Specific gravity of a fluid is the ratio of its density to the density of a well-known fluid. Compare the specific gravity of water, mercury and oil. Take S_w = specific gravity of water, S_oil = specific gravity of oil, and S_M = specific gravity of mercury.
Try it first: pick an option, then read the explanation.
Asked in SSC JE 16.11.2022
Detailed Explanation
Specific gravity ranks fluids by relative density compared to pure water at 4°C. Comparing typical engineering fluids establishes their relative floating and settling hierarchy.
Standard specific gravity values:
\[ S_M (\text{Mercury}) \approx 13.6 \]
\[ S_W (\text{Water}) = 1.0 \]
\[ S_{\text{oil}} (\text{Typical oils}) \approx 0.8\text{ to }0.9 \]
Therefore: \(S_M > S_W > S_{\text{oil}}\).
Why “S_M > S_W > S_oil” Is Correct
Because mercury is much denser than water (13.6 vs 1.0) and oil is lighter than water (0.8–0.9 vs 1.0), the inequality is S_M > S_W > S_oil.
Why the Other Options Are Wrong
- A. S_M < S_W > S_oil: Mercury's specific gravity is 13.6, which is far greater than water (1.0), not less.
- B. S_M > S_W = S_oil: Water and oil have different specific gravities; oil floats on water because its specific gravity is less than 1.0.
- D. S_M > S_W < S_oil: S_W < S_oil is false because oil is less dense than water.
Key Point
S_mercury (13.6) > S_water (1.0) > S_oil (0.8–0.9).
Key Concepts for Students
- Substances with S < 1 float on water; substances with S > 1 sink.
- Mercury is 13.6 times heavier than water, making it ideal for compact manometers and barometers.
- Specific gravity is independent of system of units.
Practice more: this is question 19 in Properties of Fluid MCQs – Part 2. Try the whole set and check every answer instantly.