As the temperature of water increases, its viscosity 

CONCEPT:

• Viscous force (F): When a layer of fluid slips or tends to slip on adjacent layers in contact, the two layers exert tangential force on each other which tries to oppose the relative motion between them.
  • The property of a fluid due to which it opposes the relative motion between its different layers is called viscosity (or fluid friction or internal friction) and the force between the layers opposing the relative motion is called viscous force.

The force acting between the different layers of a fluid is given by:

\(F = - \eta A\frac{{dv}}{{dx}}\)

Where η = coefficient of viscosity, A = area of the plane and dv/dx = velocity gradient.

• A negative sign is employed because viscous force acts in a direction opposite to the flow of liquid.

EXPLANATION:

• With an increase in pressure, the viscosity of liquids (except water) increases while that of gases is practically independent of pressure. The viscosity of water decreases with an increase in pressure.
• The viscosity of liquid decreases with an increase of temperature, because the cohesive force between the liquid molecules decreases with an increase of temperature. Hence option 2 is correct.
• The relation between the coefficient of viscosity and temperature is given by Andrade formula –

\(\eta = \frac{{A{e^{C\frac{\rho }{T}\;}}}}{{{\rho ^{\frac{{ - 1}}{3}}}}}\)

Where T= Absolute temperature of a liquid, ρ = density of a liquid, A and C are constants.