Define the specific gravity of a fluid. What is its unit?

Hint:The ratio of a substance's density (mass per unit volume) to the density of a particular reference material is known as relative density or specific gravity. For liquids, the reference is almost always water at its densest temperature (\[4{\text{ }}^\circ C\]); for gases, the reference is air at room temperature (\[20{\text{ }}^\circ C\]). In scientific circles, the phrase "relative density" is frequently used.

Complete answer:
A fluid's specific gravity is defined as the ratio of its density to the density of water at \[4{\text{ }}^\circ C\]. It does not have a unit. Because it is the ratio of densities or weights, relative density or specific gravity is a dimensionless number. The term "relative density of substance with regard to reference" refers to the density of the substance in relation to the reference material.

If the reference isn't given, it's usually presumed to be water at \[4{\text{ }}^\circ C\] (or 3.98 degrees Celsius, which is the temperature at which water achieves its maximum density). Water has a density of \[1000{\text{ }}kg{m^{ - 3}}\] in SI units, which makes relative density calculations simple: the object's density just has to be divided by 1000 or 1, depending on the units.
\[RD = \dfrac{{{\rho _{{\text{substance}}}}}}{{{\rho _{{\text{reference}}}}}}{\mkern 1mu} \]

If the relative density of a material is less than one, it is less dense than the reference; if it is more than one, it is denser. The densities are identical if the relative density is precisely 1; that is, equal volumes of the two substances have the same mass. A substance having a relative density (or specific gravity) less than 1 will float in water if the reference material is water. An ice cube, for example, with a relative density of 0.91 will float. A material sinks if its relative density is larger than one.

Note:Relative density can also be used to calculate a material's buoyancy in a fluid or gas, or to determine the density of an unknown substance based on the density of a known substance. Geologists and mineralogists frequently utilise relative density to estimate the mineral composition of a rock or other sample. It is used by gemologists to help in the identification of gemstones. Water is selected as a reference because field measurements are easier to do (see below for examples of measurement methods).