JEE Main · 2021 · 25 Jul 2021 · Shift 2 · Chapter_pending

Density rises when heavier atoms are fitted onto the same molecular framework, because the mass gained outweighs the small extra volume the substituent occupies. The four compounds drawn are benzene, chlorobenzene, 1,3-dichlorobenzene and 1-bromo-3-chlorobenzene, of molar mass 78, 112.5, 147 and 191.5 g mol${}^{-1}$ on a ring of essentially unchanged size. Bromine, the heaviest substituent present, lifts the density most, two chlorine atoms come next, a single chlorine next, and the unsubstituted ring is lightest. The decreasing order of density is 1-bromo-3-chlorobenzene, then 1,3-dichlorobenzene, then chlorobenzene, then benzene.