Viscosity is the property of the liquid by virtue of which it opposes the relative motion between its adjacent layers.
(A) The viscosity of an ideal fluid is zero.
(B) With the rise in temperature, the viscosity of liquids increase and that of gases decrease.
(C) The viscosity of gases is much less than that of liquids.
(D) Viscosity of liquid generally increase with increase in pressure.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (A), (C) and (D) only
In a liquid, viscosity comes mainly from the attraction between molecules of neighbouring layers; in a gas, from molecules moving between layers and carrying momentum with them. This difference decides how each responds to temperature.
(A) The viscosity of an ideal fluid is zero. Correct. An ideal fluid is defined as one with no internal friction, so its layers slide over each other freely.
(B) With the rise in temperature, the viscosity of liquids increase and that of gases decrease. Incorrect. It is the other way round: heating weakens the attraction in a liquid, so its viscosity falls, while gas molecules move faster and carry more momentum between layers, so a gas’s viscosity rises. Warm honey flows more easily than cold honey.
(C) The viscosity of gases is much less than that of liquids. Correct. Gas molecules are far apart and hardly attract each other, so gases are much less viscous; the viscosity of air is about one-fiftieth of that of water.
(D) Viscosity of liquid generally increase with increase in pressure. Correct. Pressure pushes the molecules closer together, so they slide past each other less easily; for most liquids the viscosity rises with pressure.