resulting in the lowering of the viscosity of the liquid. The variation with temperature can be expressed by the following relationship.

η = Aexp.(E/RT)………….(i)

Where η is the coefficient of viscosity if the liquid

E os the activation energy for the viscous flow of the liquid.

R is universal gas constant (8.314k-1 Mol-1)

A is constant.

Equation (i) can be written as

logη=logA+ E/2.303R* 1/T……………………….(ii)

comparing equation (ii) with

y=mx+c (equation of straight line)

A plot of logη against 1/T gives a straight line having a slope equal to E/2.303R,

Thus, the activation energy for the viscous flow of a liquid can be obtained from the slope of the straight line,i.e,

E/2.303R = Slope

E=Slope*2.303*R