Question

Viscosity of a liquid increases by:
A.Increase in Temperature
B.Decrease in molecular size
C.Increase in molecular size
D.None of the above

Answer 1

Viscosity of a liquid is the resistance to its flow or change in shape. It can also be described as the internal friction of the fluid. This property is dependent on both the molecular weight of the fluid as well as the temperature of the fluid.

Complete step by step answer:
The viscosity of a liquid decreases rapidly with the increase in temperature and this is because the molecular force of attraction between the molecules decreases in the liquid and hence the power to resist the flow loosens up. Hence, option A is wrong.
Viscosity of the liquid is directly proportional to the size of the molecules or molecular size, the larger the molecule, the higher is the time required to move and hence more is the viscosity. Hence, option B is incorrect.
Continuing from the above point, liquids with large molecular size should have high molecular weight and hence should be more viscous compared than a low molecular weight liquid. So, option C is correct.

Hence, option C is the correct option.

Note:
As gases can flow too, they are also considered to be fluids and for the gases, the viscosity increases with the increase in temperature, in contrast to that observed for liquids. This is because increase in temperature increases the speed of the gas molecules and hence the resistance also increases.