Relative Density

Key notes :

Definition of Relative Density:

  • Relative density (RD) is the ratio of the density of a substance to the density of a reference substance, usually water (for liquids and solids) or air (for gases).
  • It is also called specific gravity.

Formula:

  • The formula for relative density is:

Relative Density = Density of Substance / Density of Water

  • It is a dimensionless quantity because it’s a ratio of two similar quantities.

No Units:

  • Since relative density is a ratio, it does not have any units. It is simply a number.

Reference Substance (Water) :

  • For liquids and solids, the density of water ( which is 1 g/cm³ or 1000 kg/m³ at 4°C ) is used as the reference.
  • If RD > 1: The substance is denser than water and will sink in water.
  • If RD < 1: The substance is less dense than water and will float.

Importance of Relative Density:

  • It is used to compare the densities of different materials without needing to know their actual density values.
  • It helps determine whether an object will float or sink in a liquid.

Applications:

  • Floating and Sinking: Relative density helps in predicting whether an object will float or sink in water (e.g., ships, icebergs).
  • Hydrometer: A device used to measure the relative density of liquids.
  • Quality Control: Used in industries to compare the quality of materials (e.g., oil, metal).

Example Calculation:

  • If the density of a material is 2.5 g/cm³ and the density of water is 1 g/cm³, then the relative density of the material is:

RD = 2.5 / 1 = 2.5

  • This means the material is 2.5 times denser than water.

Factors Affecting Relative Density:

  • Temperature: The density of both the substance and the reference liquid (water) can change with temperature.
  • Impurities: The presence of impurities can alter the density and hence affect the relative density.

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