How would you use density to identify an unknown substance?

Most elements and compounds have a very specific density value. If you figured out the density the density of the unknown substance it can be compared to all the known densities, until a match is found. E. g. if you found a liquid that had the density of 1g/cm^3 its water whereas if its 0.789g/cm^3 its pure ethanol

Density is calculated by the equation "Dentisy = mass of the substance / Volume of the substance".