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10 Concept of Density and Specific Gravity


  • The density of a substance is defined as mass of the substance per unit volume.

That is:

Density = Mass of the substanceVolume of the substance


Density = Mass Volume


  • SI unit of density is ‘kilograms per cubic metre’.
  • If the density of a substance is more than the density of water, then the substance will be heavier then water and hence sink in water.
  • If the density of a substance is less than the density of water, then the substance will be lighter than water and hence float in water.

Concept of density

  • Every substance has different density which is the amount of mass in unit volume of matter.
  • Perform the following steps:
    • Step 1: Pour one-quarter cup of honey,
    • Step 2: Pour one-quarter cup of coloured water gently on top of honey.
    • Step 3: Pour one-quarter cup of coconut oil on top of the coloured water.
  • The different liquids are present in different layers.
  • This is because different substances have different density.
  • In other words, for the same volume different substances weigh differently.
  • Heavier substances tend to settle at the bottom, like honey and lighter substances like oil tend to float at the top.

Relative Density/ Specific gravity:

  • Density of liquid water at room temperature and pressure is 1gram per 1 cubic cm.
  • This value is treated as a standard and density of any other material (usually liquids) is calculated relative to the density of water, this is called relative densityor specific gravity.
  • Hence, specific gravity is the ratio of the mass of a substance to that of a reference substance. The density of honey is approx.
    1.42 grams/cm3

    , so its specific gravity would be 1.42/1 = 1.42.

  • Specific gravity is a ratio; therefore it does not have a unit.
  • It is a dimensionless physical quantity.
  • If the specific gravity of a substance is below 1 then it will float and if it is greater than 1 it will sink.
  • The specific gravity of a substance enables us to know if it will float or sink and gives idea about relative mass or relative density.
  • Archimedes principle states that if a body is submerged partially or fully inside a fluid, the body will experience a force that will be equal to the weight of the fluid displaced.
  • This principle is used in designing of ships, in lactometers to determine the quality of milk and to determine purity of gold and other metals.

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