This experiment determines the density and relative density (specific gravity) of an unknown liquid by the pycnometric method. The R.D. bottle is a calibrated borosilicate glass vessel with a ground-glass capillary stopper that automatically expels excess liquid and eliminates air bubbles, ensuring a precisely reproducible volume at each filling. Three sequential weighings on a 4 decimal place analytical balance — empty bottle (w1), bottle filled with distilled water (w2), and bottle filled with the unknown liquid (w3) — provide all data required. The density is calculated from the ratio rho(liquid) = (w3 – w1)/(w2 – w1) g/mL, exploiting the identical volume for both liquids and the known reference density of water (0.9971 g/mL at 25°C). Relative density equals the liquid density divided by water density and is numerically equal to the density at 25°C. A worked example using benzene demonstrates the calculation, error analysis, and identity inference by density matching. The chapter provides a full safety and handling profile for benzene as a Group 1 IARC carcinogen with flash point minus 2°C, requiring fume hood work and collection in halogenated organic waste.