The ratio of specific weight of a liquid to the specific weight of pure water at a standard temperature is called:
Try it first: pick an option, then read the explanation.
Asked in SSC JE 2012
Detailed Explanation
In fluid mechanics, comparing a liquid's weight density to a standard reference fluid provides a convenient dimensionless ratio.
\[ \text{Specific gravity } (S) = \frac{\text{Specific weight of liquid}}{\text{Specific weight of water at } 4^\circ\text{C}} = \frac{\gamma_{\text{liquid}}}{\gamma_{\text{water}}} = \frac{\rho_{\text{liquid}}}{\rho_{\text{water}}} \]
Why “specific gravity of liquid” Is Correct
By definition, the ratio of specific weight (or density) of a liquid to that of pure water at 4°C is the specific gravity (relative density) of the liquid.
| Property | Formula | SI unit | Standard value (water, 4°C) |
|---|---|---|---|
| Density ρ | m/V | kg/m³ | 1000 kg/m³ |
| Specific weight γ | ρg | N/m³ | 9810 N/m³ |
| Specific gravity S | ρ/ρ_water | — | 1.0 |
| Specific volume v | 1/ρ | m³/kg | 0.001 m³/kg |
| Bulk modulus K | −dp/(dV/V) | Pa | 2.06 GPa |
| Compressibility β | 1/K | Pa⁻¹ | 4.85 × 10⁻¹⁰ Pa⁻¹ |
Why the Other Options Are Wrong
- A. density of liquid: Density of liquid is mass per unit volume (kg/m³), which has units.
- C. compressibility of liquid: Compressibility is the reciprocal of bulk modulus (Pa⁻¹).
- D. surface tension of liquid: Surface tension is force per unit length (N/m).
Key Point
Specific gravity = Specific weight of liquid / Specific weight of water at standard temperature.
Key Concepts for Students
- Pure water at 4°C has specific weight γ = 9810 N/m³ and density ρ = 1000 kg/m³.
- Specific gravity is a pure dimensionless number.
- Also called relative density.
Practice more: this is question 75 in Properties of Fluid MCQs – Part 5. Try the whole set and check every answer instantly.