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".
