What is specific gravity?

Short Answer:

Specific gravity is the ratio of the density of a substance to the density of a reference substance, usually water for liquids and solids, and air for gases. It has no unit because it is a ratio of two similar quantities. Specific gravity helps in identifying materials and comparing their heaviness with water.

In simple words, specific gravity shows how heavy or light a material is compared to water. If the specific gravity of a material is greater than 1, it means the material is heavier than water; if it is less than 1, the material is lighter and will float on water.

Detailed Explanation :

Specific Gravity

Specific gravity is a very important physical property used to compare the heaviness of different materials. It is defined as the ratio of the density of a substance to the density of a reference substance. For solids and liquids, the reference substance is usually water at 4°C, where the density of water is 1000 kg/m³. For gases, the reference substance is air or hydrogen under standard conditions.

Mathematically, the specific gravity (SG) is expressed as:

Since it is a ratio of two densities having the same units, specific gravity has no unit and is therefore called a dimensionless quantity.

For example, if a material has a density of 8000 kg/m³ and water has a density of 1000 kg/m³, then the specific gravity is:

This means the material is 8 times heavier than water.

Explanation with Examples

  1. Water: The specific gravity of pure water is 1.
  2. Iron: The specific gravity of iron is about 7.87, which means iron is 7.87 times heavier than water.
  3. Wood: Some types of wood have a specific gravity less than 1 (around 0.6–0.9), so they float on water.
  4. Mercury: The specific gravity of mercury is 13.6, meaning it is 13.6 times heavier than water.

From these examples, we can easily understand that substances with a specific gravity greater than 1 will sink in water, and those with a specific gravity less than 1 will float.

Importance of Specific Gravity in Engineering

  1. Material Identification:
    Specific gravity helps engineers identify and classify materials. For example, in metallurgy, it helps distinguish between metals and alloys.
  2. Fluid Mechanics:
    In fluid mechanics, specific gravity is used to compare different fluids and understand how they behave under pressure and flow conditions.
  3. Hydraulic Engineering:
    It is used in the design of hydraulic machines like pumps, turbines, and fluid measuring instruments to understand how fluids of different weights behave.
  4. Civil Engineering:
    Specific gravity of construction materials like sand, cement, and aggregates is important for designing concrete mixes and ensuring strength and stability.
  5. Petroleum and Chemical Industry:
    Specific gravity is used to check the purity of liquids like oil, acids, and alcohol. A small change in specific gravity indicates the presence of impurities.

Measurement of Specific Gravity

There are different instruments and methods used for measuring specific gravity:

  1. Hydrometer:
    Used for measuring the specific gravity of liquids directly by floating the instrument in the liquid.
  2. Pycnometer:
    A small flask used for accurate determination of specific gravity of solids and liquids.
  3. Specific Gravity Bottle:
    Used for measuring the specific gravity of liquids by comparing the weight of the bottle filled with liquid and water.

Difference Between Density and Specific Gravity

  • Density is mass per unit volume (kg/m³), while specific gravity is a ratio comparing the density of a substance to that of a reference material.
  • Density has units (kg/m³), but specific gravity has no unit.
  • Specific gravity helps in easy comparison without worrying about unit conversions.

Applications of Specific Gravity

  • Used in determining whether a material will float or sink in a fluid.
  • Helpful in determining fuel quality, battery condition, and purity of fluids.
  • Used in soil mechanics to understand soil composition and strength.
  • Important for selecting materials in mechanical, civil, and chemical engineering projects.
Conclusion

Specific gravity is a ratio that shows how heavy a material is compared to a reference substance, usually water. It has no unit and is widely used in engineering to identify materials, analyze fluids, and determine material behavior. Knowing the specific gravity helps in designing safe and efficient machines, structures, and systems. It is a simple but very useful concept in all fields of engineering and science.