Viscosity of water in comparison to mercury is:
Try it first: pick an option, then read the explanation.
Asked in SSC JE 4 MARCH 2017 Morning Shift
Detailed Explanation
Although mercury is a heavy liquid metal with 13.6 times the mass density of water, its dynamic viscosity is only moderately higher than water.
Dynamic viscosity comparison at standard room temperature (20°C):
\[ \mu_{\text{water}} \approx 1.002\text{ cP} = 1.002 \times 10^{-3}\text{ Pa}\cdot\text{s} \]
\[ \mu_{\text{mercury}} \approx 1.55\text{ cP} = 1.55 \times 10^{-3}\text{ Pa}\cdot\text{s} \]
Therefore: \(\mu_{\text{water}} < \mu_{\text{mercury}}\).
Why “Lower” Is Correct
Water has dynamic viscosity around 1.0 cP while mercury has dynamic viscosity around 1.55 cP, meaning water's viscosity is lower than mercury's.
Why the Other Options Are Wrong
- A. Higher: Water's dynamic viscosity is roughly two-thirds of mercury's, not higher.
- C. Same: Water and mercury have distinctly different dynamic viscosities.
- D. Higher/lower depending on temperature: Across all typical liquid temperatures, water has lower dynamic viscosity than mercury.
Key Point
Dynamic viscosity of water (1.0 cP) is lower than that of mercury (1.55 cP).
Key Concepts for Students
- Note on kinematic viscosity: ν = μ/ρ. Since mercury is 13.6 times denser, its kinematic viscosity is much lower than water's.
- ν_mercury ≈ 1.55 / 13.6 ≈ 0.114 cSt, whereas ν_water ≈ 1.0 cSt.
- Dynamic viscosity reflects internal shear friction, not mass density.
Practice more: this is question 61 in Properties of Fluid MCQs – Part 5. Try the whole set and check every answer instantly.