# What is the difference between "density" and "relative density"?

Density is $\text{Mass"/"Unit Volume}$; relative density is the density compared to a reference substance (usually water) under standard conditions.
$\text{Density, } \rho$ $=$ $\text{Mass"/"Volume}$, and usually is quoted with units $g \cdot m {L}^{-} 1$ or $g \cdot c {m}^{-} 3$. Relative density or specific gravity ( https://en.wikipedia.org/wiki/Relative_density) is the density compared to a reference substance, that is water, which has a density of $1 \cdot g \cdot c {m}^{-} 3$; the relative density is thus a dimensionless quantity. If relative density is $>$ $1$, the object should sink; if relative density is $<$ $1$, the object should float. Why?