Pressure
Pressure is the force acting on a surface per unit area in a direction perpendicular to it. It is

a scalar quantity and its SI unit is N / m2 named pascal ( Pa ) in honour of the French scientist Blasé Pascal. Its dimensional formula is M 1 L - 1 T - 2. Thus,

Pressure, P (Pa) = Force, F (N) . Area, A (m2 )

A bigger unit of pressure is ‘bar’. 1 bar = 10 5 Pa.
1 atmosphere pressure (atm) = 1.013×105 Pa or N/m2 = 760 mm (76 cm ) of Hg column.

Density:
Density is the ratio of mass to volume of an object. It is a scalar quantity and its S I unit is

kg /m3.

Liquids are almost incompressible. Hence, the density of a liquid remains almost constant at a given temperature for small change in the value of pressure. Gases are compressible. Hence, the volume of gas decreases and density increases with increase of pressure.

Relative density / Specific density / Specific gravity:
“Relative density also known as specific density or specific gravity of a given substance is

the ratio of its density to the density of water at 277 K ( i.e., 4° C ).” It is a dimensionless quantity and hence does not have a unit. Also,

Relative ( specific ) density of an object =Mass of an object/Mass of the same volume of water at 277 K