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Relative Density or Specific Gravity

The relative density (RD) of a material is the ratio of the density of the substance to the density of water at 4°C.
The relative density of a substance Description: 59474.png

where 1000 is density of water.

Since RD is the ratio of similar quantities, it is a number and has no units. For example, RD of iron is 7.8 means if equal volumes of iron and water are taken, iron would be 7.8 times as heavy as water.
Hydrometer measures the relative density (RD) of a liquid using the relation:
Description: 60322.png
It is based on the principle of floatation. A special type of hydrometer is used to measure the density of acid in a car battery.
Lactometer is a specially designed hydrometer used to find relative density of milk and hence for testing purity of milk.

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