The density of a substance is defined as the mass per unit volume.
Its S.I. unit is kg∕m3.
Density = Mass / Volume.
An object that is partly, or completely, submerged experiences a greater Pressure on its bottom surface than on its top surface. This causes a resultant force upwards. This force is called upthrust.
Upthrust = Apparent loss of weight of object = Weight in the air – Weight in liquid.