Question

Which of the following substances has the greatest viscosity?
A) Mercury
B) Water
C) Kerosene
D) Glycerine

Answer 1

Hint: Viscosity is the measure of resistance that is offered by a given fluid when it is subjected to deformation or is allowed to flow. Both gases and liquids exhibit this property but in liquids, viscosity is much more pronounced, and can highly affect its mechanical as well as physical properties. Mercury, Water, Kerosene, and Glycerine are all liquid at room temperature.

Complete step by step solution:
Viscosity is also sometimes termed as “fluid friction”.
The main factor that affects the viscosity of a liquid is the intermolecular force of attraction. But there are other factors too, like- shape of individual molecules or the temperature.
All of the given substances are liquid at room temperature, so, on looking at their absolute viscosity values at room temperature (300K), we see:
Mercury - $0.0015{\text{ N s }}{{\text{m}}^{ - 2}}$
Water- $0.00089{\text{ N s }}{{\text{m}}^{ - 2}}$
Kerosene- $0.00164{\text{ N s }}{{\text{m}}^{ - 2}}$
Glycerine- $0.950{\text{ N s }}{{\text{m}}^{ - 2}}$

By looking at these values, it can be concluded that Glycerine is the most viscous substance from the given list.

Hence, the answer is option (D).

Additional Information:
Viscosity is defined as a ratio of-
1. Shearing stress: This is stress which tries to deform a body while acting tangential to its surface. When the topmost layer of the liquid moves forward with velocity, the bottom layers create resistance inflow due to attractive force, this generates shear stress. And,

2. Velocity Gradient: Since in a liquid, different layers of the liquid move with different velocity, faster on the surface to almost zero on the bottom, a velocity gradient is defined as the difference between velocities of two such layers.
The absolute viscosity ($\eta $) of a liquid can be determined by dropping a sphere into a liquid, which gives us a formula-
$\eta = \dfrac{{2g{a^2}(\vartriangle \rho )}}{{9v}}$ Where, g=acceleration due to gravity, a = Radius of the sphere, and v= Velocity with which the sphere moves through a liquid. The unit of viscosity is Pa S (Pascal Second).

Note: One should remember the values of the viscosity for such questions. If not remembered, it can also be solved by eliminating the options as we see these liquids in our daily lives. Water, mercury, and kerosene are easily flowable liquids so they can be eliminated. So, glycerine is found to be the most viscous liquid.