Bromine vapour is heavier than air , even then it goes up ( against gravity ) and mixes with air in the upper jar . Why it is so ?
Bromine vapour settles in the lower jar at first, but once the separator is removed convection current carry them upwards, and gaseous diffusion causes mixing with air even without convection. This happens because gas molecules are in a state of constant motion, colliding with other molecules causing them gradually to diffuse or "spread" in all directions. The bromine fumes gradually spread into all the space available - which in this case is a jar.