Question

There are 3 liquids, water, honey and air. The decreasing order of viscosity if these liquids are-
1. Water > ​Honey > Air
2. Water < ​Honey < Air
3. Honey > Water > Air
4. ​Honey < Water < Air

Answer 1

Correct Answer - Option 3 : Honey > Water > Air

CONCEPT:

​Viscosity:

• S I unit: Pascal seconds (Pa·s) or kg·m−1·s−1.
• It is the property of a liquid representing the internal resistance of its different layers to motion.
• The forces between the layers opposing motion are known as Viscous force.
• It is the internal property of liquids & gases which is produced by the cohesive forces of the molecules.
• Generally, viscosity falls rapidly with temperature & it rises with rising pressure except in water.
• The viscosity of the fluid at rest will be 1 Poise.
• It is independent of the state of the fluid.

​EXPLANATION:

​The viscosity of various substances are:

Substance	Viscosity (Pa·s)
Water (at 20oC)	1.0
Milk (at 25oC)	2.0
Liquid Honey (at 20oc)	10
Air (at 15oC)	1.81 x 10-5
Blood (at 37oC)	3 - 4
Glycerin (at 20oC)	1420

The correct order is Honey > water > air

• It is controlled by the strength of intermolecular forces & shapes of the molecules of a fluid.
• Fluids whose molecules are polar or can form hydrogen bonds are usually more viscous than nonpolar substances.
• The viscosity of a liquid always decreases as temperature increases.
• As the molecules gain more energy, they can escape from their intermolecular forces more easily.

• The viscosity of a liquid is measured by a Viscometer or Rheometer.
• According to Newton, the viscous force acting between any two layers of the fluid is proportional to the velocity gradient (dv/dx) in the direction perpendicular to the layers & area (A) of the layer.
  • If the viscous force F is acting between two layers of fluid of area A & have a velocity gradient dv/dx between them, then:
    • F = -n A dv / dx. 
      • ​F / A = - n dv/dx.  i.e: Shear stress is directly proportional to Shear strain.
      • F / A directly proportional dv/dx, where
        (dv/dx) = velocity gradient & n= coefficient of viscosity.
• Factors on which viscosity depends:
  • Distance between the two adjacent layers.
  • Area of contact between the two adjacent layers.
  • The velocity difference between the two adjacent layers.