Bromine vapour is heavier than air. Even so, it spreads upwards in the following experiment. Why? Place an open gas jar of air upside down on an open gas jar containing a few drops of red-brown bromine. The red-brown colour spreads upwards.

Bromine vapour is heavier than air. Even so, it spreads upwards in the given experiment because of the process of diffusion . An open gas jar of air when placed upside down on an open gas jar containing a few drops of red-brown bromine. The red-brown colour spreads upwards because particles of bromine vapour mix among the particles of air. In this experiment, particles mix by colliding with each other and bouncing off in all directions. This mixing process is called diffusion. The overall result is the flow of particles from where they are more concentrated to where they are less concentrated, until they are evenly spread out. As Bromine vapour is heavier than air, it indicates that the former is more concentrated, and hence the particles flow from the former to the latter, until it is evenly spread out. In rock and other solids, the particles are not free to move around. But in liquids and gases, they move freely. As they move they collide with each other; and bounce off in all directions. The particle moves randomly, changing direction every time it hits another particle. This is called as random motion. Hence, solid substances do not exhibit diffusion. Only gases and liquids can exhibit diffusion.

Diffusion is a mixing process in which the particles mix by colliding with each other and bouncing off in all directions. The overall result is the flow of particles from where they are more concentrated to where they are less concentrated, until they are evenly spread out. In rock and other solids, the particles are not free to move around. But in liquids and gases, they move freely. As they move they collide with each other; and bounce off in all directions. The particle moves randomly, changing direction every time it hits another particle. This is called as random motion. Hence, solid substances do not exhibit diffusion. Only gases and liquids can exhibit diffusion.