Difference Between Density and Specific Gravity

The key difference between density and specific gravity is that density is an absolute value, while specific gravity is a relative value for a substance.

Density and specific gravity are the two commonly used terms that are often confused with each other. Particularly, in industries, these terms are widely important to weigh different substance and to calculate the concentration of liquids. Temperature and pressure are very important in calculating the two.

CONTENTS

1. Overview and Key Difference
2. What is Density 
3. What is Specific Gravity
4. Side by Side Comparison –  Density vs Specific Gravity in Tabular Form
6. Summary

What is Density?

Mass density, which we commonly call density of a material, is its mass per unit volume. We use the symbol P to denote density, where its SI unit is kilogram per cubic meter. Temperature and pressure are two factors that affect the density; for instance, when we increase the pressure, the volume of an object decreases, which enhances the density of that particular object. Similarly, if we increase the temperature of an object, its density decreases as the volume increases. The density of water is 1.0 g/ml. If the density of an object is less than water’s density,  it will float on the surface of the water and vice versa.

Figure 01: Whether an object will float or sink depends on its own density and the density of the liquid it is placed in.

If you increase the amount of a substance, it will increase the mass but will not affect the density. This means the density of a substance is unique and absolute.

What is Specific Gravity?

Specific gravity is a relative value, so it has no units. It is, in fact, another term for relative density. It is the ratio of the density of a substance to the density of a reference substance, which is always water for solid and liquid, for gas it is an equal volume of air or hydrogen. We can denote specific gravity as SG.

Figure 02: SG of water at different temperatures (here, SG is given by “ρ”)

Temperature and pressure are important in measuring SG, as it is a measure at the standard pressure of 1 atm and 4°C temperature. Temperature can vary in industries, as every industry has its own standards and needs. Specific gravity is useful in industries to calculate the concentration of solutions, which are manufactured for specific purposes. Further, we consider the specific gravity of water at standard temperature and pressure as 1, so if the SG of a substance is 5, it means it is five times denser than water. That substance will sink to the bottom of the water. Similarly, if the SG of a substance is less than one, it will float on the surface of the water.

What are the Similarities Between Density and Specific Gravity?

• When measuring the density and specific gravity, we use the masses of materials; therefore, these terms depends on temperature and pressure because mass is affected by temperature and pressure.
• We take the density and SG of water as 1.
• In some cases, we express density value as the SG value as well.

What is the Difference Between Density and Specific Gravity?

Density is the material per unit volume. Specific gravity is the ratio of the density of a substance to the density of a reference substance. The key difference between density and specific gravity is that density is an absolute value, while specific gravity is a relative value for a substance. Usually, we denote density as “p” and specific gravity as “SG”. Further, the unit of measurement for density is kilogram per cubic meter, while SG has no unit since it is a ratio.

Summary – Density vs Specific Gravity

Density is the material per unit volume while specific gravity is the ratio of the density of a substance to the density of a reference substance. The key difference between density and specific gravity is that density is an absolute value, while specific gravity is a relative value for a substance.

Reference:

1. White Plumeria Flower on Water” (CC0) via Pexels
2. “Specific gravity of water” By Janhuisman42 – Own work (CC BY-SA 3.0) via Commons Wikimedia