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Gravitation

10 Concept of Density and Specific Gravity

DENSITY

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

That is:

• 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.

, 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.