'Density' is defined as (mass) / (volume). You need enough information to be able to measure or calculate the mass and the volume of the object.
Answer:
A hydrometer is an instrument used to determine specific gravity. It operates based on the Archimedes principle that a solid body displaces its own weight within a liquid in which it floats. Hydrometers can be divided into two general classes: liquids heavier than water and liquids lighter than water
