Question

if you have 2 jar placed over each other . lower jar 'A' contains CO2 and upper jar 'B' contains Bromine . what will happen ? will diffusion take place or not

Answer 1

The end effect will result into diffusion. This is because bromine is denser than CO2. The experiment will happen as follows:

The bromine gas will quickly sink into the lower jar then rise up again slowly and diffuse and mix up with the CO2. Leaving both jars filled with the characteristic brown color of bromine. In the normal occasion where bromine and air is used,with bromine in the lower jar, diffusion still takes place even though bromine is five times denser than air. 
Therefore diffusion would still take place in this scenario.