Introduction

Specific gravity is an important physical property used to compare the density of a material with the density of water. It indicates whether a material is heavier or lighter than water. Specific gravity is widely used in material identification, fluid mechanics, metallurgy, and engineering design.

Unlike density, specific gravity is a dimensionless quantity because it is a ratio of two densities.

Definition

Specific Gravity (Relative Density):
Specific gravity is the ratio of the density of a material to the density of water at standard temperature (usually 4°C).

SG = \frac{\rho_{material}}{\rho_{water}}

Where:

• $SG$ = Specific gravity

• $\rho_{material}$ = Density of the material

• $\rho_{water}$ = Density of water

Since both numerator and denominator have the same units, specific gravity has no unit.

Core Concept Explanation

Specific gravity helps engineers determine how heavy a material is compared to water.

Important interpretations:

• SG > 1 → Material is heavier than water (will sink).

• SG < 1 → Material is lighter than water (will float).

• SG = 1 → Density equal to water.

Example:

Thus, wood floats in water while metals usually sink.

Important Classifications

1. Specific Gravity of Solids

Determined by comparing the density of the solid material with water.

Common for metals, alloys, and engineering materials.

2. Specific Gravity of Liquids

Determined using instruments such as:

• Hydrometer

• Pycnometer

Used for liquids like oils and fuels.

3. Specific Gravity of Gases

Defined relative to air instead of water.

Example:

Specific gravity of gas = Density of gas / Density of air.

Key Principles / Concepts

1. Ratio Concept

Specific gravity is always a ratio of densities.

2. No Unit

Because it is a ratio, specific gravity is dimensionless.

3. Temperature Effect

Specific gravity varies slightly with temperature because density changes with temperature.

Important Comparisons

Properties / Characteristics

Important characteristics of specific gravity:

• Dimensionless quantity

• Indicates relative heaviness of materials

• Depends on density

• Slightly affected by temperature

Typical values:

Applications in Engineering

Specific gravity is widely used in engineering practice.

1. Material Identification
Helps identify unknown materials.

2. Metallurgy
Used in metal and alloy characterization.

3. Fluid Mechanics
Used in analysis of liquids and gases.

4. Petroleum Industry
Used to measure density of fuels and oils.

Exam-Focused Points

Important facts frequently asked in JE/AE exams:

• Specific gravity = density of material / density of water.

• It is dimensionless (no unit).

• Water density reference temperature = 4°C.

• SG > 1 → material sinks in water.

• SG < 1 → material floats in water.

Common Exam Traps

Trap 1

Writing units for specific gravity.

Correct concept: specific gravity has no unit.

Trap 2

Confusing density with specific gravity.

Density → kg/m³
Specific gravity → ratio (dimensionless)

Trap 3

Using air instead of water as reference for solids/liquids.

Correct reference:

• Water for solids and liquids

• Air for gases

Example Competitive Exam Questions

Question: What is specific gravity?
Answer: The ratio of the density of a material to the density of water.

Question: Does specific gravity have units?
Answer: No, it is dimensionless.

Question: What happens if the specific gravity of a material is greater than 1?
Answer: The material sinks in water.

Question: What is the reference substance for specific gravity of solids?
Answer: Water.

Question: At what temperature is water density taken as reference for specific gravity?
Answer: 4°C.

Quick Revision Summary

• Specific gravity = ratio of density of material to density of water.

• Formula shown above.

• Dimensionless (no unit).

• SG > 1 → sinks in water.

• SG < 1 → floats in water.

• Reference temperature for water = 4°C.