Specific gravity (Comprehensive explanation)

Introduction

In science and engineering, the concept of specific gravity is widely used to compare substances in a simple, dimensionless way. Instead of dealing with absolute density values (which require units), specific gravity expresses how heavy or light a substance is compared to a standard reference.

For solids and liquids, the reference is usually water at 4°C (because at this temperature water reaches its maximum density, 1000 kg/m³). For gases, the reference is air at standard conditions.

Specific gravity has been applied for centuries—from ancient methods of gold testing to modern applications in civil engineering, medical diagnostics, chemical industries, petroleum refining, food quality control, and environmental science.

Definition of specific gravity

Specific Gravity (SG) is defined as the ratio of the density of a substance to the density of water (or air, for gases).

Standard Formula of specifc gravity

SG= Density of Current substance / Density of reference SG (Mostly water at 4 deg C)

Key Characteristics:

1. It is dimensionless (no units).
2. It provides a quick comparison between materials.
   SG > 1 → substance is heavier than water.
   SG < 1 → substance is lighter than water.
3. SG helps identify substances and verify their purity
4. Unlike density, SG is easier to compare across substances because it’s a pure number
5. Indicator of Buoyancy
   Determines whether an object will float or sink in water:
   SG < 1 → floats
   SG > 1 → sinks

what factors effect specific gravity value:

What is the specific gravity of water:

The specific gravity of pure water is always taken as 1.000 at 4°C.

At other temperatures, the density of water changes slightly:

• At 0°C → 0.9998 g/cm³
• At 20°C → 0.9982 g/cm³
• At 100°C → 0.958 g/cm³

But in practice, we still approximate SG of water = 1 (unless very high precision is needed).

List of Specific Gravitys of various substance:

The following table gives approximate values of specific gravity for common substances, useful as a quick reference.

NOTE: These are the standard values (Values may vary slightly depending on temperature, purity, and composition.)

Methods of Calculating Specific gravity

1. From Density
   • Measure or look up density of the material.
   • Divide by the density of water (1000 kg/m³ at 4°C).
   • Example: Density of cement = 3150 kg/m³ → SG = 3.15.
2. Archimedes’ Principle (Solids)
   • Weigh the object in air (W₁).
   • Weigh the object submerged in water (W₂).
   • Formula:

1. Liquid Comparison
   • Fill a container with equal volumes of liquid and water.
   • Weigh both and take the ratio.

How to calculate specific gravity for different cases

Problem 1: Using Density

A liquid has a density of 850 kg/m³. Calculate its specific gravity.

SG=850/1000=0.85

Answer: SG = 0.85 (typical of diesel).

Problem 2: Solid Object

If a stone weighs 120 g in air and 90 g in water. Find its SG.

Archimedes Principal formula: (W1)/(W1-W2) = Specific gravity

W1 = 120 g
W2 = 90 g
SG= (120) / (120−90) = 4

Answer: Specifc gravity of solid object = 4

Problem 3: Medical Example

Urine sample density = 1.025 g/cm³. Find the specific gravity?.

SG=1.025 / 1.000=1.025

Answer: Specific gravity of urine = 1.025 (normal)

Problem 4: Cement

Cement density = 3150 kg/m³. Find the specific gravity?

SG= (3150) / (1000) = 3.15

Answer: Specific gravity of cement density is = 3.15

Problem 5: Gas Example

Density of oxygen = 1.429 kg/m³, density of air = 1.225 kg/m³.

SG= (1.429) / (1.225) = 1.17

Answer: Oxygen is 1.17 times heavier than air.

Methods of Measuring specific gravity

A. Laboratory / Instrument-Based Techniques

• Hydrometer: Floats in liquids, reading marked on the stem gives SG.
• Pycnometer (Specific Gravity Bottle): A small, calibrated glass bottle used for precise density measurement.
• Digital Density Meters: Automated and highly accurate instruments.
• Urinometer: Specialized hydrometer for urine SG in medicine.

B. Field / Simple Techniques

• Displacement Method: Volume of displaced water gives density → SG.
• Weight in Air and Water: Simple balance method.
• Household Approximation: Use kitchen scale and measuring cup.

Also read: Engineering Measuring Instruments: Definition, Types, Working, Applications & Advantages with Table of 40+ measuring instruments pdf

Applications of Specific Gravity

1. Civil Engineering
   • Identifying quality of cement, soil, sand, and aggregates.
   • Essential in mix design and soil classification.
2. Medical Field
   • Urine SG indicates hydration and kidney function.
   • Blood plasma SG in diagnostic tests.
3. Petroleum & Chemical Industries
   • API gravity (in petroleum) derived from SG.
   • Determines fuel quality and classification.
4. Food & Beverage Industry
   • Alcohol concentration in beer, wine, spirits.
   • Checking purity of milk and honey.
5. Environmental Science
   • Understanding oil spills (floating vs. sinking oils).
   • Studying groundwater and soil contamination.
6. Everyday Uses
   • Checking car battery acid (SG indicates charge).
   • Explaining why ice floats in water.

FAQs of Specific gravity

Conclusion

Specific gravity is one of the most versatile and practical concepts in science and engineering. It eliminates the need for units, simplifies comparison, and has real-world applications ranging from construction quality control to medical diagnosis and fuel testing.

By mastering its calculation, measurement methods, and interpretation, students and professionals can quickly evaluate the physical properties of substances and apply this knowledge across industries.