Test: Viscosity & Surface Tension (Old NCERT) - NEET MCQ

A simple way to form a drop is to allow liquid to flow slowly from the lower end of a vertical tube of small diameter. The surface tension of the liquid causes the liquid to hang from the tube, forming a pendant. When the drop exceeds a certain size it is no longer stable and detaches itself. The falling liquid is also a drop held together by surface tension.

In general, surface tension decreases when temperature increases because cohesive forces decrease with an increase of molecular thermal activity. The influence of the surrounding environment is due to the adhesive action liquid molecules have at the interface.

The magnitude of surface tension of liquid depends on the attractive forces between the molecules. In ethanol and water, we have hydrogen bonding. However water has more effective H-bonding owing to its small size. Hexane has induced dipole-induced dipole force which is a weak force. So the order is 
III<II<I

Vapour pressure depends on both molecular interaction and thermal energy within molecular attraction vapour pressure decreases and with increase in thermal energy vapour pressure increases.
Surface tension depends on both molecular interaction and thermal energy with increase in molecular interaction surface tension increases.
Viscosity is also depends on Molecular interaction, with increase in molecular attraction viscosity increases.

Vapor pressure or equilibrium vapor pressure is defined as the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases at a given temperature in a closed system. The equilibrium vapor pressure is an indication of a liquid's evaporation rate.

The correct answer is option D
The SI unit of viscosity is the pascal second [Pa s].
The most common unit of viscosity is the dyne second per square centimeter[dyne s/cm2],
Which is given the name poise [P] after the French physiologist Jean Poiseuille
We know that 1 Pa = 1Nm^-2 and 1N = 1kgms^-2
So, from these relations we can get,
1 Pa-s = 1Nm-2s = 1 kgm-2 s-1
Ten poise equal one pascal second [Pa s] making the centipoise [cP] and millipascal second [mPa s] identical.
Hence, 10 poise = 1 kg m^-1 s^-1

Work done = surface tension×Change in area
Surface tension = 73 dyne cm^-1 or 73 x 10^-3 N/m
Area = 0.10 m²
So, work done = 73×10^-3 x 0.10
= 73 x 10^-4 J or 73×10^-4 x 10⁷ erg
= 7.3×10⁴ erg

Hence Option A is correct

Viscosity = kg m^-1 s^-1
Surface tension = N m^-1
Surface energy = J m^-2
For latent heat, we have J kg^-1 and not as in p.

The spherical shape is the shape in which it is most stable due to its surface tension. On earth also it is applicable. A drop of water wants to be in a spherical shape but is affected by other forces, so it becomes distorted. The shape is due to surface tension and not pressure. In space there is no force available to distort the spherical shape.
Adding soaps and detergents that disrupt the intermolecular attractions between adjacent water molecules can reduce the surface tension of water. Because they affect the surface properties of a liquid, soaps, and detergents are called surface-active agents, or surfactants.
 Surface tension decreases when the temperature increases because cohesive forces decrease with an increase in molecular thermal activity.

a) True, cohesive forces are intermolecular forces between like molecules and adhesive forces between unlike molecules.
b) True, only due to cohesive force, all the molecules of droplets(which are like molecule) are attracted towards each other to form drop
c) True, if the cohesive force becomes weak as to adhesive forces, then there will be no force to bind water molecules and the drop will collapse and spread into film.
d) True, It's only due to cohesive force that mercury doesn’t wet the glass.

Viscosity of liquids decreases as the temperature rises because at high temperature, molecules have high kinetic energy and can overcome the intermolecular forces to slip past one another between the layers.

In water and glycerine, the intermolecular force that exists is  Hydrogen bonding.
In the case of Hexane, Dispersion forces / London forces exists. 
The order of viscosities of these liquids is hexane < water < glycerine.
Hexane has weakest intermolecular forces and glycerine the strongest. ( three OH groups). 
Therefore, hexane has minimum viscosity and glycerine has maximum viscosity.

Assertion is false but reason is true.
Liquid tend to reduce number of molecules or surface tension at there surface, that's why small liquid drops have spherical shape.

Both statements are true and statement IIis the correct explanation of statement I. As temperature is increased, less work is required to extend the surface of a liquid therefore surface tension decreases. The reason behind is that with increase in temperature, molecular motion increases and so intermolecular forces become less effective.

With increase in temperature, viscosity increases in gases while it decreases in liquids. So both the statements are correct.